Add m-esr52 at 52.6.0

73 files changed, 15195 insertions, 0 deletions

diff --git a/toolkit/components/osfile/NativeOSFileInternals.cpp b/toolkit/components/osfile/NativeOSFileInternals.cpp
new file mode 100644
index 000000000..e4725d390
--- /dev/null
+++ b/toolkit/components/osfile/NativeOSFileInternals.cpp
@@ -0,0 +1,916 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * Native implementation of some OS.File operations.
+ */
+
+#include "nsString.h"
+#include "nsNetCID.h"
+#include "nsThreadUtils.h"
+#include "nsXPCOMCID.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsServiceManagerUtils.h"
+#include "nsProxyRelease.h"
+
+#include "nsINativeOSFileInternals.h"
+#include "NativeOSFileInternals.h"
+#include "mozilla/dom/NativeOSFileInternalsBinding.h"
+
+#include "nsIUnicodeDecoder.h"
+#include "nsIEventTarget.h"
+
+#include "mozilla/dom/EncodingUtils.h"
+#include "mozilla/DebugOnly.h"
+#include "mozilla/Scoped.h"
+#include "mozilla/HoldDropJSObjects.h"
+#include "mozilla/TimeStamp.h"
+
+#include "prio.h"
+#include "prerror.h"
+#include "private/pprio.h"
+
+#include "jsapi.h"
+#include "jsfriendapi.h"
+#include "js/Utility.h"
+#include "xpcpublic.h"
+
+#include <algorithm>
+#if defined(XP_UNIX)
+#include <unistd.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <sys/stat.h>
+#include <sys/uio.h>
+#endif // defined (XP_UNIX)
+
+#if defined(XP_WIN)
+#include <windows.h>
+#endif // defined (XP_WIN)
+
+namespace mozilla {
+
+MOZ_TYPE_SPECIFIC_SCOPED_POINTER_TEMPLATE(ScopedPRFileDesc, PRFileDesc, PR_Close)
+
+namespace {
+
+// Utilities for safely manipulating ArrayBuffer contents even in the
+// absence of a JSContext.
+
+/**
+ * The C buffer underlying to an ArrayBuffer. Throughout the code, we manipulate
+ * this instead of a void* buffer, as this lets us transfer data across threads
+ * and into JavaScript without copy.
+ */
+struct ArrayBufferContents {
+  /**
+   * The data of the ArrayBuffer. This is the pointer manipulated to
+   * read/write the contents of the buffer.
+   */
+  uint8_t* data;
+  /**
+   * The number of bytes in the ArrayBuffer.
+   */
+  size_t nbytes;
+};
+
+/**
+ * RAII for ArrayBufferContents.
+ */
+struct ScopedArrayBufferContentsTraits {
+  typedef ArrayBufferContents type;
+  const static type empty() {
+    type result = {0, 0};
+    return result;
+  }
+  static void release(type ptr) {
+    js_free(ptr.data);
+    ptr.data = nullptr;
+    ptr.nbytes = 0;
+  }
+};
+
+struct MOZ_NON_TEMPORARY_CLASS ScopedArrayBufferContents: public Scoped<ScopedArrayBufferContentsTraits> {
+  explicit ScopedArrayBufferContents(MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM):
+    Scoped<ScopedArrayBufferContentsTraits>(MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM_TO_PARENT)
+  { }
+  explicit ScopedArrayBufferContents(const ArrayBufferContents& v
+                            MOZ_GUARD_OBJECT_NOTIFIER_PARAM):
+    Scoped<ScopedArrayBufferContentsTraits>(v MOZ_GUARD_OBJECT_NOTIFIER_PARAM_TO_PARENT)
+  { }
+  ScopedArrayBufferContents& operator=(ArrayBufferContents ptr) {
+    Scoped<ScopedArrayBufferContentsTraits>::operator=(ptr);
+    return *this;
+  }
+
+  /**
+   * Request memory for this ArrayBufferContent. This memory may later
+   * be used to create an ArrayBuffer object (possibly on another
+   * thread) without copy.
+   *
+   * @return true In case of success, false otherwise.
+   */
+  bool Allocate(uint32_t length) {
+    dispose();
+    ArrayBufferContents& value = rwget();
+    void *ptr = js_calloc(1, length);
+    if (ptr) {
+      value.data = (uint8_t *) ptr;
+      value.nbytes = length;
+      return true;
+    }
+    return false;
+  }
+private:
+  explicit ScopedArrayBufferContents(ScopedArrayBufferContents& source) = delete;
+  ScopedArrayBufferContents& operator=(ScopedArrayBufferContents& source) = delete;
+};
+
+///////// Cross-platform issues
+
+// Platform specific constants. As OS.File always uses OS-level
+// errors, we need to map a few high-level errors to OS-level
+// constants.
+#if defined(XP_UNIX)
+#define OS_ERROR_NOMEM ENOMEM
+#define OS_ERROR_INVAL EINVAL
+#define OS_ERROR_TOO_LARGE EFBIG
+#define OS_ERROR_RACE EIO
+#elif defined(XP_WIN)
+#define OS_ERROR_NOMEM ERROR_NOT_ENOUGH_MEMORY
+#define OS_ERROR_INVAL ERROR_BAD_ARGUMENTS
+#define OS_ERROR_TOO_LARGE ERROR_FILE_TOO_LARGE
+#define OS_ERROR_RACE ERROR_SHARING_VIOLATION
+#else
+#error "We do not have platform-specific constants for this platform"
+#endif
+
+///////// Results of OS.File operations
+
+/**
+ * Base class for results passed to the callbacks.
+ *
+ * This base class implements caching of JS values returned to the client.
+ * We make use of this caching in derived classes e.g. to avoid accidents
+ * when we transfer data allocated on another thread into JS. Note that
+ * this caching can lead to cycles (e.g. if a client adds a back-reference
+ * in the JS value), so we implement all Cycle Collector primitives in
+ * AbstractResult.
+ */
+class AbstractResult: public nsINativeOSFileResult {
+public:
+  NS_DECL_NSINATIVEOSFILERESULT
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS(AbstractResult)
+
+  /**
+   * Construct the result object. Must be called on the main thread
+   * as the AbstractResult is cycle-collected.
+   *
+   * @param aStartDate The instant at which the operation was
+   * requested.  Used to collect Telemetry statistics.
+   */
+  explicit AbstractResult(TimeStamp aStartDate)
+    : mStartDate(aStartDate)
+  {
+    MOZ_ASSERT(NS_IsMainThread());
+    mozilla::HoldJSObjects(this);
+  }
+
+  /**
+   * Setup the AbstractResult once data is available.
+   *
+   * @param aDispatchDate The instant at which the IO thread received
+   * the operation request. Used to collect Telemetry statistics.
+   * @param aExecutionDuration The duration of the operation on the
+   * IO thread.
+   */
+  void Init(TimeStamp aDispatchDate,
+            TimeDuration aExecutionDuration) {
+    MOZ_ASSERT(!NS_IsMainThread());
+
+    mDispatchDuration = (aDispatchDate - mStartDate);
+    mExecutionDuration = aExecutionDuration;
+  }
+
+  /**
+   * Drop any data that could lead to a cycle.
+   */
+  void DropJSData() {
+    mCachedResult = JS::UndefinedValue();
+  }
+
+protected:
+  virtual ~AbstractResult() {
+    MOZ_ASSERT(NS_IsMainThread());
+    DropJSData();
+    mozilla::DropJSObjects(this);
+  }
+
+  virtual nsresult GetCacheableResult(JSContext *cx, JS::MutableHandleValue aResult) = 0;
+
+private:
+  TimeStamp mStartDate;
+  TimeDuration mDispatchDuration;
+  TimeDuration mExecutionDuration;
+  JS::Heap<JS::Value> mCachedResult;
+};
+
+NS_IMPL_CYCLE_COLLECTING_ADDREF(AbstractResult)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(AbstractResult)
+
+NS_IMPL_CYCLE_COLLECTION_CLASS(AbstractResult)
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(AbstractResult)
+  NS_INTERFACE_MAP_ENTRY(nsINativeOSFileResult)
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+NS_IMPL_CYCLE_COLLECTION_TRACE_BEGIN(AbstractResult)
+  NS_IMPL_CYCLE_COLLECTION_TRACE_JS_MEMBER_CALLBACK(mCachedResult)
+NS_IMPL_CYCLE_COLLECTION_TRACE_END
+
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(AbstractResult)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE_SCRIPT_OBJECTS
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(AbstractResult)
+  tmp->DropJSData();
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+
+NS_IMETHODIMP
+AbstractResult::GetDispatchDurationMS(double *aDispatchDuration)
+{
+  *aDispatchDuration = mDispatchDuration.ToMilliseconds();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+AbstractResult::GetExecutionDurationMS(double *aExecutionDuration)
+{
+  *aExecutionDuration = mExecutionDuration.ToMilliseconds();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+AbstractResult::GetResult(JSContext *cx, JS::MutableHandleValue aResult)
+{
+  if (mCachedResult.isUndefined()) {
+    nsresult rv = GetCacheableResult(cx, aResult);
+    if (NS_FAILED(rv)) {
+      return rv;
+    }
+    mCachedResult = aResult;
+    return NS_OK;
+  }
+  aResult.set(mCachedResult);
+  return NS_OK;
+}
+
+/**
+ * Return a result as a string.
+ *
+ * In this implementation, attribute |result| is a string. Strings are
+ * passed to JS without copy.
+ */
+class StringResult final : public AbstractResult
+{
+public:
+  explicit StringResult(TimeStamp aStartDate)
+    : AbstractResult(aStartDate)
+  {
+  }
+
+  /**
+   * Initialize the object once the contents of the result as available.
+   *
+   * @param aContents The string to pass to JavaScript. Ownership of the
+   * string and its contents is passed to StringResult. The string must
+   * be valid UTF-16.
+   */
+  void Init(TimeStamp aDispatchDate,
+            TimeDuration aExecutionDuration,
+            nsString& aContents) {
+    AbstractResult::Init(aDispatchDate, aExecutionDuration);
+    mContents = aContents;
+  }
+
+protected:
+  nsresult GetCacheableResult(JSContext* cx, JS::MutableHandleValue aResult) override;
+
+private:
+  nsString mContents;
+};
+
+nsresult
+StringResult::GetCacheableResult(JSContext* cx, JS::MutableHandleValue aResult)
+{
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(mContents.get());
+
+  // Convert mContents to a js string without copy. Note that this
+  // may have the side-effect of stealing the contents of the string
+  // from XPCOM and into JS.
+  if (!xpc::StringToJsval(cx, mContents, aResult)) {
+    return NS_ERROR_FAILURE;
+  }
+  return NS_OK;
+}
+
+
+/**
+ * Return a result as a Uint8Array.
+ *
+ * In this implementation, attribute |result| is a Uint8Array. The array
+ * is passed to JS without memory copy.
+ */
+class TypedArrayResult final : public AbstractResult
+{
+public:
+  explicit TypedArrayResult(TimeStamp aStartDate)
+    : AbstractResult(aStartDate)
+  {
+  }
+
+  /**
+   * @param aContents The contents to pass to JS. Calling this method.
+   * transmits ownership of the ArrayBufferContents to the TypedArrayResult.
+   * Do not reuse this value anywhere else.
+   */
+  void Init(TimeStamp aDispatchDate,
+            TimeDuration aExecutionDuration,
+            ArrayBufferContents aContents) {
+    AbstractResult::Init(aDispatchDate, aExecutionDuration);
+    mContents = aContents;
+  }
+
+protected:
+  nsresult GetCacheableResult(JSContext* cx, JS::MutableHandleValue aResult) override;
+private:
+  ScopedArrayBufferContents mContents;
+};
+
+nsresult
+TypedArrayResult::GetCacheableResult(JSContext* cx, JS::MutableHandle<JS::Value> aResult)
+{
+  MOZ_ASSERT(NS_IsMainThread());
+  // We cannot simply construct a typed array using contents.data as
+  // this would allow us to have several otherwise unrelated
+  // ArrayBuffers with the same underlying C buffer. As this would be
+  // very unsafe, we need to cache the result once we have it.
+
+  const ArrayBufferContents& contents = mContents.get();
+  MOZ_ASSERT(contents.data);
+
+  JS::Rooted<JSObject*>
+    arrayBuffer(cx, JS_NewArrayBufferWithContents(cx, contents.nbytes, contents.data));
+  if (!arrayBuffer) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  JS::Rooted<JSObject*>
+    result(cx, JS_NewUint8ArrayWithBuffer(cx, arrayBuffer,
+                                          0, contents.nbytes));
+  if (!result) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+  // The memory of contents has been allocated on a thread that
+  // doesn't have a JSRuntime, hence without a context. Now that we
+  // have a context, attach the memory to where it belongs.
+  JS_updateMallocCounter(cx, contents.nbytes);
+  mContents.forget();
+
+  aResult.setObject(*result);
+  return NS_OK;
+}
+
+//////// Callback events
+
+/**
+ * An event used to notify asynchronously of an error.
+ */
+class ErrorEvent final : public Runnable {
+public:
+  /**
+   * @param aOnSuccess The success callback.
+   * @param aOnError The error callback.
+   * @param aDiscardedResult The discarded result.
+   * @param aOperation The name of the operation, used for error reporting.
+   * @param aOSError The OS error of the operation, as returned by errno/
+   * GetLastError().
+   *
+   * Note that we pass both the success callback and the error
+   * callback, as well as the discarded result to ensure that they are
+   * all released on the main thread, rather than on the IO thread
+   * (which would hopefully segfault). Also, we pass the callbacks as
+   * alread_AddRefed to ensure that we do not manipulate main-thread
+   * only refcounters off the main thread.
+   */
+  ErrorEvent(nsMainThreadPtrHandle<nsINativeOSFileSuccessCallback>& aOnSuccess,
+             nsMainThreadPtrHandle<nsINativeOSFileErrorCallback>& aOnError,
+             already_AddRefed<AbstractResult>& aDiscardedResult,
+             const nsACString& aOperation,
+             int32_t aOSError)
+    : mOnSuccess(aOnSuccess)
+    , mOnError(aOnError)
+    , mDiscardedResult(aDiscardedResult)
+    , mOSError(aOSError)
+    , mOperation(aOperation)
+    {
+      MOZ_ASSERT(!NS_IsMainThread());
+    }
+
+  NS_IMETHOD Run() override {
+    MOZ_ASSERT(NS_IsMainThread());
+    (void)mOnError->Complete(mOperation, mOSError);
+
+    // Ensure that the callbacks are released on the main thread.
+    mOnSuccess = nullptr;
+    mOnError = nullptr;
+    mDiscardedResult = nullptr;
+
+    return NS_OK;
+  }
+ private:
+  // The callbacks. Maintained as nsMainThreadPtrHandle as they are generally
+  // xpconnect values, which cannot be manipulated with nsCOMPtr off
+  // the main thread. We store both the success callback and the
+  // error callback to ensure that they are safely released on the
+  // main thread.
+  nsMainThreadPtrHandle<nsINativeOSFileSuccessCallback> mOnSuccess;
+  nsMainThreadPtrHandle<nsINativeOSFileErrorCallback> mOnError;
+  RefPtr<AbstractResult> mDiscardedResult;
+  int32_t mOSError;
+  nsCString mOperation;
+};
+
+/**
+ * An event used to notify of a success.
+ */
+class SuccessEvent final : public Runnable {
+public:
+  /**
+   * @param aOnSuccess The success callback.
+   * @param aOnError The error callback.
+   *
+   * Note that we pass both the success callback and the error
+   * callback to ensure that they are both released on the main
+   * thread, rather than on the IO thread (which would hopefully
+   * segfault). Also, we pass them as alread_AddRefed to ensure that
+   * we do not manipulate xpconnect refcounters off the main thread
+   * (which is illegal).
+   */
+  SuccessEvent(nsMainThreadPtrHandle<nsINativeOSFileSuccessCallback>& aOnSuccess,
+               nsMainThreadPtrHandle<nsINativeOSFileErrorCallback>& aOnError,
+               already_AddRefed<nsINativeOSFileResult>& aResult)
+    : mOnSuccess(aOnSuccess)
+    , mOnError(aOnError)
+    , mResult(aResult)
+    {
+      MOZ_ASSERT(!NS_IsMainThread());
+    }
+
+  NS_IMETHOD Run() override {
+    MOZ_ASSERT(NS_IsMainThread());
+    (void)mOnSuccess->Complete(mResult);
+
+    // Ensure that the callbacks are released on the main thread.
+    mOnSuccess = nullptr;
+    mOnError = nullptr;
+    mResult = nullptr;
+
+    return NS_OK;
+  }
+ private:
+  // The callbacks. Maintained as nsMainThreadPtrHandle as they are generally
+  // xpconnect values, which cannot be manipulated with nsCOMPtr off
+  // the main thread. We store both the success callback and the
+  // error callback to ensure that they are safely released on the
+  // main thread.
+  nsMainThreadPtrHandle<nsINativeOSFileSuccessCallback> mOnSuccess;
+  nsMainThreadPtrHandle<nsINativeOSFileErrorCallback> mOnError;
+  RefPtr<nsINativeOSFileResult> mResult;
+};
+
+
+//////// Action events
+
+/**
+ * Base class shared by actions.
+ */
+class AbstractDoEvent: public Runnable {
+public:
+  AbstractDoEvent(nsMainThreadPtrHandle<nsINativeOSFileSuccessCallback>& aOnSuccess,
+                  nsMainThreadPtrHandle<nsINativeOSFileErrorCallback>& aOnError)
+    : mOnSuccess(aOnSuccess)
+    , mOnError(aOnError)
+#if defined(DEBUG)
+    , mResolved(false)
+#endif // defined(DEBUG)
+  {
+    MOZ_ASSERT(NS_IsMainThread());
+  }
+
+  /**
+   * Fail, asynchronously.
+   */
+  void Fail(const nsACString& aOperation,
+            already_AddRefed<AbstractResult>&& aDiscardedResult,
+            int32_t aOSError = 0) {
+    Resolve();
+    RefPtr<ErrorEvent> event = new ErrorEvent(mOnSuccess,
+                                                mOnError,
+                                                aDiscardedResult,
+                                                aOperation,
+                                                aOSError);
+    nsresult rv = NS_DispatchToMainThread(event);
+    if (NS_FAILED(rv)) {
+      // Last ditch attempt to release on the main thread - some of
+      // the members of event are not thread-safe, so letting the
+      // pointer go out of scope would cause a crash.
+      NS_ReleaseOnMainThread(event.forget());
+    }
+  }
+
+  /**
+   * Succeed, asynchronously.
+   */
+  void Succeed(already_AddRefed<nsINativeOSFileResult>&& aResult) {
+    Resolve();
+    RefPtr<SuccessEvent> event = new SuccessEvent(mOnSuccess,
+                                                    mOnError,
+                                                    aResult);
+    nsresult rv = NS_DispatchToMainThread(event);
+    if (NS_FAILED(rv)) {
+      // Last ditch attempt to release on the main thread - some of
+      // the members of event are not thread-safe, so letting the
+      // pointer go out of scope would cause a crash.
+      NS_ReleaseOnMainThread(event.forget());
+    }
+
+  }
+
+private:
+
+  /**
+   * Mark the event as complete, for debugging purposes.
+   */
+  void Resolve() {
+#if defined(DEBUG)
+    MOZ_ASSERT(!mResolved);
+    mResolved = true;
+#endif // defined(DEBUG)
+  }
+
+private:
+  nsMainThreadPtrHandle<nsINativeOSFileSuccessCallback> mOnSuccess;
+  nsMainThreadPtrHandle<nsINativeOSFileErrorCallback> mOnError;
+#if defined(DEBUG)
+  // |true| once the action is complete
+  bool mResolved;
+#endif // defined(DEBUG)
+};
+
+/**
+ * An abstract event implementing reading from a file.
+ *
+ * Concrete subclasses are responsible for handling the
+ * data obtained from the file and possibly post-processing it.
+ */
+class AbstractReadEvent: public AbstractDoEvent {
+public:
+  /**
+   * @param aPath The path of the file.
+   */
+  AbstractReadEvent(const nsAString& aPath,
+                    const uint64_t aBytes,
+                    nsMainThreadPtrHandle<nsINativeOSFileSuccessCallback>& aOnSuccess,
+                    nsMainThreadPtrHandle<nsINativeOSFileErrorCallback>& aOnError)
+    : AbstractDoEvent(aOnSuccess, aOnError)
+    , mPath(aPath)
+    , mBytes(aBytes)
+  {
+    MOZ_ASSERT(NS_IsMainThread());
+  }
+
+  NS_IMETHOD Run() override {
+    MOZ_ASSERT(!NS_IsMainThread());
+    TimeStamp dispatchDate = TimeStamp::Now();
+
+    nsresult rv = BeforeRead();
+    if (NS_FAILED(rv)) {
+      // Error reporting is handled by BeforeRead();
+      return NS_OK;
+    }
+
+    ScopedArrayBufferContents buffer;
+    rv = Read(buffer);
+    if (NS_FAILED(rv)) {
+      // Error reporting is handled by Read();
+      return NS_OK;
+    }
+
+    AfterRead(dispatchDate, buffer);
+    return NS_OK;
+  }
+
+ private:
+  /**
+   * Read synchronously.
+   *
+   * Must be called off the main thread.
+   *
+   * @param aBuffer The destination buffer.
+   */
+  nsresult Read(ScopedArrayBufferContents& aBuffer)
+  {
+    MOZ_ASSERT(!NS_IsMainThread());
+
+    ScopedPRFileDesc file;
+#if defined(XP_WIN)
+    // On Windows, we can't use PR_OpenFile because it doesn't
+    // handle UTF-16 encoding, which is pretty bad. In addition,
+    // PR_OpenFile opens files without sharing, which is not the
+    // general semantics of OS.File.
+    HANDLE handle =
+      ::CreateFileW(mPath.get(),
+                    GENERIC_READ,
+                    FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE,
+                    /*Security attributes*/nullptr,
+                    OPEN_EXISTING,
+                    FILE_ATTRIBUTE_NORMAL | FILE_FLAG_SEQUENTIAL_SCAN,
+                    /*Template file*/ nullptr);
+
+    if (handle == INVALID_HANDLE_VALUE) {
+      Fail(NS_LITERAL_CSTRING("open"), nullptr, ::GetLastError());
+      return NS_ERROR_FAILURE;
+    }
+
+    file = PR_ImportFile((PROsfd)handle);
+    if (!file) {
+      // |file| is closed by PR_ImportFile
+      Fail(NS_LITERAL_CSTRING("ImportFile"), nullptr, PR_GetOSError());
+      return NS_ERROR_FAILURE;
+    }
+
+#else
+    // On other platforms, PR_OpenFile will do.
+    NS_ConvertUTF16toUTF8 path(mPath);
+    file = PR_OpenFile(path.get(), PR_RDONLY, 0);
+    if (!file) {
+      Fail(NS_LITERAL_CSTRING("open"), nullptr, PR_GetOSError());
+      return NS_ERROR_FAILURE;
+    }
+
+#endif // defined(XP_XIN)
+
+    PRFileInfo64 stat;
+    if (PR_GetOpenFileInfo64(file, &stat) != PR_SUCCESS) {
+      Fail(NS_LITERAL_CSTRING("stat"), nullptr, PR_GetOSError());
+      return NS_ERROR_FAILURE;
+    }
+
+    uint64_t bytes = std::min((uint64_t)stat.size, mBytes);
+    if (bytes > UINT32_MAX) {
+      Fail(NS_LITERAL_CSTRING("Arithmetics"), nullptr, OS_ERROR_INVAL);
+      return NS_ERROR_FAILURE;
+    }
+
+    if (!aBuffer.Allocate(bytes)) {
+      Fail(NS_LITERAL_CSTRING("allocate"), nullptr, OS_ERROR_NOMEM);
+      return NS_ERROR_FAILURE;
+    }
+
+    uint64_t total_read = 0;
+    int32_t just_read = 0;
+    char* dest_chars = reinterpret_cast<char*>(aBuffer.rwget().data);
+    do {
+      just_read = PR_Read(file, dest_chars + total_read,
+                          std::min(uint64_t(PR_INT32_MAX), bytes - total_read));
+      if (just_read == -1) {
+        Fail(NS_LITERAL_CSTRING("read"), nullptr, PR_GetOSError());
+        return NS_ERROR_FAILURE;
+      }
+      total_read += just_read;
+    } while (just_read != 0 && total_read < bytes);
+    if (total_read != bytes) {
+      // We seem to have a race condition here.
+      Fail(NS_LITERAL_CSTRING("read"), nullptr, OS_ERROR_RACE);
+      return NS_ERROR_FAILURE;
+    }
+
+    return NS_OK;
+  }
+
+protected:
+  /**
+   * Any steps that need to be taken before reading.
+   *
+   * In case of error, this method should call Fail() and return
+   * a failure code.
+   */
+  virtual
+  nsresult BeforeRead() {
+    return NS_OK;
+  }
+
+  /**
+   * Proceed after reading.
+   */
+  virtual
+  void AfterRead(TimeStamp aDispatchDate, ScopedArrayBufferContents& aBuffer) = 0;
+
+ protected:
+  const nsString mPath;
+  const uint64_t mBytes;
+};
+
+/**
+ * An implementation of a Read event that provides the data
+ * as a TypedArray.
+ */
+class DoReadToTypedArrayEvent final : public AbstractReadEvent {
+public:
+  DoReadToTypedArrayEvent(const nsAString& aPath,
+                          const uint32_t aBytes,
+                          nsMainThreadPtrHandle<nsINativeOSFileSuccessCallback>& aOnSuccess,
+                          nsMainThreadPtrHandle<nsINativeOSFileErrorCallback>& aOnError)
+    : AbstractReadEvent(aPath, aBytes,
+                        aOnSuccess, aOnError)
+    , mResult(new TypedArrayResult(TimeStamp::Now()))
+  { }
+
+  ~DoReadToTypedArrayEvent() {
+    // If AbstractReadEvent::Run() has bailed out, we may need to cleanup
+    // mResult, which is main-thread only data
+    if (!mResult) {
+      return;
+    }
+    NS_ReleaseOnMainThread(mResult.forget());
+  }
+
+protected:
+  void AfterRead(TimeStamp aDispatchDate,
+                 ScopedArrayBufferContents& aBuffer) override {
+    MOZ_ASSERT(!NS_IsMainThread());
+    mResult->Init(aDispatchDate, TimeStamp::Now() - aDispatchDate, aBuffer.forget());
+    Succeed(mResult.forget());
+  }
+
+ private:
+  RefPtr<TypedArrayResult> mResult;
+};
+
+/**
+ * An implementation of a Read event that provides the data
+ * as a JavaScript string.
+ */
+class DoReadToStringEvent final : public AbstractReadEvent {
+public:
+  DoReadToStringEvent(const nsAString& aPath,
+                      const nsACString& aEncoding,
+                      const uint32_t aBytes,
+                      nsMainThreadPtrHandle<nsINativeOSFileSuccessCallback>& aOnSuccess,
+                      nsMainThreadPtrHandle<nsINativeOSFileErrorCallback>& aOnError)
+    : AbstractReadEvent(aPath, aBytes, aOnSuccess, aOnError)
+    , mEncoding(aEncoding)
+    , mResult(new StringResult(TimeStamp::Now()))
+  { }
+
+  ~DoReadToStringEvent() {
+    // If AbstraactReadEvent::Run() has bailed out, we may need to cleanup
+    // mResult, which is main-thread only data
+    if (!mResult) {
+      return;
+    }
+    NS_ReleaseOnMainThread(mResult.forget());
+  }
+
+protected:
+  nsresult BeforeRead() override {
+    // Obtain the decoder. We do this before reading to avoid doing
+    // any unnecessary I/O in case the name of the encoding is incorrect.
+    MOZ_ASSERT(!NS_IsMainThread());
+    nsAutoCString encodingName;
+    if (!dom::EncodingUtils::FindEncodingForLabel(mEncoding, encodingName)) {
+      Fail(NS_LITERAL_CSTRING("Decode"), mResult.forget(), OS_ERROR_INVAL);
+      return NS_ERROR_FAILURE;
+    }
+    mDecoder = dom::EncodingUtils::DecoderForEncoding(encodingName);
+    if (!mDecoder) {
+      Fail(NS_LITERAL_CSTRING("DecoderForEncoding"), mResult.forget(), OS_ERROR_INVAL);
+      return NS_ERROR_FAILURE;
+    }
+
+    return NS_OK;
+  }
+
+  void AfterRead(TimeStamp aDispatchDate,
+                 ScopedArrayBufferContents& aBuffer) override {
+    MOZ_ASSERT(!NS_IsMainThread());
+
+    int32_t maxChars;
+    const char* sourceChars = reinterpret_cast<const char*>(aBuffer.get().data);
+    int32_t sourceBytes = aBuffer.get().nbytes;
+    if (sourceBytes < 0) {
+      Fail(NS_LITERAL_CSTRING("arithmetics"), mResult.forget(), OS_ERROR_TOO_LARGE);
+      return;
+    }
+
+    nsresult rv = mDecoder->GetMaxLength(sourceChars, sourceBytes, &maxChars);
+    if (NS_FAILED(rv)) {
+      Fail(NS_LITERAL_CSTRING("GetMaxLength"), mResult.forget(), OS_ERROR_INVAL);
+      return;
+    }
+
+    if (maxChars < 0) {
+      Fail(NS_LITERAL_CSTRING("arithmetics"), mResult.forget(), OS_ERROR_TOO_LARGE);
+      return;
+    }
+
+    nsString resultString;
+    resultString.SetLength(maxChars);
+    if (resultString.Length() != (nsString::size_type)maxChars) {
+      Fail(NS_LITERAL_CSTRING("allocation"), mResult.forget(), OS_ERROR_TOO_LARGE);
+      return;
+    }
+
+
+    rv = mDecoder->Convert(sourceChars, &sourceBytes,
+                           resultString.BeginWriting(), &maxChars);
+    MOZ_ASSERT(NS_SUCCEEDED(rv));
+    resultString.SetLength(maxChars);
+
+    mResult->Init(aDispatchDate, TimeStamp::Now() - aDispatchDate, resultString);
+    Succeed(mResult.forget());
+  }
+
+ private:
+  nsCString mEncoding;
+  nsCOMPtr<nsIUnicodeDecoder> mDecoder;
+  RefPtr<StringResult> mResult;
+};
+
+} // namespace
+
+// The OS.File service
+
+NS_IMPL_ISUPPORTS(NativeOSFileInternalsService, nsINativeOSFileInternalsService);
+
+NS_IMETHODIMP
+NativeOSFileInternalsService::Read(const nsAString& aPath,
+                                   JS::HandleValue aOptions,
+                                   nsINativeOSFileSuccessCallback *aOnSuccess,
+                                   nsINativeOSFileErrorCallback *aOnError,
+                                   JSContext* cx)
+{
+  // Extract options
+  nsCString encoding;
+  uint64_t bytes = UINT64_MAX;
+
+  if (aOptions.isObject()) {
+    dom::NativeOSFileReadOptions dict;
+    if (!dict.Init(cx, aOptions)) {
+      return NS_ERROR_INVALID_ARG;
+    }
+
+    if (dict.mEncoding.WasPassed()) {
+      CopyUTF16toUTF8(dict.mEncoding.Value(), encoding);
+    }
+
+    if (dict.mBytes.WasPassed() && !dict.mBytes.Value().IsNull()) {
+      bytes = dict.mBytes.Value().Value();
+    }
+  }
+
+  // Prepare the off main thread event and dispatch it
+  nsCOMPtr<nsINativeOSFileSuccessCallback> onSuccess(aOnSuccess);
+  nsMainThreadPtrHandle<nsINativeOSFileSuccessCallback> onSuccessHandle(
+    new nsMainThreadPtrHolder<nsINativeOSFileSuccessCallback>(onSuccess));
+  nsCOMPtr<nsINativeOSFileErrorCallback> onError(aOnError);
+  nsMainThreadPtrHandle<nsINativeOSFileErrorCallback> onErrorHandle(
+    new nsMainThreadPtrHolder<nsINativeOSFileErrorCallback>(onError));
+
+  RefPtr<AbstractDoEvent> event;
+  if (encoding.IsEmpty()) {
+    event = new DoReadToTypedArrayEvent(aPath, bytes,
+                                        onSuccessHandle,
+                                        onErrorHandle);
+  } else {
+    event = new DoReadToStringEvent(aPath, encoding, bytes,
+                                    onSuccessHandle,
+                                    onErrorHandle);
+  }
+
+  nsresult rv;
+  nsCOMPtr<nsIEventTarget> target = do_GetService(NS_STREAMTRANSPORTSERVICE_CONTRACTID, &rv);
+
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+  return target->Dispatch(event, NS_DISPATCH_NORMAL);
+}
+
+} // namespace mozilla
+
diff --git a/toolkit/components/osfile/NativeOSFileInternals.h b/toolkit/components/osfile/NativeOSFileInternals.h
new file mode 100644
index 000000000..2da929357
--- /dev/null
+++ b/toolkit/components/osfile/NativeOSFileInternals.h
@@ -0,0 +1,25 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_nativeosfileinternalservice_h__
+#define mozilla_nativeosfileinternalservice_h__
+
+#include "nsINativeOSFileInternals.h"
+#include "mozilla/Attributes.h"
+
+namespace mozilla {
+
+class NativeOSFileInternalsService final : public nsINativeOSFileInternalsService {
+public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSINATIVEOSFILEINTERNALSSERVICE
+private:
+  ~NativeOSFileInternalsService() {}
+  // Avoid accidental use of built-in operator=
+  void operator=(const NativeOSFileInternalsService& other) = delete;
+};
+
+} // namespace mozilla
+
+#endif // mozilla_finalizationwitnessservice_h__
diff --git a/toolkit/components/osfile/modules/moz.build b/toolkit/components/osfile/modules/moz.build
new file mode 100644
index 000000000..7a0580ca3
--- /dev/null
+++ b/toolkit/components/osfile/modules/moz.build
@@ -0,0 +1,22 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+EXTRA_JS_MODULES.osfile += [
+    'osfile_async_front.jsm',
+    'osfile_async_worker.js',
+    'osfile_native.jsm',
+    'osfile_shared_allthreads.jsm',
+    'osfile_shared_front.jsm',
+    'osfile_unix_allthreads.jsm',
+    'osfile_unix_back.jsm',
+    'osfile_unix_front.jsm',
+    'osfile_win_allthreads.jsm',
+    'osfile_win_back.jsm',
+    'osfile_win_front.jsm',
+    'ospath.jsm',
+    'ospath_unix.jsm',
+    'ospath_win.jsm',
+]
diff --git a/toolkit/components/osfile/modules/osfile_async_front.jsm b/toolkit/components/osfile/modules/osfile_async_front.jsm
new file mode 100644
index 000000000..181471cd8
--- /dev/null
+++ b/toolkit/components/osfile/modules/osfile_async_front.jsm
@@ -0,0 +1,1533 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * Asynchronous front-end for OS.File.
+ *
+ * This front-end is meant to be imported from the main thread. In turn,
+ * it spawns one worker (perhaps more in the future) and delegates all
+ * disk I/O to this worker.
+ *
+ * Documentation note: most of the functions and methods in this module
+ * return promises. For clarity, we denote as follows a promise that may resolve
+ * with type |A| and some value |value| or reject with type |B| and some
+ * reason |reason|
+ * @resolves {A} value
+ * @rejects {B} reason
+ */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = ["OS"];
+
+const Cu = Components.utils;
+const Ci = Components.interfaces;
+
+var SharedAll = {};
+Cu.import("resource://gre/modules/osfile/osfile_shared_allthreads.jsm", SharedAll);
+Cu.import("resource://gre/modules/XPCOMUtils.jsm", this);
+Cu.import("resource://gre/modules/Timer.jsm", this);
+
+
+// Boilerplate, to simplify the transition to require()
+var LOG = SharedAll.LOG.bind(SharedAll, "Controller");
+var isTypedArray = SharedAll.isTypedArray;
+
+// The constructor for file errors.
+var SysAll = {};
+if (SharedAll.Constants.Win) {
+  Cu.import("resource://gre/modules/osfile/osfile_win_allthreads.jsm", SysAll);
+} else if (SharedAll.Constants.libc) {
+  Cu.import("resource://gre/modules/osfile/osfile_unix_allthreads.jsm", SysAll);
+} else {
+  throw new Error("I am neither under Windows nor under a Posix system");
+}
+var OSError = SysAll.Error;
+var Type = SysAll.Type;
+
+var Path = {};
+Cu.import("resource://gre/modules/osfile/ospath.jsm", Path);
+
+// The library of promises.
+Cu.import("resource://gre/modules/Promise.jsm", this);
+Cu.import("resource://gre/modules/Task.jsm", this);
+
+// The implementation of communications
+Cu.import("resource://gre/modules/PromiseWorker.jsm", this);
+Cu.import("resource://gre/modules/Services.jsm", this);
+Cu.import("resource://gre/modules/TelemetryStopwatch.jsm", this);
+Cu.import("resource://gre/modules/AsyncShutdown.jsm", this);
+var Native = Cu.import("resource://gre/modules/osfile/osfile_native.jsm", {});
+
+
+// It's possible for osfile.jsm to get imported before the profile is
+// set up. In this case, some path constants aren't yet available.
+// Here, we make them lazy loaders.
+
+function lazyPathGetter(constProp, dirKey) {
+  return function () {
+    let path;
+    try {
+      path = Services.dirsvc.get(dirKey, Ci.nsIFile).path;
+      delete SharedAll.Constants.Path[constProp];
+      SharedAll.Constants.Path[constProp] = path;
+    } catch (ex) {
+      // Ignore errors if the value still isn't available. Hopefully
+      // the next access will return it.
+    }
+
+    return path;
+  };
+}
+
+for (let [constProp, dirKey] of [
+  ["localProfileDir", "ProfLD"],
+  ["profileDir", "ProfD"],
+  ["userApplicationDataDir", "UAppData"],
+  ["winAppDataDir", "AppData"],
+  ["winStartMenuProgsDir", "Progs"],
+  ]) {
+
+  if (constProp in SharedAll.Constants.Path) {
+    continue;
+  }
+
+  LOG("Installing lazy getter for OS.Constants.Path." + constProp +
+      " because it isn't defined and profile may not be loaded.");
+  Object.defineProperty(SharedAll.Constants.Path, constProp, {
+    get: lazyPathGetter(constProp, dirKey),
+  });
+}
+
+/**
+ * Return a shallow clone of the enumerable properties of an object.
+ */
+var clone = SharedAll.clone;
+
+/**
+ * Extract a shortened version of an object, fit for logging.
+ *
+ * This function returns a copy of the original object in which all
+ * long strings, Arrays, TypedArrays, ArrayBuffers are removed and
+ * replaced with placeholders. Use this function to sanitize objects
+ * if you wish to log them or to keep them in memory.
+ *
+ * @param {*} obj The obj to shorten.
+ * @return {*} array A shorter object, fit for logging.
+ */
+function summarizeObject(obj) {
+  if (!obj) {
+    return null;
+  }
+  if (typeof obj == "string") {
+    if (obj.length > 1024) {
+      return {"Long string": obj.length};
+    }
+    return obj;
+  }
+  if (typeof obj == "object") {
+    if (Array.isArray(obj)) {
+      if (obj.length > 32) {
+        return {"Long array": obj.length};
+      }
+      return obj.map(summarizeObject);
+    }
+    if ("byteLength" in obj) {
+      // Assume TypedArray or ArrayBuffer
+      return {"Binary Data": obj.byteLength};
+    }
+    let result = {};
+    for (let k of Object.keys(obj)) {
+      result[k] = summarizeObject(obj[k]);
+    }
+    return result;
+  }
+  return obj;
+}
+
+// In order to expose Scheduler to the unfiltered Cu.import return value variant
+// on B2G we need to save it to `this`.  This does not make it public;
+// EXPORTED_SYMBOLS still controls that in all cases.
+var Scheduler = this.Scheduler = {
+
+  /**
+   * |true| once we have sent at least one message to the worker.
+   * This field is unaffected by resetting the worker.
+   */
+  launched: false,
+
+  /**
+   * |true| once shutdown has begun i.e. we should reject any
+   * message, including resets.
+   */
+  shutdown: false,
+
+  /**
+   * A promise resolved once all currently pending operations are complete.
+   *
+   * This promise is never rejected and the result is always undefined.
+   */
+  queue: Promise.resolve(),
+
+  /**
+   * A promise resolved once all currently pending `kill` operations
+   * are complete.
+   *
+   * This promise is never rejected and the result is always undefined.
+   */
+  _killQueue: Promise.resolve(),
+
+  /**
+   * Miscellaneous debugging information
+   */
+  Debugging: {
+    /**
+     * The latest message sent and still waiting for a reply.
+     */
+    latestSent: undefined,
+
+    /**
+     * The latest reply received, or null if we are waiting for a reply.
+     */
+    latestReceived: undefined,
+
+    /**
+     * Number of messages sent to the worker. This includes the
+     * initial SET_DEBUG, if applicable.
+     */
+    messagesSent: 0,
+
+    /**
+     * Total number of messages ever queued, including the messages
+     * sent.
+     */
+    messagesQueued: 0,
+
+    /**
+     * Number of messages received from the worker.
+     */
+    messagesReceived: 0,
+  },
+
+  /**
+   * A timer used to automatically shut down the worker after some time.
+   */
+  resetTimer: null,
+
+  /**
+   * The worker to which to send requests.
+   *
+   * If the worker has never been created or has been reset, this is a
+   * fresh worker, initialized with osfile_async_worker.js.
+   *
+   * @type {PromiseWorker}
+   */
+  get worker() {
+    if (!this._worker) {
+      // Either the worker has never been created or it has been
+      // reset.  In either case, it is time to instantiate the worker.
+      this._worker = new BasePromiseWorker("resource://gre/modules/osfile/osfile_async_worker.js");
+      this._worker.log = LOG;
+      this._worker.ExceptionHandlers["OS.File.Error"] = OSError.fromMsg;
+    }
+    return this._worker;
+  },
+
+  _worker: null,
+
+  /**
+   * Prepare to kill the OS.File worker after a few seconds.
+   */
+  restartTimer: function(arg) {
+    let delay;
+    try {
+      delay = Services.prefs.getIntPref("osfile.reset_worker_delay");
+    } catch(e) {
+      // Don't auto-shutdown if we don't have a delay preference set.
+      return;
+    }
+
+    if (this.resetTimer) {
+      clearTimeout(this.resetTimer);
+    }
+    this.resetTimer = setTimeout(
+      () => Scheduler.kill({reset: true, shutdown: false}),
+      delay
+    );
+  },
+
+  /**
+   * Shutdown OS.File.
+   *
+   * @param {*} options
+   * - {boolean} shutdown If |true|, reject any further request. Otherwise,
+   *   further requests will resurrect the worker.
+   * - {boolean} reset If |true|, instruct the worker to shutdown if this
+   *   would not cause leaks. Otherwise, assume that the worker will be shutdown
+   *   through some other mean.
+   */
+  kill: function({shutdown, reset}) {
+    // Grab the kill queue to make sure that we
+    // cannot be interrupted by another call to `kill`.
+    let killQueue = this._killQueue;
+
+    // Deactivate the queue, to ensure that no message is sent
+    // to an obsolete worker (we reactivate it in the `finally`).
+    // This needs to be done right now so that we maintain relative
+    // ordering with calls to post(), etc.
+    let deferred = Promise.defer();
+    let savedQueue = this.queue;
+    this.queue = deferred.promise;
+
+    return this._killQueue = Task.spawn(function*() {
+
+      yield killQueue;
+      // From this point, and until the end of the Task, we are the
+      // only call to `kill`, regardless of any `yield`.
+
+      yield savedQueue;
+
+      try {
+        // Enter critical section: no yield in this block
+        // (we want to make sure that we remain the only
+        // request in the queue).
+
+        if (!this.launched || this.shutdown || !this._worker) {
+          // Nothing to kill
+          this.shutdown = this.shutdown || shutdown;
+          this._worker = null;
+          return null;
+        }
+
+        // Exit critical section
+
+        let message = ["Meta_shutdown", [reset]];
+
+        Scheduler.latestReceived = [];
+        Scheduler.latestSent = [Date.now(),
+          Task.Debugging.generateReadableStack(new Error().stack),
+          ...message];
+
+        // Wait for result
+        let resources;
+        try {
+          resources = yield this._worker.post(...message);
+
+          Scheduler.latestReceived = [Date.now(), message];
+        } catch (ex) {
+          LOG("Could not dispatch Meta_reset", ex);
+          // It's most likely a programmer error, but we'll assume that
+          // the worker has been shutdown, as it's less risky than the
+          // opposite stance.
+          resources = {openedFiles: [], openedDirectoryIterators: [], killed: true};
+
+          Scheduler.latestReceived = [Date.now(), message, ex];
+        }
+
+        let {openedFiles, openedDirectoryIterators, killed} = resources;
+        if (!reset
+          && (openedFiles && openedFiles.length
+            || ( openedDirectoryIterators && openedDirectoryIterators.length))) {
+          // The worker still holds resources. Report them.
+
+          let msg = "";
+          if (openedFiles.length > 0) {
+            msg += "The following files are still open:\n" +
+              openedFiles.join("\n");
+          }
+          if (openedDirectoryIterators.length > 0) {
+            msg += "The following directory iterators are still open:\n" +
+              openedDirectoryIterators.join("\n");
+          }
+
+          LOG("WARNING: File descriptors leaks detected.\n" + msg);
+        }
+
+        // Make sure that we do not leave an invalid |worker| around.
+        if (killed || shutdown) {
+          this._worker = null;
+        }
+
+        this.shutdown = shutdown;
+
+        return resources;
+
+      } finally {
+        // Resume accepting messages. If we have set |shutdown| to |true|,
+        // any pending/future request will be rejected. Otherwise, any
+        // pending/future request will spawn a new worker if necessary.
+        deferred.resolve();
+      }
+
+    }.bind(this));
+  },
+
+  /**
+   * Push a task at the end of the queue.
+   *
+   * @param {function} code A function returning a Promise.
+   * This function will be executed once all the previously
+   * pushed tasks have completed.
+   * @return {Promise} A promise with the same behavior as
+   * the promise returned by |code|.
+   */
+  push: function(code) {
+    let promise = this.queue.then(code);
+    // By definition, |this.queue| can never reject.
+    this.queue = promise.then(null, () => undefined);
+    // Fork |promise| to ensure that uncaught errors are reported
+    return promise.then(null, null);
+  },
+
+  /**
+   * Post a message to the worker thread.
+   *
+   * @param {string} method The name of the method to call.
+   * @param {...} args The arguments to pass to the method. These arguments
+   * must be clonable.
+   * @return {Promise} A promise conveying the result/error caused by
+   * calling |method| with arguments |args|.
+   */
+  post: function post(method, args = undefined, closure = undefined) {
+    if (this.shutdown) {
+      LOG("OS.File is not available anymore. The following request has been rejected.",
+        method, args);
+      return Promise.reject(new Error("OS.File has been shut down. Rejecting post to " + method));
+    }
+    let firstLaunch = !this.launched;
+    this.launched = true;
+
+    if (firstLaunch && SharedAll.Config.DEBUG) {
+      // If we have delayed sending SET_DEBUG, do it now.
+      this.worker.post("SET_DEBUG", [true]);
+      Scheduler.Debugging.messagesSent++;
+    }
+
+    Scheduler.Debugging.messagesQueued++;
+    return this.push(Task.async(function*() {
+      if (this.shutdown) {
+	LOG("OS.File is not available anymore. The following request has been rejected.",
+	  method, args);
+	throw new Error("OS.File has been shut down. Rejecting request to " + method);
+      }
+
+      // Update debugging information. As |args| may be quite
+      // expensive, we only keep a shortened version of it.
+      Scheduler.Debugging.latestReceived = null;
+      Scheduler.Debugging.latestSent = [Date.now(), method, summarizeObject(args)];
+
+      // Don't kill the worker just yet
+      Scheduler.restartTimer();
+
+
+      let reply;
+      try {
+        try {
+          Scheduler.Debugging.messagesSent++;
+          Scheduler.Debugging.latestSent = Scheduler.Debugging.latestSent.slice(0, 2);
+          reply = yield this.worker.post(method, args, closure);
+          Scheduler.Debugging.latestReceived = [Date.now(), summarizeObject(reply)];
+          return reply;
+        } finally {
+          Scheduler.Debugging.messagesReceived++;
+        }
+      } catch (error) {
+        Scheduler.Debugging.latestReceived = [Date.now(), error.message, error.fileName, error.lineNumber];
+        throw error;
+      } finally {
+        if (firstLaunch) {
+          Scheduler._updateTelemetry();
+        }
+        Scheduler.restartTimer();
+      }
+    }.bind(this)));
+  },
+
+  /**
+   * Post Telemetry statistics.
+   *
+   * This is only useful on first launch.
+   */
+  _updateTelemetry: function() {
+    let worker = this.worker;
+    let workerTimeStamps = worker.workerTimeStamps;
+    if (!workerTimeStamps) {
+      // If the first call to OS.File results in an uncaught errors,
+      // the timestamps are absent. As this case is a developer error,
+      // let's not waste time attempting to extract telemetry from it.
+      return;
+    }
+    let HISTOGRAM_LAUNCH = Services.telemetry.getHistogramById("OSFILE_WORKER_LAUNCH_MS");
+    HISTOGRAM_LAUNCH.add(worker.workerTimeStamps.entered - worker.launchTimeStamp);
+
+    let HISTOGRAM_READY = Services.telemetry.getHistogramById("OSFILE_WORKER_READY_MS");
+    HISTOGRAM_READY.add(worker.workerTimeStamps.loaded - worker.launchTimeStamp);
+  }
+};
+
+const PREF_OSFILE_LOG = "toolkit.osfile.log";
+const PREF_OSFILE_LOG_REDIRECT = "toolkit.osfile.log.redirect";
+
+/**
+ * Safely read a PREF_OSFILE_LOG preference.
+ * Returns a value read or, in case of an error, oldPref or false.
+ *
+ * @param bool oldPref
+ *        An optional value that the DEBUG flag was set to previously.
+ */
+function readDebugPref(prefName, oldPref = false) {
+  let pref = oldPref;
+  try {
+    pref = Services.prefs.getBoolPref(prefName);
+  } catch (x) {
+    // In case of an error when reading a pref keep it as is.
+  }
+  // If neither pref nor oldPref were set, default it to false.
+  return pref;
+};
+
+/**
+ * Listen to PREF_OSFILE_LOG changes and update gShouldLog flag
+ * appropriately.
+ */
+Services.prefs.addObserver(PREF_OSFILE_LOG,
+  function prefObserver(aSubject, aTopic, aData) {
+    SharedAll.Config.DEBUG = readDebugPref(PREF_OSFILE_LOG, SharedAll.Config.DEBUG);
+    if (Scheduler.launched) {
+      // Don't start the worker just to set this preference.
+      Scheduler.post("SET_DEBUG", [SharedAll.Config.DEBUG]);
+    }
+  }, false);
+SharedAll.Config.DEBUG = readDebugPref(PREF_OSFILE_LOG, false);
+
+Services.prefs.addObserver(PREF_OSFILE_LOG_REDIRECT,
+  function prefObserver(aSubject, aTopic, aData) {
+    SharedAll.Config.TEST = readDebugPref(PREF_OSFILE_LOG_REDIRECT, OS.Shared.TEST);
+  }, false);
+SharedAll.Config.TEST = readDebugPref(PREF_OSFILE_LOG_REDIRECT, false);
+
+
+/**
+ * If |true|, use the native implementaiton of OS.File methods
+ * whenever possible. Otherwise, force the use of the JS version.
+ */
+var nativeWheneverAvailable = true;
+const PREF_OSFILE_NATIVE = "toolkit.osfile.native";
+Services.prefs.addObserver(PREF_OSFILE_NATIVE,
+  function prefObserver(aSubject, aTopic, aData) {
+    nativeWheneverAvailable = readDebugPref(PREF_OSFILE_NATIVE, nativeWheneverAvailable);
+  }, false);
+
+
+// Update worker's DEBUG flag if it's true.
+// Don't start the worker just for this, though.
+if (SharedAll.Config.DEBUG && Scheduler.launched) {
+  Scheduler.post("SET_DEBUG", [true]);
+}
+
+// Observer topics used for monitoring shutdown
+const WEB_WORKERS_SHUTDOWN_TOPIC = "web-workers-shutdown";
+
+// Preference used to configure test shutdown observer.
+const PREF_OSFILE_TEST_SHUTDOWN_OBSERVER =
+  "toolkit.osfile.test.shutdown.observer";
+
+AsyncShutdown.webWorkersShutdown.addBlocker(
+  "OS.File: flush pending requests, warn about unclosed files, shut down service.",
+  Task.async(function*() {
+    // Give clients a last chance to enqueue requests.
+    yield Barriers.shutdown.wait({crashAfterMS: null});
+
+    // Wait until all requests are complete and kill the worker.
+    yield Scheduler.kill({reset: false, shutdown: true});
+  }),
+  () => {
+    let details = Barriers.getDetails();
+    details.clients = Barriers.shutdown.state;
+    return details;
+  }
+);
+
+
+// Attaching an observer for PREF_OSFILE_TEST_SHUTDOWN_OBSERVER to enable or
+// disable the test shutdown event observer.
+// Note: By default the PREF_OSFILE_TEST_SHUTDOWN_OBSERVER is unset.
+// Note: This is meant to be used for testing purposes only.
+Services.prefs.addObserver(PREF_OSFILE_TEST_SHUTDOWN_OBSERVER,
+  function prefObserver() {
+    // The temporary phase topic used to trigger the unclosed
+    // phase warning.
+    let TOPIC = null;
+    try {
+      TOPIC = Services.prefs.getCharPref(
+        PREF_OSFILE_TEST_SHUTDOWN_OBSERVER);
+    } catch (x) {
+    }
+    if (TOPIC) {
+      // Generate a phase, add a blocker.
+      // Note that this can work only if AsyncShutdown itself has been
+      // configured for testing by the testsuite.
+      let phase = AsyncShutdown._getPhase(TOPIC);
+      phase.addBlocker(
+        "(for testing purposes) OS.File: warn about unclosed files",
+        () => Scheduler.kill({shutdown: false, reset: false})
+      );
+    }
+  }, false);
+
+/**
+ * Representation of a file, with asynchronous methods.
+ *
+ * @param {*} fdmsg The _message_ representing the platform-specific file
+ * handle.
+ *
+ * @constructor
+ */
+var File = function File(fdmsg) {
+  // FIXME: At the moment, |File| does not close on finalize
+  // (see bug 777715)
+  this._fdmsg = fdmsg;
+  this._closeResult = null;
+  this._closed = null;
+};
+
+
+File.prototype = {
+  /**
+   * Close a file asynchronously.
+   *
+   * This method is idempotent.
+   *
+   * @return {promise}
+   * @resolves {null}
+   * @rejects {OS.File.Error}
+   */
+  close: function close() {
+    if (this._fdmsg != null) {
+      let msg = this._fdmsg;
+      this._fdmsg = null;
+      return this._closeResult =
+        Scheduler.post("File_prototype_close", [msg], this);
+    }
+    return this._closeResult;
+  },
+
+  /**
+   * Fetch information about the file.
+   *
+   * @return {promise}
+   * @resolves {OS.File.Info} The latest information about the file.
+   * @rejects {OS.File.Error}
+   */
+  stat: function stat() {
+    return Scheduler.post("File_prototype_stat", [this._fdmsg], this).then(
+      File.Info.fromMsg
+    );
+  },
+
+  /**
+   * Write bytes from a buffer to this file.
+   *
+   * Note that, by default, this function may perform several I/O
+   * operations to ensure that the buffer is fully written.
+   *
+   * @param {Typed array | C pointer} buffer The buffer in which the
+   * the bytes are stored. The buffer must be large enough to
+   * accomodate |bytes| bytes. Using the buffer before the operation
+   * is complete is a BAD IDEA.
+   * @param {*=} options Optionally, an object that may contain the
+   * following fields:
+   * - {number} bytes The number of |bytes| to write from the buffer. If
+   * unspecified, this is |buffer.byteLength|. Note that |bytes| is required
+   * if |buffer| is a C pointer.
+   *
+   * @return {number} The number of bytes actually written.
+   */
+  write: function write(buffer, options = {}) {
+    // If |buffer| is a typed array and there is no |bytes| options,
+    // we need to extract the |byteLength| now, as it will be lost
+    // by communication.
+    // Options might be a nullish value, so better check for that before using
+    // the |in| operator.
+    if (isTypedArray(buffer) && !(options && "bytes" in options)) {
+      // Preserve reference to option |outExecutionDuration|, if it is passed.
+      options = clone(options, ["outExecutionDuration"]);
+      options.bytes = buffer.byteLength;
+    }
+    return Scheduler.post("File_prototype_write",
+      [this._fdmsg,
+       Type.void_t.in_ptr.toMsg(buffer),
+       options],
+       buffer/*Ensure that |buffer| is not gc-ed*/);
+  },
+
+  /**
+   * Read bytes from this file to a new buffer.
+   *
+   * @param {number=} bytes If unspecified, read all the remaining bytes from
+   * this file. If specified, read |bytes| bytes, or less if the file does not
+   * contain that many bytes.
+   * @param {JSON} options
+   * @return {promise}
+   * @resolves {Uint8Array} An array containing the bytes read.
+   */
+  read: function read(nbytes, options = {}) {
+    let promise = Scheduler.post("File_prototype_read",
+      [this._fdmsg,
+       nbytes, options]);
+    return promise.then(
+      function onSuccess(data) {
+        return new Uint8Array(data.buffer, data.byteOffset, data.byteLength);
+      });
+  },
+
+  /**
+   * Return the current position in the file, as bytes.
+   *
+   * @return {promise}
+   * @resolves {number} The current position in the file,
+   * as a number of bytes since the start of the file.
+   */
+  getPosition: function getPosition() {
+    return Scheduler.post("File_prototype_getPosition",
+      [this._fdmsg]);
+  },
+
+  /**
+   * Set the current position in the file, as bytes.
+   *
+   * @param {number} pos A number of bytes.
+   * @param {number} whence The reference position in the file,
+   * which may be either POS_START (from the start of the file),
+   * POS_END (from the end of the file) or POS_CUR (from the
+   * current position in the file).
+   *
+   * @return {promise}
+   */
+  setPosition: function setPosition(pos, whence) {
+    return Scheduler.post("File_prototype_setPosition",
+      [this._fdmsg, pos, whence]);
+  },
+
+  /**
+   * Flushes the file's buffers and causes all buffered data
+   * to be written.
+   * Disk flushes are very expensive and therefore should be used carefully,
+   * sparingly and only in scenarios where it is vital that data survives
+   * system crashes. Even though the function will be executed off the
+   * main-thread, it might still affect the overall performance of any running
+   * application.
+   *
+   * @return {promise}
+   */
+  flush: function flush() {
+    return Scheduler.post("File_prototype_flush",
+      [this._fdmsg]);
+  },
+
+  /**
+   * Set the file's access permissions.  This does nothing on Windows.
+   *
+   * This operation is likely to fail if applied to a file that was
+   * not created by the currently running program (more precisely,
+   * if it was created by a program running under a different OS-level
+   * user account).  It may also fail, or silently do nothing, if the
+   * filesystem containing the file does not support access permissions.
+   *
+   * @param {*=} options Object specifying the requested permissions:
+   *
+   * - {number} unixMode The POSIX file mode to set on the file.  If omitted,
+   *  the POSIX file mode is reset to the default used by |OS.file.open|.  If
+   *  specified, the permissions will respect the process umask as if they
+   *  had been specified as arguments of |OS.File.open|, unless the
+   *  |unixHonorUmask| parameter tells otherwise.
+   * - {bool} unixHonorUmask If omitted or true, any |unixMode| value is
+   *  modified by the process umask, as |OS.File.open| would have done.  If
+   *  false, the exact value of |unixMode| will be applied.
+   */
+  setPermissions: function setPermissions(options = {}) {
+    return Scheduler.post("File_prototype_setPermissions",
+                          [this._fdmsg, options]);
+  }
+};
+
+
+if (SharedAll.Constants.Sys.Name != "Android" && SharedAll.Constants.Sys.Name != "Gonk") {
+   /**
+   * Set the last access and modification date of the file.
+   * The time stamp resolution is 1 second at best, but might be worse
+   * depending on the platform.
+   *
+   * WARNING: This method is not implemented on Android/B2G. On Android/B2G,
+   * you should use File.setDates instead.
+   *
+   * @return {promise}
+   * @rejects {TypeError}
+   * @rejects {OS.File.Error}
+   */
+  File.prototype.setDates = function(accessDate, modificationDate) {
+    return Scheduler.post("File_prototype_setDates",
+      [this._fdmsg, accessDate, modificationDate], this);
+  };
+}
+
+
+/**
+ * Open a file asynchronously.
+ *
+ * @return {promise}
+ * @resolves {OS.File}
+ * @rejects {OS.Error}
+ */
+File.open = function open(path, mode, options) {
+  return Scheduler.post(
+    "open", [Type.path.toMsg(path), mode, options],
+    path
+  ).then(
+    function onSuccess(msg) {
+      return new File(msg);
+    }
+  );
+};
+
+/**
+ * Creates and opens a file with a unique name. By default, generate a random HEX number and use it to create a unique new file name.
+ *
+ * @param {string} path The path to the file.
+ * @param {*=} options Additional options for file opening. This
+ * implementation interprets the following fields:
+ *
+ * - {number} humanReadable If |true|, create a new filename appending a decimal number. ie: filename-1.ext, filename-2.ext.
+ *  If |false| use HEX numbers ie: filename-A65BC0.ext
+ * - {number} maxReadableNumber Used to limit the amount of tries after a failed
+ *  file creation. Default is 20.
+ *
+ * @return {Object} contains A file object{file} and the path{path}.
+ * @throws {OS.File.Error} If the file could not be opened.
+ */
+File.openUnique = function openUnique(path, options) {
+  return Scheduler.post(
+      "openUnique", [Type.path.toMsg(path), options],
+      path
+    ).then(
+    function onSuccess(msg) {
+      return {
+        path: msg.path,
+        file: new File(msg.file)
+      };
+    }
+  );
+};
+
+/**
+ * Get the information on the file.
+ *
+ * @return {promise}
+ * @resolves {OS.File.Info}
+ * @rejects {OS.Error}
+ */
+File.stat = function stat(path, options) {
+  return Scheduler.post(
+    "stat", [Type.path.toMsg(path), options],
+    path).then(File.Info.fromMsg);
+};
+
+
+/**
+ * Set the last access and modification date of the file.
+ * The time stamp resolution is 1 second at best, but might be worse
+ * depending on the platform.
+ *
+ * @return {promise}
+ * @rejects {TypeError}
+ * @rejects {OS.File.Error}
+ */
+File.setDates = function setDates(path, accessDate, modificationDate) {
+  return Scheduler.post("setDates",
+                        [Type.path.toMsg(path), accessDate, modificationDate],
+                        this);
+};
+
+/**
+ * Set the file's access permissions.  This does nothing on Windows.
+ *
+ * This operation is likely to fail if applied to a file that was
+ * not created by the currently running program (more precisely,
+ * if it was created by a program running under a different OS-level
+ * user account).  It may also fail, or silently do nothing, if the
+ * filesystem containing the file does not support access permissions.
+ *
+ * @param {string} path The path to the file.
+ * @param {*=} options Object specifying the requested permissions:
+ *
+ * - {number} unixMode The POSIX file mode to set on the file.  If omitted,
+ *  the POSIX file mode is reset to the default used by |OS.file.open|.  If
+ *  specified, the permissions will respect the process umask as if they
+ *  had been specified as arguments of |OS.File.open|, unless the
+ *  |unixHonorUmask| parameter tells otherwise.
+ * - {bool} unixHonorUmask If omitted or true, any |unixMode| value is
+ *  modified by the process umask, as |OS.File.open| would have done.  If
+ *  false, the exact value of |unixMode| will be applied.
+ */
+File.setPermissions = function setPermissions(path, options = {}) {
+  return Scheduler.post("setPermissions",
+                        [Type.path.toMsg(path), options]);
+};
+
+/**
+ * Fetch the current directory
+ *
+ * @return {promise}
+ * @resolves {string} The current directory, as a path usable with OS.Path
+ * @rejects {OS.Error}
+ */
+File.getCurrentDirectory = function getCurrentDirectory() {
+  return Scheduler.post(
+    "getCurrentDirectory"
+  ).then(Type.path.fromMsg);
+};
+
+/**
+ * Change the current directory
+ *
+ * @param {string} path The OS-specific path to the current directory.
+ * You should use the methods of OS.Path and the constants of OS.Constants.Path
+ * to build OS-specific paths in a portable manner.
+ *
+ * @return {promise}
+ * @resolves {null}
+ * @rejects {OS.Error}
+ */
+File.setCurrentDirectory = function setCurrentDirectory(path) {
+  return Scheduler.post(
+    "setCurrentDirectory", [Type.path.toMsg(path)], path
+  );
+};
+
+/**
+ * Copy a file to a destination.
+ *
+ * @param {string} sourcePath The platform-specific path at which
+ * the file may currently be found.
+ * @param {string} destPath The platform-specific path at which the
+ * file should be copied.
+ * @param {*=} options An object which may contain the following fields:
+ *
+ * @option {bool} noOverwrite - If true, this function will fail if
+ * a file already exists at |destPath|. Otherwise, if this file exists,
+ * it will be erased silently.
+ *
+ * @rejects {OS.File.Error} In case of any error.
+ *
+ * General note: The behavior of this function is defined only when
+ * it is called on a single file. If it is called on a directory, the
+ * behavior is undefined and may not be the same across all platforms.
+ *
+ * General note: The behavior of this function with respect to metadata
+ * is unspecified. Metadata may or may not be copied with the file. The
+ * behavior may not be the same across all platforms.
+*/
+File.copy = function copy(sourcePath, destPath, options) {
+  return Scheduler.post("copy", [Type.path.toMsg(sourcePath),
+    Type.path.toMsg(destPath), options], [sourcePath, destPath]);
+};
+
+/**
+ * Move a file to a destination.
+ *
+ * @param {string} sourcePath The platform-specific path at which
+ * the file may currently be found.
+ * @param {string} destPath The platform-specific path at which the
+ * file should be moved.
+ * @param {*=} options An object which may contain the following fields:
+ *
+ * @option {bool} noOverwrite - If set, this function will fail if
+ * a file already exists at |destPath|. Otherwise, if this file exists,
+ * it will be erased silently.
+ *
+ * @returns {Promise}
+ * @rejects {OS.File.Error} In case of any error.
+ *
+ * General note: The behavior of this function is defined only when
+ * it is called on a single file. If it is called on a directory, the
+ * behavior is undefined and may not be the same across all platforms.
+ *
+ * General note: The behavior of this function with respect to metadata
+ * is unspecified. Metadata may or may not be moved with the file. The
+ * behavior may not be the same across all platforms.
+ */
+File.move = function move(sourcePath, destPath, options) {
+  return Scheduler.post("move", [Type.path.toMsg(sourcePath),
+    Type.path.toMsg(destPath), options], [sourcePath, destPath]);
+};
+
+/**
+ * Create a symbolic link to a source.
+ *
+ * @param {string} sourcePath The platform-specific path to which
+ * the symbolic link should point.
+ * @param {string} destPath The platform-specific path at which the
+ * symbolic link should be created.
+ *
+ * @returns {Promise}
+ * @rejects {OS.File.Error} In case of any error.
+ */
+if (!SharedAll.Constants.Win) {
+  File.unixSymLink = function unixSymLink(sourcePath, destPath) {
+    return Scheduler.post("unixSymLink", [Type.path.toMsg(sourcePath),
+      Type.path.toMsg(destPath)], [sourcePath, destPath]);
+  };
+}
+
+/**
+ * Gets the number of bytes available on disk to the current user.
+ *
+ * @param {string} Platform-specific path to a directory on the disk to
+ * query for free available bytes.
+ *
+ * @return {number} The number of bytes available for the current user.
+ * @throws {OS.File.Error} In case of any error.
+ */
+File.getAvailableFreeSpace = function getAvailableFreeSpace(sourcePath) {
+  return Scheduler.post("getAvailableFreeSpace",
+    [Type.path.toMsg(sourcePath)], sourcePath
+  ).then(Type.uint64_t.fromMsg);
+};
+
+/**
+ * Remove an empty directory.
+ *
+ * @param {string} path The name of the directory to remove.
+ * @param {*=} options Additional options.
+ *   - {bool} ignoreAbsent If |true|, do not fail if the
+ *     directory does not exist yet.
+ */
+File.removeEmptyDir = function removeEmptyDir(path, options) {
+  return Scheduler.post("removeEmptyDir",
+    [Type.path.toMsg(path), options], path);
+};
+
+/**
+ * Remove an existing file.
+ *
+ * @param {string} path The name of the file.
+ * @param {*=} options Additional options.
+ *   - {bool} ignoreAbsent If |false|, throw an error if the file does
+ *     not exist. |true| by default.
+ *
+ * @throws {OS.File.Error} In case of I/O error.
+ */
+File.remove = function remove(path, options) {
+  return Scheduler.post("remove",
+    [Type.path.toMsg(path), options], path);
+};
+
+
+
+/**
+ * Create a directory and, optionally, its parent directories.
+ *
+ * @param {string} path The name of the directory.
+ * @param {*=} options Additional options.
+ *
+ * - {string} from If specified, the call to |makeDir| creates all the
+ * ancestors of |path| that are descendants of |from|. Note that |path|
+ * must be a descendant of |from|, and that |from| and its existing
+ * subdirectories present in |path|  must be user-writeable.
+ * Example:
+ *   makeDir(Path.join(profileDir, "foo", "bar"), { from: profileDir });
+ *  creates directories profileDir/foo, profileDir/foo/bar
+ * - {bool} ignoreExisting If |false|, throw an error if the directory
+ * already exists. |true| by default. Ignored if |from| is specified.
+ * - {number} unixMode Under Unix, if specified, a file creation mode,
+ * as per libc function |mkdir|. If unspecified, dirs are
+ * created with a default mode of 0700 (dir is private to
+ * the user, the user can read, write and execute). Ignored under Windows
+ * or if the file system does not support file creation modes.
+ * - {C pointer} winSecurity Under Windows, if specified, security
+ * attributes as per winapi function |CreateDirectory|. If
+ * unspecified, use the default security descriptor, inherited from
+ * the parent directory. Ignored under Unix or if the file system
+ * does not support security descriptors.
+ */
+File.makeDir = function makeDir(path, options) {
+  return Scheduler.post("makeDir",
+    [Type.path.toMsg(path), options], path);
+};
+
+/**
+ * Return the contents of a file
+ *
+ * @param {string} path The path to the file.
+ * @param {number=} bytes Optionally, an upper bound to the number of bytes
+ * to read. DEPRECATED - please use options.bytes instead.
+ * @param {JSON} options Additional options.
+ * - {boolean} sequential A flag that triggers a population of the page cache
+ * with data from a file so that subsequent reads from that file would not
+ * block on disk I/O. If |true| or unspecified, inform the system that the
+ * contents of the file will be read in order. Otherwise, make no such
+ * assumption. |true| by default.
+ * - {number} bytes An upper bound to the number of bytes to read.
+ * - {string} compression If "lz4" and if the file is compressed using the lz4
+ * compression algorithm, decompress the file contents on the fly.
+ *
+ * @resolves {Uint8Array} A buffer holding the bytes
+ * read from the file.
+ */
+File.read = function read(path, bytes, options = {}) {
+  if (typeof bytes == "object") {
+    // Passing |bytes| as an argument is deprecated.
+    // We should now be passing it as a field of |options|.
+    options = bytes || {};
+  } else {
+    options = clone(options, ["outExecutionDuration"]);
+    if (typeof bytes != "undefined") {
+      options.bytes = bytes;
+    }
+  }
+
+  if (options.compression || !nativeWheneverAvailable) {
+    // We need to use the JS implementation.
+    let promise = Scheduler.post("read",
+      [Type.path.toMsg(path), bytes, options], path);
+    return promise.then(
+      function onSuccess(data) {
+        if (typeof data == "string") {
+          return data;
+        }
+        return new Uint8Array(data.buffer, data.byteOffset, data.byteLength);
+      });
+  }
+
+  // Otherwise, use the native implementation.
+  return Scheduler.push(() => Native.read(path, options));
+};
+
+/**
+ * Find outs if a file exists.
+ *
+ * @param {string} path The path to the file.
+ *
+ * @return {bool} true if the file exists, false otherwise.
+ */
+File.exists = function exists(path) {
+  return Scheduler.post("exists",
+    [Type.path.toMsg(path)], path);
+};
+
+/**
+ * Write a file, atomically.
+ *
+ * By opposition to a regular |write|, this operation ensures that,
+ * until the contents are fully written, the destination file is
+ * not modified.
+ *
+ * Limitation: In a few extreme cases (hardware failure during the
+ * write, user unplugging disk during the write, etc.), data may be
+ * corrupted. If your data is user-critical (e.g. preferences,
+ * application data, etc.), you may wish to consider adding options
+ * |tmpPath| and/or |flush| to reduce the likelihood of corruption, as
+ * detailed below. Note that no combination of options can be
+ * guaranteed to totally eliminate the risk of corruption.
+ *
+ * @param {string} path The path of the file to modify.
+ * @param {Typed Array | C pointer} buffer A buffer containing the bytes to write.
+ * @param {*=} options Optionally, an object determining the behavior
+ * of this function. This object may contain the following fields:
+ * - {number} bytes The number of bytes to write. If unspecified,
+ * |buffer.byteLength|. Required if |buffer| is a C pointer.
+ * - {string} tmpPath If |null| or unspecified, write all data directly
+ * to |path|. If specified, write all data to a temporary file called
+ * |tmpPath| and, once this write is complete, rename the file to
+ * replace |path|. Performing this additional operation is a little
+ * slower but also a little safer.
+ * - {bool} noOverwrite - If set, this function will fail if a file already
+ * exists at |path|.
+ * - {bool} flush - If |false| or unspecified, return immediately once the
+ * write is complete. If |true|, before writing, force the operating system
+ * to write its internal disk buffers to the disk. This is considerably slower
+ * (not just for the application but for the whole system) but also safer:
+ * if the system shuts down improperly (typically due to a kernel freeze
+ * or a power failure) or if the device is disconnected before the buffer
+ * is flushed, the file has more chances of not being corrupted.
+ * - {string} backupTo - If specified, backup the destination file as |backupTo|.
+ * Note that this function renames the destination file before overwriting it.
+ * If the process or the operating system freezes or crashes
+ * during the short window between these operations,
+ * the destination file will have been moved to its backup.
+ *
+ * @return {promise}
+ * @resolves {number} The number of bytes actually written.
+ */
+File.writeAtomic = function writeAtomic(path, buffer, options = {}) {
+  // Copy |options| to avoid modifying the original object but preserve the
+  // reference to |outExecutionDuration| option if it is passed.
+  options = clone(options, ["outExecutionDuration"]);
+  // As options.tmpPath is a path, we need to encode it as |Type.path| message
+  if ("tmpPath" in options) {
+    options.tmpPath = Type.path.toMsg(options.tmpPath);
+  };
+  if (isTypedArray(buffer) && (!("bytes" in options))) {
+    options.bytes = buffer.byteLength;
+  };
+  let refObj = {};
+  TelemetryStopwatch.start("OSFILE_WRITEATOMIC_JANK_MS", refObj);
+  let promise = Scheduler.post("writeAtomic",
+    [Type.path.toMsg(path),
+     Type.void_t.in_ptr.toMsg(buffer),
+     options], [options, buffer, path]);
+  TelemetryStopwatch.finish("OSFILE_WRITEATOMIC_JANK_MS", refObj);
+  return promise;
+};
+
+File.removeDir = function(path, options = {}) {
+  return Scheduler.post("removeDir",
+    [Type.path.toMsg(path), options], path);
+};
+
+/**
+ * Information on a file, as returned by OS.File.stat or
+ * OS.File.prototype.stat
+ *
+ * @constructor
+ */
+File.Info = function Info(value) {
+  // Note that we can't just do this[k] = value[k] because our
+  // prototype defines getters for all of these fields.
+  for (let k in value) {
+    if (k != "creationDate") {
+      Object.defineProperty(this, k, {value: value[k]});
+    }
+  }
+  Object.defineProperty(this, "_deprecatedCreationDate", {value: value["creationDate"]});
+};
+File.Info.prototype = SysAll.AbstractInfo.prototype;
+
+// Deprecated
+Object.defineProperty(File.Info.prototype, "creationDate", {
+  get: function creationDate() {
+    let {Deprecated} = Cu.import("resource://gre/modules/Deprecated.jsm", {});
+    Deprecated.warning("Field 'creationDate' is deprecated.", "https://developer.mozilla.org/en-US/docs/JavaScript_OS.File/OS.File.Info#Cross-platform_Attributes");
+    return this._deprecatedCreationDate;
+  }
+});
+
+File.Info.fromMsg = function fromMsg(value) {
+  return new File.Info(value);
+};
+
+/**
+ * Get worker's current DEBUG flag.
+ * Note: This is used for testing purposes.
+ */
+File.GET_DEBUG = function GET_DEBUG() {
+  return Scheduler.post("GET_DEBUG");
+};
+
+/**
+ * Iterate asynchronously through a directory
+ *
+ * @constructor
+ */
+var DirectoryIterator = function DirectoryIterator(path, options) {
+  /**
+   * Open the iterator on the worker thread
+   *
+   * @type {Promise}
+   * @resolves {*} A message accepted by the methods of DirectoryIterator
+   * in the worker thread
+   * @rejects {StopIteration} If all entries have already been visited
+   * or the iterator has been closed.
+   */
+  this.__itmsg = Scheduler.post(
+    "new_DirectoryIterator", [Type.path.toMsg(path), options],
+    path
+  );
+  this._isClosed = false;
+};
+DirectoryIterator.prototype = {
+  iterator: function () {
+    return this;
+  },
+  __iterator__: function () {
+    return this;
+  },
+
+  // Once close() is called, _itmsg should reject with a
+  // StopIteration. However, we don't want to create the promise until
+  // it's needed because it might never be used. In that case, we
+  // would get a warning on the console.
+  get _itmsg() {
+    if (!this.__itmsg) {
+      this.__itmsg = Promise.reject(StopIteration);
+    }
+    return this.__itmsg;
+  },
+
+  /**
+   * Determine whether the directory exists.
+   *
+   * @resolves {boolean}
+   */
+  exists: function exists() {
+    return this._itmsg.then(
+      function onSuccess(iterator) {
+        return Scheduler.post("DirectoryIterator_prototype_exists", [iterator]);
+      }
+    );
+  },
+  /**
+   * Get the next entry in the directory.
+   *
+   * @return {Promise}
+   * @resolves {OS.File.Entry}
+   * @rejects {StopIteration} If all entries have already been visited.
+   */
+  next: function next() {
+    let self = this;
+    let promise = this._itmsg;
+
+    // Get the iterator, call _next
+    promise = promise.then(
+      function withIterator(iterator) {
+        return self._next(iterator);
+      });
+
+    return promise;
+  },
+  /**
+   * Get several entries at once.
+   *
+   * @param {number=} length If specified, the number of entries
+   * to return. If unspecified, return all remaining entries.
+   * @return {Promise}
+   * @resolves {Array} An array containing the |length| next entries.
+   */
+  nextBatch: function nextBatch(size) {
+    if (this._isClosed) {
+      return Promise.resolve([]);
+    }
+    let promise = this._itmsg;
+    promise = promise.then(
+      function withIterator(iterator) {
+        return Scheduler.post("DirectoryIterator_prototype_nextBatch", [iterator, size]);
+      });
+    promise = promise.then(
+      function withEntries(array) {
+        return array.map(DirectoryIterator.Entry.fromMsg);
+      });
+    return promise;
+  },
+  /**
+   * Apply a function to all elements of the directory sequentially.
+   *
+   * @param {Function} cb This function will be applied to all entries
+   * of the directory. It receives as arguments
+   *  - the OS.File.Entry corresponding to the entry;
+   *  - the index of the entry in the enumeration;
+   *  - the iterator itself - return |iterator.close()| to stop the loop.
+   *
+   * If the callback returns a promise, iteration waits until the
+   * promise is resolved before proceeding.
+   *
+   * @return {Promise} A promise resolved once the loop has reached
+   * its end.
+   */
+  forEach: function forEach(cb, options) {
+    if (this._isClosed) {
+      return Promise.resolve();
+    }
+
+    let self = this;
+    let position = 0;
+    let iterator;
+
+    // Grab iterator
+    let promise = this._itmsg.then(
+      function(aIterator) {
+        iterator = aIterator;
+      }
+    );
+
+    // Then iterate
+    let loop = function loop() {
+      if (self._isClosed) {
+        return Promise.resolve();
+      }
+      return self._next(iterator).then(
+        function onSuccess(value) {
+          return Promise.resolve(cb(value, position++, self)).then(loop);
+        },
+        function onFailure(reason) {
+          if (reason == StopIteration) {
+            return;
+          }
+          throw reason;
+        }
+      );
+    };
+
+    return promise.then(loop);
+  },
+  /**
+   * Auxiliary method: fetch the next item
+   *
+   * @rejects {StopIteration} If all entries have already been visited
+   * or the iterator has been closed.
+   */
+  _next: function _next(iterator) {
+    if (this._isClosed) {
+      return this._itmsg;
+    }
+    let self = this;
+    let promise = Scheduler.post("DirectoryIterator_prototype_next", [iterator]);
+    promise = promise.then(
+      DirectoryIterator.Entry.fromMsg,
+      function onReject(reason) {
+        if (reason == StopIteration) {
+          self.close();
+          throw StopIteration;
+        }
+        throw reason;
+      });
+    return promise;
+  },
+  /**
+   * Close the iterator
+   */
+  close: function close() {
+    if (this._isClosed) {
+      return Promise.resolve();
+    }
+    this._isClosed = true;
+    let self = this;
+    return this._itmsg.then(
+      function withIterator(iterator) {
+        // Set __itmsg to null so that the _itmsg getter returns a
+        // rejected StopIteration promise if it's ever used.
+        self.__itmsg = null;
+        return Scheduler.post("DirectoryIterator_prototype_close", [iterator]);
+      }
+    );
+  }
+};
+
+DirectoryIterator.Entry = function Entry(value) {
+  return value;
+};
+DirectoryIterator.Entry.prototype = Object.create(SysAll.AbstractEntry.prototype);
+
+DirectoryIterator.Entry.fromMsg = function fromMsg(value) {
+  return new DirectoryIterator.Entry(value);
+};
+
+File.resetWorker = function() {
+  return Task.spawn(function*() {
+    let resources = yield Scheduler.kill({shutdown: false, reset: true});
+    if (resources && !resources.killed) {
+        throw new Error("Could not reset worker, this would leak file descriptors: " + JSON.stringify(resources));
+    }
+  });
+};
+
+// Constants
+File.POS_START = SysAll.POS_START;
+File.POS_CURRENT = SysAll.POS_CURRENT;
+File.POS_END = SysAll.POS_END;
+
+// Exports
+File.Error = OSError;
+File.DirectoryIterator = DirectoryIterator;
+
+this.OS = {};
+this.OS.File = File;
+this.OS.Constants = SharedAll.Constants;
+this.OS.Shared = {
+  LOG: SharedAll.LOG,
+  Type: SysAll.Type,
+  get DEBUG() {
+    return SharedAll.Config.DEBUG;
+  },
+  set DEBUG(x) {
+    return SharedAll.Config.DEBUG = x;
+  }
+};
+Object.freeze(this.OS.Shared);
+this.OS.Path = Path;
+
+// Returns a resolved promise when all the queued operation have been completed.
+Object.defineProperty(OS.File, "queue", {
+  get: function() {
+    return Scheduler.queue;
+  }
+});
+
+// `true` if this is a content process, `false` otherwise.
+// It would be nicer to go through `Services.appInfo`, but some tests need to be
+// able to replace that field with a custom implementation before it is first
+// called.
+const isContent = Components.classes["@mozilla.org/xre/app-info;1"].getService(Ci.nsIXULRuntime).processType == Ci.nsIXULRuntime.PROCESS_TYPE_CONTENT;
+
+/**
+ * Shutdown barriers, to let clients register to be informed during shutdown.
+ */
+var Barriers = {
+  shutdown: new AsyncShutdown.Barrier("OS.File: Waiting for clients before full shutdown"),
+  /**
+   * Return the shutdown state of OS.File
+   */
+  getDetails: function() {
+    let result = {
+      launched: Scheduler.launched,
+      shutdown: Scheduler.shutdown,
+      worker: !!Scheduler._worker,
+      pendingReset: !!Scheduler.resetTimer,
+      latestSent: Scheduler.Debugging.latestSent,
+      latestReceived: Scheduler.Debugging.latestReceived,
+      messagesSent: Scheduler.Debugging.messagesSent,
+      messagesReceived: Scheduler.Debugging.messagesReceived,
+      messagesQueued: Scheduler.Debugging.messagesQueued,
+      DEBUG: SharedAll.Config.DEBUG,
+    };
+    // Convert dates to strings for better readability
+    for (let key of ["latestSent", "latestReceived"]) {
+      if (result[key] && typeof result[key][0] == "number") {
+        result[key][0] = Date(result[key][0]);
+      }
+    }
+    return result;
+  }
+};
+
+function setupShutdown(phaseName) {
+  Barriers[phaseName] = new AsyncShutdown.Barrier(`OS.File: Waiting for clients before ${phaseName}`),
+  File[phaseName] = Barriers[phaseName].client;
+
+  // Auto-flush OS.File during `phaseName`. This ensures that any I/O
+  // that has been queued *before* `phaseName` is properly completed.
+  // To ensure that I/O queued *during* `phaseName` change is completed,
+  // clients should register using AsyncShutdown.addBlocker.
+  AsyncShutdown[phaseName].addBlocker(
+    `OS.File: flush I/O queued before ${phaseName}`,
+    Task.async(function*() {
+      // Give clients a last chance to enqueue requests.
+      yield Barriers[phaseName].wait({crashAfterMS: null});
+
+      // Wait until all currently enqueued requests are completed.
+      yield Scheduler.queue;
+    }),
+    () => {
+      let details = Barriers.getDetails();
+      details.clients = Barriers[phaseName].state;
+      return details;
+    }
+  );
+}
+
+// profile-before-change only exists in the parent, and OS.File should
+// not be used in the child process anyways.
+if (!isContent) {
+  setupShutdown("profileBeforeChange")
+}
+File.shutdown = Barriers.shutdown.client;
diff --git a/toolkit/components/osfile/modules/osfile_async_worker.js b/toolkit/components/osfile/modules/osfile_async_worker.js
new file mode 100644
index 000000000..84287c75e
--- /dev/null
+++ b/toolkit/components/osfile/modules/osfile_async_worker.js
@@ -0,0 +1,407 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+
+if (this.Components) {
+  throw new Error("This worker can only be loaded from a worker thread");
+}
+
+// Worker thread for osfile asynchronous front-end
+
+(function(exports) {
+  "use strict";
+
+  // Timestamps, for use in Telemetry.
+  // The object is set to |null| once it has been sent
+  // to the main thread.
+  let timeStamps = {
+    entered: Date.now(),
+    loaded: null
+  };
+
+  importScripts("resource://gre/modules/osfile.jsm");
+
+  let PromiseWorker = require("resource://gre/modules/workers/PromiseWorker.js");
+  let SharedAll = require("resource://gre/modules/osfile/osfile_shared_allthreads.jsm");
+  let LOG = SharedAll.LOG.bind(SharedAll, "Agent");
+
+  let worker = new PromiseWorker.AbstractWorker();
+  worker.dispatch = function(method, args = []) {
+    return Agent[method](...args);
+  },
+  worker.log = LOG;
+  worker.postMessage = function(message, ...transfers) {
+    if (timeStamps) {
+      message.timeStamps = timeStamps;
+      timeStamps = null;
+    }
+    self.postMessage(message, ...transfers);
+  };
+  worker.close = function() {
+    self.close();
+  };
+  let Meta = PromiseWorker.Meta;
+
+  self.addEventListener("message", msg => worker.handleMessage(msg));
+
+ /**
+  * A data structure used to track opened resources
+  */
+  let ResourceTracker = function ResourceTracker() {
+   // A number used to generate ids
+   this._idgen = 0;
+   // A map from id to resource
+   this._map = new Map();
+  };
+  ResourceTracker.prototype = {
+   /**
+    * Get a resource from its unique identifier.
+    */
+   get: function(id) {
+     let result = this._map.get(id);
+     if (result == null) {
+       return result;
+     }
+     return result.resource;
+   },
+   /**
+    * Remove a resource from its unique identifier.
+    */
+   remove: function(id) {
+     if (!this._map.has(id)) {
+       throw new Error("Cannot find resource id " + id);
+     }
+     this._map.delete(id);
+   },
+   /**
+    * Add a resource, return a new unique identifier
+    *
+    * @param {*} resource A resource.
+    * @param {*=} info Optional information. For debugging purposes.
+    *
+    * @return {*} A unique identifier. For the moment, this is a number,
+    * but this might not remain the case forever.
+    */
+   add: function(resource, info) {
+     let id = this._idgen++;
+     this._map.set(id, {resource:resource, info:info});
+     return id;
+   },
+   /**
+    * Return a list of all open resources i.e. the ones still present in
+    * ResourceTracker's _map.
+    */
+   listOpenedResources: function listOpenedResources() {
+     return Array.from(this._map, ([id, resource]) => resource.info.path);
+   }
+  };
+
+ /**
+  * A map of unique identifiers to opened files.
+  */
+  let OpenedFiles = new ResourceTracker();
+
+ /**
+  * Execute a function in the context of a given file.
+  *
+  * @param {*} id A unique identifier, as used by |OpenFiles|.
+  * @param {Function} f A function to call.
+  * @param {boolean} ignoreAbsent If |true|, the error is ignored. Otherwise, the error causes an exception.
+  * @return The return value of |f()|
+  *
+  * This function attempts to get the file matching |id|. If
+  * the file exists, it executes |f| within the |this| set
+  * to the corresponding file. Otherwise, it throws an error.
+  */
+  let withFile = function withFile(id, f, ignoreAbsent) {
+   let file = OpenedFiles.get(id);
+   if (file == null) {
+     if (!ignoreAbsent) {
+       throw OS.File.Error.closed("accessing file");
+     }
+     return undefined;
+   }
+   return f.call(file);
+  };
+
+  let OpenedDirectoryIterators = new ResourceTracker();
+  let withDir = function withDir(fd, f, ignoreAbsent) {
+   let file = OpenedDirectoryIterators.get(fd);
+   if (file == null) {
+     if (!ignoreAbsent) {
+       throw OS.File.Error.closed("accessing directory");
+     }
+     return undefined;
+   }
+   if (!(file instanceof File.DirectoryIterator)) {
+     throw new Error("file is not a directory iterator " + file.__proto__.toSource());
+   }
+   return f.call(file);
+  };
+
+  let Type = exports.OS.Shared.Type;
+
+  let File = exports.OS.File;
+
+ /**
+  * The agent.
+  *
+  * It is in charge of performing method-specific deserialization
+  * of messages, calling the function/method of OS.File and serializing
+  * back the results.
+  */
+  let Agent = {
+   // Update worker's OS.Shared.DEBUG flag message from controller.
+   SET_DEBUG: function(aDEBUG) {
+     SharedAll.Config.DEBUG = aDEBUG;
+   },
+   // Return worker's current OS.Shared.DEBUG value to controller.
+   // Note: This is used for testing purposes.
+   GET_DEBUG: function() {
+     return SharedAll.Config.DEBUG;
+   },
+   /**
+    * Execute shutdown sequence, returning data on leaked file descriptors.
+    *
+    * @param {bool} If |true|, kill the worker if this would not cause
+    * leaks.
+    */
+   Meta_shutdown: function(kill) {
+     let result = {
+       openedFiles: OpenedFiles.listOpenedResources(),
+       openedDirectoryIterators: OpenedDirectoryIterators.listOpenedResources(),
+       killed: false // Placeholder
+     };
+
+     // Is it safe to kill the worker?
+     let safe = result.openedFiles.length == 0
+           && result.openedDirectoryIterators.length == 0;
+     result.killed = safe && kill;
+
+     return new Meta(result, {shutdown: result.killed});
+   },
+   // Functions of OS.File
+   stat: function stat(path, options) {
+     return exports.OS.File.Info.toMsg(
+       exports.OS.File.stat(Type.path.fromMsg(path), options));
+   },
+   setPermissions: function setPermissions(path, options = {}) {
+     return exports.OS.File.setPermissions(Type.path.fromMsg(path), options);
+   },
+   setDates: function setDates(path, accessDate, modificationDate) {
+     return exports.OS.File.setDates(Type.path.fromMsg(path), accessDate,
+                                     modificationDate);
+   },
+   getCurrentDirectory: function getCurrentDirectory() {
+     return exports.OS.Shared.Type.path.toMsg(File.getCurrentDirectory());
+   },
+   setCurrentDirectory: function setCurrentDirectory(path) {
+     File.setCurrentDirectory(exports.OS.Shared.Type.path.fromMsg(path));
+   },
+   copy: function copy(sourcePath, destPath, options) {
+     return File.copy(Type.path.fromMsg(sourcePath),
+       Type.path.fromMsg(destPath), options);
+   },
+   move: function move(sourcePath, destPath, options) {
+     return File.move(Type.path.fromMsg(sourcePath),
+       Type.path.fromMsg(destPath), options);
+   },
+   getAvailableFreeSpace: function getAvailableFreeSpace(sourcePath) {
+     return Type.uint64_t.toMsg(
+       File.getAvailableFreeSpace(Type.path.fromMsg(sourcePath)));
+   },
+   makeDir: function makeDir(path, options) {
+     return File.makeDir(Type.path.fromMsg(path), options);
+   },
+   removeEmptyDir: function removeEmptyDir(path, options) {
+     return File.removeEmptyDir(Type.path.fromMsg(path), options);
+   },
+   remove: function remove(path, options) {
+     return File.remove(Type.path.fromMsg(path), options);
+   },
+   open: function open(path, mode, options) {
+     let filePath = Type.path.fromMsg(path);
+     let file = File.open(filePath, mode, options);
+     return OpenedFiles.add(file, {
+       // Adding path information to keep track of opened files
+       // to report leaks when debugging.
+       path: filePath
+     });
+   },
+   openUnique: function openUnique(path, options) {
+     let filePath = Type.path.fromMsg(path);
+     let openedFile = OS.Shared.AbstractFile.openUnique(filePath, options);
+     let resourceId = OpenedFiles.add(openedFile.file, {
+       // Adding path information to keep track of opened files
+       // to report leaks when debugging.
+       path: openedFile.path
+     });
+
+     return {
+       path: openedFile.path,
+       file: resourceId
+     };
+   },
+   read: function read(path, bytes, options) {
+     let data = File.read(Type.path.fromMsg(path), bytes, options);
+     if (typeof data == "string") {
+       return data;
+     }
+     return new Meta({
+         buffer: data.buffer,
+         byteOffset: data.byteOffset,
+         byteLength: data.byteLength
+     }, {
+       transfers: [data.buffer]
+     });
+   },
+   exists: function exists(path) {
+     return File.exists(Type.path.fromMsg(path));
+   },
+   writeAtomic: function writeAtomic(path, buffer, options) {
+     if (options.tmpPath) {
+       options.tmpPath = Type.path.fromMsg(options.tmpPath);
+     }
+     return File.writeAtomic(Type.path.fromMsg(path),
+                             Type.voidptr_t.fromMsg(buffer),
+                             options
+                            );
+   },
+   removeDir: function(path, options) {
+     return File.removeDir(Type.path.fromMsg(path), options);
+   },
+   new_DirectoryIterator: function new_DirectoryIterator(path, options) {
+     let directoryPath = Type.path.fromMsg(path);
+     let iterator = new File.DirectoryIterator(directoryPath, options);
+     return OpenedDirectoryIterators.add(iterator, {
+       // Adding path information to keep track of opened directory
+       // iterators to report leaks when debugging.
+       path: directoryPath
+     });
+   },
+   // Methods of OS.File
+   File_prototype_close: function close(fd) {
+     return withFile(fd,
+       function do_close() {
+         try {
+           return this.close();
+         } finally {
+           OpenedFiles.remove(fd);
+         }
+     });
+   },
+   File_prototype_stat: function stat(fd) {
+     return withFile(fd,
+       function do_stat() {
+         return exports.OS.File.Info.toMsg(this.stat());
+       });
+   },
+   File_prototype_setPermissions: function setPermissions(fd, options = {}) {
+     return withFile(fd,
+       function do_setPermissions() {
+         return this.setPermissions(options);
+       });
+   },
+   File_prototype_setDates: function setDates(fd, accessTime, modificationTime) {
+     return withFile(fd,
+       function do_setDates() {
+         return this.setDates(accessTime, modificationTime);
+       });
+   },
+   File_prototype_read: function read(fd, nbytes, options) {
+     return withFile(fd,
+       function do_read() {
+         let data = this.read(nbytes, options);
+         return new Meta({
+             buffer: data.buffer,
+             byteOffset: data.byteOffset,
+             byteLength: data.byteLength
+         }, {
+           transfers: [data.buffer]
+         });
+       }
+     );
+   },
+   File_prototype_readTo: function readTo(fd, buffer, options) {
+     return withFile(fd,
+       function do_readTo() {
+         return this.readTo(exports.OS.Shared.Type.voidptr_t.fromMsg(buffer),
+         options);
+       });
+   },
+   File_prototype_write: function write(fd, buffer, options) {
+     return withFile(fd,
+       function do_write() {
+         return this.write(exports.OS.Shared.Type.voidptr_t.fromMsg(buffer),
+         options);
+       });
+   },
+   File_prototype_setPosition: function setPosition(fd, pos, whence) {
+     return withFile(fd,
+       function do_setPosition() {
+         return this.setPosition(pos, whence);
+       });
+   },
+   File_prototype_getPosition: function getPosition(fd) {
+     return withFile(fd,
+       function do_getPosition() {
+         return this.getPosition();
+       });
+   },
+   File_prototype_flush: function flush(fd) {
+     return withFile(fd,
+       function do_flush() {
+         return this.flush();
+       });
+   },
+   // Methods of OS.File.DirectoryIterator
+   DirectoryIterator_prototype_next: function next(dir) {
+     return withDir(dir,
+       function do_next() {
+         try {
+           return File.DirectoryIterator.Entry.toMsg(this.next());
+         } catch (x) {
+           if (x == StopIteration) {
+             OpenedDirectoryIterators.remove(dir);
+           }
+           throw x;
+         }
+       }, false);
+   },
+   DirectoryIterator_prototype_nextBatch: function nextBatch(dir, size) {
+     return withDir(dir,
+       function do_nextBatch() {
+         let result;
+         try {
+           result = this.nextBatch(size);
+         } catch (x) {
+           OpenedDirectoryIterators.remove(dir);
+           throw x;
+         }
+         return result.map(File.DirectoryIterator.Entry.toMsg);
+       }, false);
+   },
+   DirectoryIterator_prototype_close: function close(dir) {
+     return withDir(dir,
+       function do_close() {
+         this.close();
+         OpenedDirectoryIterators.remove(dir);
+       }, true);// ignore error to support double-closing |DirectoryIterator|
+   },
+   DirectoryIterator_prototype_exists: function exists(dir) {
+     return withDir(dir,
+       function do_exists() {
+         return this.exists();
+       });
+   }
+  };
+  if (!SharedAll.Constants.Win) {
+    Agent.unixSymLink = function unixSymLink(sourcePath, destPath) {
+      return File.unixSymLink(Type.path.fromMsg(sourcePath),
+        Type.path.fromMsg(destPath));
+    };
+  }
+
+  timeStamps.loaded = Date.now();
+})(this);
diff --git a/toolkit/components/osfile/modules/osfile_native.jsm b/toolkit/components/osfile/modules/osfile_native.jsm
new file mode 100644
index 000000000..16cd3c92a
--- /dev/null
+++ b/toolkit/components/osfile/modules/osfile_native.jsm
@@ -0,0 +1,70 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * Native (xpcom) implementation of key OS.File functions
+ */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = ["read"];
+
+var {results: Cr, utils: Cu, interfaces: Ci} = Components;
+
+var SharedAll = Cu.import("resource://gre/modules/osfile/osfile_shared_allthreads.jsm", {});
+
+var SysAll = {};
+if (SharedAll.Constants.Win) {
+  Cu.import("resource://gre/modules/osfile/osfile_win_allthreads.jsm", SysAll);
+} else if (SharedAll.Constants.libc) {
+  Cu.import("resource://gre/modules/osfile/osfile_unix_allthreads.jsm", SysAll);
+} else {
+  throw new Error("I am neither under Windows nor under a Posix system");
+}
+var {Promise} = Cu.import("resource://gre/modules/Promise.jsm", {});
+var {XPCOMUtils} = Cu.import("resource://gre/modules/XPCOMUtils.jsm", {});
+
+/**
+ * The native service holding the implementation of the functions.
+ */
+XPCOMUtils.defineLazyServiceGetter(this,
+  "Internals",
+  "@mozilla.org/toolkit/osfile/native-internals;1",
+  "nsINativeOSFileInternalsService");
+
+/**
+ * Native implementation of OS.File.read
+ *
+ * This implementation does not handle option |compression|.
+ */
+this.read = function(path, options = {}) {
+  // Sanity check on types of options
+  if ("encoding" in options && typeof options.encoding != "string") {
+    return Promise.reject(new TypeError("Invalid type for option encoding"));
+  }
+  if ("compression" in options && typeof options.compression != "string") {
+    return Promise.reject(new TypeError("Invalid type for option compression"));
+  }
+  if ("bytes" in options && typeof options.bytes != "number") {
+    return Promise.reject(new TypeError("Invalid type for option bytes"));
+  }
+
+  let deferred = Promise.defer();
+  Internals.read(path,
+    options,
+    function onSuccess(success) {
+      success.QueryInterface(Ci.nsINativeOSFileResult);
+      if ("outExecutionDuration" in options) {
+        options.outExecutionDuration =
+          success.executionDurationMS +
+          (options.outExecutionDuration || 0);
+      }
+      deferred.resolve(success.result);
+    },
+    function onError(operation, oserror) {
+      deferred.reject(new SysAll.Error(operation, oserror, path));
+    }
+  );
+  return deferred.promise;
+};
diff --git a/toolkit/components/osfile/modules/osfile_shared_allthreads.jsm b/toolkit/components/osfile/modules/osfile_shared_allthreads.jsm
new file mode 100644
index 000000000..c5c505102
--- /dev/null
+++ b/toolkit/components/osfile/modules/osfile_shared_allthreads.jsm
@@ -0,0 +1,1315 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+/**
+ * OS.File utilities used by all threads.
+ *
+ * This module defines:
+ * - logging;
+ * - the base constants;
+ * - base types and primitives for declaring new types;
+ * - primitives for importing C functions;
+ * - primitives for dealing with integers, pointers, typed arrays;
+ * - the base class OSError;
+ * - a few additional utilities.
+ */
+
+// Boilerplate used to be able to import this module both from the main
+// thread and from worker threads.
+
+// Since const is lexically scoped, hoist the
+// conditionally-useful definition ourselves.
+const Cu = typeof Components != "undefined" ? Components.utils : undefined;
+const Ci = typeof Components != "undefined" ? Components.interfaces : undefined;
+const Cc = typeof Components != "undefined" ? Components.classes : undefined;
+
+/**
+ * A constructor for messages that require transfers instead of copies.
+ *
+ * See BasePromiseWorker.Meta.
+ *
+ * @constructor
+ */
+var Meta;
+if (typeof Components != "undefined") {
+  // Global definition of |exports|, to keep everybody happy.
+  // In non-main thread, |exports| is provided by the module
+  // loader.
+  this.exports = {};
+
+  Cu.import("resource://gre/modules/Services.jsm", this);
+  Meta = Cu.import("resource://gre/modules/PromiseWorker.jsm", {}).BasePromiseWorker.Meta;
+} else {
+  importScripts("resource://gre/modules/workers/require.js");
+  Meta = require("resource://gre/modules/workers/PromiseWorker.js").Meta;
+}
+
+var EXPORTED_SYMBOLS = [
+  "LOG",
+  "clone",
+  "Config",
+  "Constants",
+  "Type",
+  "HollowStructure",
+  "OSError",
+  "Library",
+  "declareFFI",
+  "declareLazy",
+  "declareLazyFFI",
+  "normalizeBufferArgs",
+  "projectValue",
+  "isArrayBuffer",
+  "isTypedArray",
+  "defineLazyGetter",
+  "OS" // Warning: this exported symbol will disappear
+];
+
+////////////////////// Configuration of OS.File
+
+var Config = {
+  /**
+   * If |true|, calls to |LOG| are shown. Otherwise, they are hidden.
+   *
+   * This configuration option is controlled by preference "toolkit.osfile.log".
+   */
+  DEBUG: false,
+
+  /**
+   * TEST
+   */
+  TEST: false
+};
+exports.Config = Config;
+
+////////////////////// OS Constants
+
+if (typeof Components != "undefined") {
+  // On the main thread, OS.Constants is defined by a xpcom
+  // component. On other threads, it is available automatically
+  Cu.import("resource://gre/modules/ctypes.jsm");
+  Cc["@mozilla.org/net/osfileconstantsservice;1"].
+    getService(Ci.nsIOSFileConstantsService).init();
+}
+
+exports.Constants = OS.Constants;
+
+///////////////////// Utilities
+
+// Define a lazy getter for a property
+var defineLazyGetter = function defineLazyGetter(object, name, getter) {
+  Object.defineProperty(object, name, {
+    configurable: true,
+    get: function lazy() {
+      delete this[name];
+      let value = getter.call(this);
+      Object.defineProperty(object, name, {
+        value: value
+      });
+      return value;
+    }
+  });
+};
+exports.defineLazyGetter = defineLazyGetter;
+
+
+///////////////////// Logging
+
+/**
+ * The default implementation of the logger.
+ *
+ * The choice of logger can be overridden with Config.TEST.
+ */
+var gLogger;
+if (typeof window != "undefined" && window.console && console.log) {
+  gLogger = console.log.bind(console, "OS");
+} else {
+  gLogger = function(...args) {
+    dump("OS " + args.join(" ") + "\n");
+  };
+}
+
+/**
+ * Attempt to stringify an argument into something useful for
+ * debugging purposes, by using |.toString()| or |JSON.stringify|
+ * if available.
+ *
+ * @param {*} arg An argument to be stringified if possible.
+ * @return {string} A stringified version of |arg|.
+ */
+var stringifyArg = function stringifyArg(arg) {
+  if (typeof arg === "string") {
+    return arg;
+  }
+  if (arg && typeof arg === "object") {
+    let argToString = "" + arg;
+
+    /**
+     * The only way to detect whether this object has a non-default
+     * implementation of |toString| is to check whether it returns
+     * '[object Object]'. Unfortunately, we cannot simply compare |arg.toString|
+     * and |Object.prototype.toString| as |arg| typically comes from another
+     * compartment.
+     */
+    if (argToString === "[object Object]") {
+      return JSON.stringify(arg, function(key, value) {
+        if (isTypedArray(value)) {
+          return "["+ value.constructor.name + " " + value.byteOffset + " " + value.byteLength + "]";
+        }
+        if (isArrayBuffer(arg)) {
+          return "[" + value.constructor.name + " " + value.byteLength + "]";
+        }
+        return value;
+      });
+    } else {
+      return argToString;
+    }
+  }
+  return arg;
+};
+
+var LOG = function (...args) {
+  if (!Config.DEBUG) {
+    // If logging is deactivated, don't log
+    return;
+  }
+
+  let logFunc = gLogger;
+  if (Config.TEST && typeof Components != "undefined") {
+    // If _TESTING_LOGGING is set, and if we are on the main thread,
+    // redirect logs to Services.console, for testing purposes
+    logFunc = function logFunc(...args) {
+      let message = ["TEST", "OS"].concat(args).join(" ");
+      Services.console.logStringMessage(message + "\n");
+    };
+  }
+  logFunc.apply(null, args.map(stringifyArg));
+};
+
+exports.LOG = LOG;
+
+/**
+ * Return a shallow clone of the enumerable properties of an object.
+ *
+ * Utility used whenever normalizing options requires making (shallow)
+ * changes to an option object. The copy ensures that we do not modify
+ * a client-provided object by accident.
+ *
+ * Note: to reference and not copy specific fields, provide an optional
+ * |refs| argument containing their names.
+ *
+ * @param {JSON} object Options to be cloned.
+ * @param {Array} refs An optional array of field names to be passed by
+ * reference instead of copying.
+ */
+var clone = function (object, refs = []) {
+  let result = {};
+  // Make a reference between result[key] and object[key].
+  let refer = function refer(result, key, object) {
+    Object.defineProperty(result, key, {
+      enumerable: true,
+      get: function() {
+        return object[key];
+      },
+      set: function(value) {
+        object[key] = value;
+      }
+    });
+  };
+  for (let k in object) {
+    if (refs.indexOf(k) < 0) {
+      result[k] = object[k];
+    } else {
+      refer(result, k, object);
+    }
+  }
+  return result;
+};
+
+exports.clone = clone;
+
+///////////////////// Abstractions above js-ctypes
+
+/**
+ * Abstraction above js-ctypes types.
+ *
+ * Use values of this type to register FFI functions. In addition to the
+ * usual features of js-ctypes, values of this type perform the necessary
+ * transformations to ensure that C errors are handled nicely, to connect
+ * resources with their finalizer, etc.
+ *
+ * @param {string} name The name of the type. Must be unique.
+ * @param {CType} implementation The js-ctypes implementation of the type.
+ *
+ * @constructor
+ */
+function Type(name, implementation) {
+  if (!(typeof name == "string")) {
+    throw new TypeError("Type expects as first argument a name, got: "
+                        + name);
+  }
+  if (!(implementation instanceof ctypes.CType)) {
+    throw new TypeError("Type expects as second argument a ctypes.CType"+
+                        ", got: " + implementation);
+  }
+  Object.defineProperty(this, "name", { value: name });
+  Object.defineProperty(this, "implementation", { value: implementation });
+}
+Type.prototype = {
+  /**
+   * Serialize a value of |this| |Type| into a format that can
+   * be transmitted as a message (not necessarily a string).
+   *
+   * In the default implementation, the method returns the
+   * value unchanged.
+   */
+  toMsg: function default_toMsg(value) {
+    return value;
+  },
+  /**
+   * Deserialize a message to a value of |this| |Type|.
+   *
+   * In the default implementation, the method returns the
+   * message unchanged.
+   */
+  fromMsg: function default_fromMsg(msg) {
+    return msg;
+  },
+  /**
+   * Import a value from C.
+   *
+   * In this default implementation, return the value
+   * unchanged.
+   */
+  importFromC: function default_importFromC(value) {
+    return value;
+  },
+
+  /**
+   * A pointer/array used to pass data to the foreign function.
+   */
+  get in_ptr() {
+    delete this.in_ptr;
+    let ptr_t = new PtrType(
+      "[in] " + this.name + "*",
+      this.implementation.ptr,
+      this);
+    Object.defineProperty(this, "in_ptr",
+    {
+      get: function() {
+        return ptr_t;
+      }
+    });
+    return ptr_t;
+  },
+
+  /**
+   * A pointer/array used to receive data from the foreign function.
+   */
+  get out_ptr() {
+    delete this.out_ptr;
+    let ptr_t = new PtrType(
+      "[out] " + this.name + "*",
+      this.implementation.ptr,
+      this);
+    Object.defineProperty(this, "out_ptr",
+    {
+      get: function() {
+        return ptr_t;
+      }
+    });
+    return ptr_t;
+  },
+
+  /**
+   * A pointer/array used to both pass data to the foreign function
+   * and receive data from the foreign function.
+   *
+   * Whenever possible, prefer using |in_ptr| or |out_ptr|, which
+   * are generally faster.
+   */
+  get inout_ptr() {
+    delete this.inout_ptr;
+    let ptr_t = new PtrType(
+      "[inout] " + this.name + "*",
+      this.implementation.ptr,
+      this);
+    Object.defineProperty(this, "inout_ptr",
+    {
+      get: function() {
+        return ptr_t;
+      }
+    });
+    return ptr_t;
+  },
+
+  /**
+   * Attach a finalizer to a type.
+   */
+  releaseWith: function releaseWith(finalizer) {
+    let parent = this;
+    let type = this.withName("[auto " + this.name + ", " + finalizer + "] ");
+    type.importFromC = function importFromC(value, operation) {
+      return ctypes.CDataFinalizer(
+        parent.importFromC(value, operation),
+        finalizer);
+    };
+    return type;
+  },
+
+  /**
+   * Lazy variant of releaseWith.
+   * Attach a finalizer lazily to a type.
+   *
+   * @param {function} getFinalizer The function that
+   * returns finalizer lazily.
+   */
+  releaseWithLazy: function releaseWithLazy(getFinalizer) {
+    let parent = this;
+    let type = this.withName("[auto " + this.name + ", (lazy)] ");
+    type.importFromC = function importFromC(value, operation) {
+      return ctypes.CDataFinalizer(
+        parent.importFromC(value, operation),
+        getFinalizer());
+    };
+    return type;
+  },
+
+  /**
+   * Return an alias to a type with a different name.
+   */
+  withName: function withName(name) {
+    return Object.create(this, {name: {value: name}});
+  },
+
+  /**
+   * Cast a C value to |this| type.
+   *
+   * Throw an error if the value cannot be casted.
+   */
+  cast: function cast(value) {
+    return ctypes.cast(value, this.implementation);
+  },
+
+  /**
+   * Return the number of bytes in a value of |this| type.
+   *
+   * This may not be defined, e.g. for |void_t|, array types
+   * without length, etc.
+   */
+  get size() {
+    return this.implementation.size;
+  }
+};
+
+/**
+ * Utility function used to determine whether an object is a typed array
+ */
+var isTypedArray = function isTypedArray(obj) {
+  return obj != null && typeof obj == "object"
+    && "byteOffset" in obj;
+};
+exports.isTypedArray = isTypedArray;
+
+/**
+ * Utility function used to determine whether an object is an ArrayBuffer.
+ */
+var isArrayBuffer = function(obj) {
+  return obj != null && typeof obj == "object" &&
+    obj.constructor.name == "ArrayBuffer";
+};
+exports.isArrayBuffer = isArrayBuffer;
+
+/**
+ * A |Type| of pointers.
+ *
+ * @param {string} name The name of this type.
+ * @param {CType} implementation The type of this pointer.
+ * @param {Type} targetType The target type.
+ */
+function PtrType(name, implementation, targetType) {
+  Type.call(this, name, implementation);
+  if (targetType == null || !targetType instanceof Type) {
+    throw new TypeError("targetType must be an instance of Type");
+  }
+  /**
+   * The type of values targeted by this pointer type.
+   */
+  Object.defineProperty(this, "targetType", {
+    value: targetType
+  });
+}
+PtrType.prototype = Object.create(Type.prototype);
+
+/**
+ * Convert a value to a pointer.
+ *
+ * Protocol:
+ * - |null| returns |null|
+ * - a string returns |{string: value}|
+ * - a typed array returns |{ptr: address_of_buffer}|
+ * - a C array returns |{ptr: address_of_buffer}|
+ * everything else raises an error
+ */
+PtrType.prototype.toMsg = function ptr_toMsg(value) {
+  if (value == null) {
+    return null;
+  }
+  if (typeof value == "string") {
+    return { string: value };
+  }
+  if (isTypedArray(value)) {
+    // Automatically transfer typed arrays
+    return new Meta({data: value}, {transfers: [value.buffer]});
+  }
+  if (isArrayBuffer(value)) {
+    // Automatically transfer array buffers
+    return new Meta({data: value}, {transfers: [value]});
+  }
+  let normalized;
+  if ("addressOfElement" in value) { // C array
+    normalized = value.addressOfElement(0);
+  } else if ("isNull" in value) { // C pointer
+    normalized = value;
+  } else {
+    throw new TypeError("Value " + value +
+      " cannot be converted to a pointer");
+  }
+  let cast = Type.uintptr_t.cast(normalized);
+  return {ptr: cast.value.toString()};
+};
+
+/**
+ * Convert a message back to a pointer.
+ */
+PtrType.prototype.fromMsg = function ptr_fromMsg(msg) {
+  if (msg == null) {
+    return null;
+  }
+  if ("string" in msg) {
+    return msg.string;
+  }
+  if ("data" in msg) {
+    return msg.data;
+  }
+  if ("ptr" in msg) {
+    let address = ctypes.uintptr_t(msg.ptr);
+    return this.cast(address);
+  }
+  throw new TypeError("Message " + msg.toSource() +
+    " does not represent a pointer");
+};
+
+exports.Type = Type;
+
+
+/*
+ * Some values are large integers on 64 bit platforms. Unfortunately,
+ * in practice, 64 bit integers cannot be manipulated in JS. We
+ * therefore project them to regular numbers whenever possible.
+ */
+
+var projectLargeInt = function projectLargeInt(x) {
+  let str = x.toString();
+  let rv = parseInt(str, 10);
+  if (rv.toString() !== str) {
+    throw new TypeError("Number " + str + " cannot be projected to a double");
+  }
+  return rv;
+};
+var projectLargeUInt = function projectLargeUInt(x) {
+  return projectLargeInt(x);
+};
+var projectValue = function projectValue(x) {
+  if (!(x instanceof ctypes.CData)) {
+    return x;
+  }
+  if (!("value" in x)) { // Sanity check
+    throw new TypeError("Number " + x.toSource() + " has no field |value|");
+  }
+  return x.value;
+};
+
+function projector(type, signed) {
+  LOG("Determining best projection for", type,
+    "(size: ", type.size, ")", signed?"signed":"unsigned");
+  if (type instanceof Type) {
+    type = type.implementation;
+  }
+  if (!type.size) {
+    throw new TypeError("Argument is not a proper C type");
+  }
+  // Determine if type is projected to Int64/Uint64
+  if (type.size == 8           // Usual case
+      // The following cases have special treatment in js-ctypes
+      // Regardless of their size, the value getter returns
+      // a Int64/Uint64
+      || type == ctypes.size_t // Special cases
+      || type == ctypes.ssize_t
+      || type == ctypes.intptr_t
+      || type == ctypes.uintptr_t
+      || type == ctypes.off_t) {
+    if (signed) {
+      LOG("Projected as a large signed integer");
+      return projectLargeInt;
+    } else {
+      LOG("Projected as a large unsigned integer");
+      return projectLargeUInt;
+    }
+  }
+  LOG("Projected as a regular number");
+  return projectValue;
+};
+exports.projectValue = projectValue;
+
+/**
+ * Get the appropriate type for an unsigned int of the given size.
+ *
+ * This function is useful to define types such as |mode_t| whose
+ * actual width depends on the OS/platform.
+ *
+ * @param {number} size The number of bytes requested.
+ */
+Type.uintn_t = function uintn_t(size) {
+  switch (size) {
+  case 1: return Type.uint8_t;
+  case 2: return Type.uint16_t;
+  case 4: return Type.uint32_t;
+  case 8: return Type.uint64_t;
+  default:
+    throw new Error("Cannot represent unsigned integers of " + size + " bytes");
+  }
+};
+
+/**
+ * Get the appropriate type for an signed int of the given size.
+ *
+ * This function is useful to define types such as |mode_t| whose
+ * actual width depends on the OS/platform.
+ *
+ * @param {number} size The number of bytes requested.
+ */
+Type.intn_t = function intn_t(size) {
+  switch (size) {
+  case 1: return Type.int8_t;
+  case 2: return Type.int16_t;
+  case 4: return Type.int32_t;
+  case 8: return Type.int64_t;
+  default:
+    throw new Error("Cannot represent integers of " + size + " bytes");
+  }
+};
+
+/**
+ * Actual implementation of common C types.
+ */
+
+/**
+ * The void value.
+ */
+Type.void_t =
+  new Type("void",
+           ctypes.void_t);
+
+/**
+ * Shortcut for |void*|.
+ */
+Type.voidptr_t =
+  new PtrType("void*",
+              ctypes.voidptr_t,
+              Type.void_t);
+
+// void* is a special case as we can cast any pointer to/from it
+// so we have to shortcut |in_ptr|/|out_ptr|/|inout_ptr| and
+// ensure that js-ctypes' casting mechanism is invoked directly
+["in_ptr", "out_ptr", "inout_ptr"].forEach(function(key) {
+  Object.defineProperty(Type.void_t, key,
+  {
+    value: Type.voidptr_t
+  });
+});
+
+/**
+ * A Type of integers.
+ *
+ * @param {string} name The name of this type.
+ * @param {CType} implementation The underlying js-ctypes implementation.
+ * @param {bool} signed |true| if this is a type of signed integers,
+ * |false| otherwise.
+ *
+ * @constructor
+ */
+function IntType(name, implementation, signed) {
+  Type.call(this, name, implementation);
+  this.importFromC = projector(implementation, signed);
+  this.project = this.importFromC;
+};
+IntType.prototype = Object.create(Type.prototype);
+IntType.prototype.toMsg = function toMsg(value) {
+  if (typeof value == "number") {
+    return value;
+  }
+  return this.project(value);
+};
+
+/**
+ * A C char (one byte)
+ */
+Type.char =
+  new Type("char",
+           ctypes.char);
+
+/**
+ * A C wide char (two bytes)
+ */
+Type.char16_t =
+  new Type("char16_t",
+           ctypes.char16_t);
+
+ /**
+  * Base string types.
+  */
+Type.cstring = Type.char.in_ptr.withName("[in] C string");
+Type.wstring = Type.char16_t.in_ptr.withName("[in] wide string");
+Type.out_cstring = Type.char.out_ptr.withName("[out] C string");
+Type.out_wstring = Type.char16_t.out_ptr.withName("[out] wide string");
+
+/**
+ * A C integer (8-bits).
+ */
+Type.int8_t =
+  new IntType("int8_t", ctypes.int8_t, true);
+
+Type.uint8_t =
+  new IntType("uint8_t", ctypes.uint8_t, false);
+
+/**
+ * A C integer (16-bits).
+ *
+ * Also known as WORD under Windows.
+ */
+Type.int16_t =
+  new IntType("int16_t", ctypes.int16_t, true);
+
+Type.uint16_t =
+  new IntType("uint16_t", ctypes.uint16_t, false);
+
+/**
+ * A C integer (32-bits).
+ *
+ * Also known as DWORD under Windows.
+ */
+Type.int32_t =
+  new IntType("int32_t", ctypes.int32_t, true);
+
+Type.uint32_t =
+  new IntType("uint32_t", ctypes.uint32_t, false);
+
+/**
+ * A C integer (64-bits).
+ */
+Type.int64_t =
+  new IntType("int64_t", ctypes.int64_t, true);
+
+Type.uint64_t =
+  new IntType("uint64_t", ctypes.uint64_t, false);
+
+ /**
+ * A C integer
+ *
+ * Size depends on the platform.
+ */
+Type.int = Type.intn_t(ctypes.int.size).
+  withName("int");
+
+Type.unsigned_int = Type.intn_t(ctypes.unsigned_int.size).
+  withName("unsigned int");
+
+/**
+ * A C long integer.
+ *
+ * Size depends on the platform.
+ */
+Type.long =
+  Type.intn_t(ctypes.long.size).withName("long");
+
+Type.unsigned_long =
+  Type.intn_t(ctypes.unsigned_long.size).withName("unsigned long");
+
+/**
+ * An unsigned integer with the same size as a pointer.
+ *
+ * Used to cast a pointer to an integer, whenever necessary.
+ */
+Type.uintptr_t =
+  Type.uintn_t(ctypes.uintptr_t.size).withName("uintptr_t");
+
+/**
+ * A boolean.
+ * Implemented as a C integer.
+ */
+Type.bool = Type.int.withName("bool");
+Type.bool.importFromC = function projectBool(x) {
+  return !!(x.value);
+};
+
+/**
+ * A user identifier.
+ *
+ * Implemented as a C integer.
+ */
+Type.uid_t =
+  Type.int.withName("uid_t");
+
+/**
+ * A group identifier.
+ *
+ * Implemented as a C integer.
+ */
+Type.gid_t =
+  Type.int.withName("gid_t");
+
+/**
+ * An offset (positive or negative).
+ *
+ * Implemented as a C integer.
+ */
+Type.off_t =
+  new IntType("off_t", ctypes.off_t, true);
+
+/**
+ * A size (positive).
+ *
+ * Implemented as a C size_t.
+ */
+Type.size_t =
+  new IntType("size_t", ctypes.size_t, false);
+
+/**
+ * An offset (positive or negative).
+ * Implemented as a C integer.
+ */
+Type.ssize_t =
+  new IntType("ssize_t", ctypes.ssize_t, true);
+
+/**
+ * Encoding/decoding strings
+ */
+Type.uencoder =
+  new Type("uencoder", ctypes.StructType("uencoder"));
+Type.udecoder =
+  new Type("udecoder", ctypes.StructType("udecoder"));
+
+/**
+ * Utility class, used to build a |struct| type
+ * from a set of field names, types and offsets.
+ *
+ * @param {string} name The name of the |struct| type.
+ * @param {number} size The total size of the |struct| type in bytes.
+ */
+function HollowStructure(name, size) {
+  if (!name) {
+    throw new TypeError("HollowStructure expects a name");
+  }
+  if (!size || size < 0) {
+    throw new TypeError("HollowStructure expects a (positive) size");
+  }
+
+  // A mapping from offsets in the struct to name/type pairs
+  // (or nothing if no field starts at that offset).
+  this.offset_to_field_info = [];
+
+  // The name of the struct
+  this.name = name;
+
+  // The size of the struct, in bytes
+  this.size = size;
+
+  // The number of paddings inserted so far.
+  // Used to give distinct names to padding fields.
+  this._paddings = 0;
+}
+HollowStructure.prototype = {
+  /**
+   * Add a field at a given offset.
+   *
+   * @param {number} offset The offset at which to insert the field.
+   * @param {string} name The name of the field.
+   * @param {CType|Type} type The type of the field.
+   */
+  add_field_at: function add_field_at(offset, name, type) {
+    if (offset == null) {
+      throw new TypeError("add_field_at requires a non-null offset");
+    }
+    if (!name) {
+      throw new TypeError("add_field_at requires a non-null name");
+    }
+    if (!type) {
+      throw new TypeError("add_field_at requires a non-null type");
+    }
+    if (type instanceof Type) {
+      type = type.implementation;
+    }
+    if (this.offset_to_field_info[offset]) {
+      throw new Error("HollowStructure " + this.name +
+                      " already has a field at offset " + offset);
+    }
+    if (offset + type.size > this.size) {
+      throw new Error("HollowStructure " + this.name +
+                      " cannot place a value of type " + type +
+                      " at offset " + offset +
+                      " without exceeding its size of " + this.size);
+    }
+    let field = {name: name, type:type};
+    this.offset_to_field_info[offset] = field;
+  },
+
+  /**
+   * Create a pseudo-field that will only serve as padding.
+   *
+   * @param {number} size The number of bytes in the field.
+   * @return {Object} An association field-name => field-type,
+   * as expected by |ctypes.StructType|.
+   */
+  _makePaddingField: function makePaddingField(size) {
+    let field = ({});
+    field["padding_" + this._paddings] =
+      ctypes.ArrayType(ctypes.uint8_t, size);
+    this._paddings++;
+    return field;
+  },
+
+  /**
+   * Convert this |HollowStructure| into a |Type|.
+   */
+  getType: function getType() {
+    // Contents of the structure, in the format expected
+    // by ctypes.StructType.
+    let struct = [];
+
+    let i = 0;
+    while (i < this.size) {
+      let currentField = this.offset_to_field_info[i];
+      if (!currentField) {
+        // No field was specified at this offset, we need to
+        // introduce some padding.
+
+        // Firstly, determine how many bytes of padding
+        let padding_length = 1;
+        while (i + padding_length < this.size
+            && !this.offset_to_field_info[i + padding_length]) {
+          ++padding_length;
+        }
+
+        // Then add the padding
+        struct.push(this._makePaddingField(padding_length));
+
+        // And proceed
+        i += padding_length;
+      } else {
+        // We have a field at this offset.
+
+        // Firstly, ensure that we do not have two overlapping fields
+        for (let j = 1; j < currentField.type.size; ++j) {
+          let candidateField = this.offset_to_field_info[i + j];
+          if (candidateField) {
+            throw new Error("Fields " + currentField.name +
+              " and " + candidateField.name +
+              " overlap at position " + (i + j));
+          }
+        }
+
+        // Then add the field
+        let field = ({});
+        field[currentField.name] = currentField.type;
+        struct.push(field);
+
+        // And proceed
+        i += currentField.type.size;
+      }
+    }
+    let result = new Type(this.name, ctypes.StructType(this.name, struct));
+    if (result.implementation.size != this.size) {
+      throw new Error("Wrong size for type " + this.name +
+          ": expected " + this.size +
+          ", found " + result.implementation.size +
+          " (" + result.implementation.toSource() + ")");
+    }
+    return result;
+  }
+};
+exports.HollowStructure = HollowStructure;
+
+/**
+ * Representation of a native library.
+ *
+ * The native library is opened lazily, during the first call to its
+ * field |library| or whenever accessing one of the methods imported
+ * with declareLazyFFI.
+ *
+ * @param {string} name A human-readable name for the library. Used
+ * for debugging and error reporting.
+ * @param {string...} candidates A list of system libraries that may
+ * represent this library. Used e.g. to try different library names
+ * on distinct operating systems ("libxul", "XUL", etc.).
+ *
+ * @constructor
+ */
+function Library(name, ...candidates) {
+  this.name = name;
+  this._candidates = candidates;
+};
+Library.prototype = Object.freeze({
+  /**
+   * The native library as a js-ctypes object.
+   *
+   * @throws {Error} If none of the candidate libraries could be opened.
+   */
+  get library() {
+    let library;
+    delete this.library;
+    for (let candidate of this._candidates) {
+      try {
+        library = ctypes.open(candidate);
+        break;
+      } catch (ex) {
+        LOG("Could not open library", candidate, ex);
+      }
+    }
+    this._candidates = null;
+    if (library) {
+      Object.defineProperty(this, "library", {
+        value: library
+      });
+      Object.freeze(this);
+      return library;
+    }
+    let error = new Error("Could not open library " + this.name);
+    Object.defineProperty(this, "library", {
+      get: function() {
+        throw error;
+      }
+    });
+    Object.freeze(this);
+    throw error;
+  },
+
+  /**
+   * Declare a function, lazily.
+   *
+   * @param {object} The object containing the function as a field.
+   * @param {string} The name of the field containing the function.
+   * @param {string} symbol The name of the function, as defined in the
+   * library.
+   * @param {ctypes.abi} abi The abi to use, or |null| for default.
+   * @param {Type} returnType The type of values returned by the function.
+   * @param {...Type} argTypes The type of arguments to the function.
+   */
+  declareLazyFFI: function(object, field, ...args) {
+    let lib = this;
+    Object.defineProperty(object, field, {
+      get: function() {
+        delete this[field];
+        let ffi = declareFFI(lib.library, ...args);
+        if (ffi) {
+          return this[field] = ffi;
+        }
+        return undefined;
+      },
+      configurable: true,
+      enumerable: true
+    });
+  },
+
+  /**
+   * Define a js-ctypes function lazily using ctypes method declare.
+   *
+   * @param {object} The object containing the function as a field.
+   * @param {string} The name of the field containing the function.
+   * @param {string} symbol The name of the function, as defined in the
+   * library.
+   * @param {ctypes.abi} abi The abi to use, or |null| for default.
+   * @param {ctypes.CType} returnType The type of values returned by the function.
+   * @param {...ctypes.CType} argTypes The type of arguments to the function.
+   */
+  declareLazy: function(object, field, ...args) {
+    let lib = this;
+    Object.defineProperty(object, field, {
+      get: function() {
+        delete this[field];
+        let ffi = lib.library.declare(...args);
+        if (ffi) {
+          return this[field] = ffi;
+        }
+        return undefined;
+      },
+      configurable: true,
+      enumerable: true
+    });
+  },
+
+  /**
+   * Define a js-ctypes function lazily using ctypes method declare,
+   * with a fallback library to use if this library can't be opened
+   * or the function cannot be declared.
+   *
+   * @param {fallbacklibrary} The fallback Library object.
+   * @param {object} The object containing the function as a field.
+   * @param {string} The name of the field containing the function.
+   * @param {string} symbol The name of the function, as defined in the
+   * library.
+   * @param {ctypes.abi} abi The abi to use, or |null| for default.
+   * @param {ctypes.CType} returnType The type of values returned by the function.
+   * @param {...ctypes.CType} argTypes The type of arguments to the function.
+   */
+  declareLazyWithFallback: function(fallbacklibrary, object, field, ...args) {
+    let lib = this;
+    Object.defineProperty(object, field, {
+      get: function() {
+        delete this[field];
+        try {
+          let ffi = lib.library.declare(...args);
+          if (ffi) {
+            return this[field] = ffi;
+          }
+        } catch (ex) {
+          // Use the fallback library and get the symbol from there.
+          fallbacklibrary.declareLazy(object, field, ...args);
+          return object[field];
+        }
+        return undefined;
+      },
+      configurable: true,
+      enumerable: true
+    });
+  },
+
+  toString: function() {
+    return "[Library " + this.name + "]";
+  }
+});
+exports.Library = Library;
+
+/**
+ * Declare a function through js-ctypes
+ *
+ * @param {ctypes.library} lib The ctypes library holding the function.
+ * @param {string} symbol The name of the function, as defined in the
+ * library.
+ * @param {ctypes.abi} abi The abi to use, or |null| for default.
+ * @param {Type} returnType The type of values returned by the function.
+ * @param {...Type} argTypes The type of arguments to the function.
+ *
+ * @return null if the function could not be defined (generally because
+ * it does not exist), or a JavaScript wrapper performing the call to C
+ * and any type conversion required.
+ */
+var declareFFI = function declareFFI(lib, symbol, abi,
+                                     returnType /*, argTypes ...*/) {
+  LOG("Attempting to declare FFI ", symbol);
+  // We guard agressively, to avoid any late surprise
+  if (typeof symbol != "string") {
+    throw new TypeError("declareFFI expects as first argument a string");
+  }
+  abi = abi || ctypes.default_abi;
+  if (Object.prototype.toString.call(abi) != "[object CABI]") {
+    // Note: This is the only known manner of checking whether an object
+    // is an abi.
+    throw new TypeError("declareFFI expects as second argument an abi or null");
+  }
+  if (!returnType.importFromC) {
+    throw new TypeError("declareFFI expects as third argument an instance of Type");
+  }
+  let signature = [symbol, abi];
+  let argtypes  = [];
+  for (let i = 3; i < arguments.length; ++i) {
+    let current = arguments[i];
+    if (!current) {
+      throw new TypeError("Missing type for argument " + ( i - 3 ) +
+                          " of symbol " + symbol);
+    }
+    if (!current.implementation) {
+      throw new TypeError("Missing implementation for argument " + (i - 3)
+                          + " of symbol " + symbol
+                          + " ( " + current.name + " )" );
+    }
+    signature.push(current.implementation);
+  }
+  try {
+    let fun = lib.declare.apply(lib, signature);
+    let result = function ffi(...args) {
+      for (let i = 0; i < args.length; i++) {
+        if (typeof args[i] == "undefined") {
+          throw new TypeError("Argument " + i + " of " + symbol + " is undefined");
+        }
+      }
+      let result = fun.apply(fun, args);
+      return returnType.importFromC(result, symbol);
+    };
+    LOG("Function", symbol, "declared");
+    return result;
+  } catch (x) {
+    // Note: Not being able to declare a function is normal.
+    // Some functions are OS (or OS version)-specific.
+    LOG("Could not declare function ", symbol, x);
+    return null;
+  }
+};
+exports.declareFFI = declareFFI;
+
+/**
+ * Define a lazy getter to a js-ctypes function using declareFFI.
+ *
+ * @param {object} The object containing the function as a field.
+ * @param {string} The name of the field containing the function.
+ * @param {ctypes.library} lib The ctypes library holding the function.
+ * @param {string} symbol The name of the function, as defined in the
+ * library.
+ * @param {ctypes.abi} abi The abi to use, or |null| for default.
+ * @param {Type} returnType The type of values returned by the function.
+ * @param {...Type} argTypes The type of arguments to the function.
+ */
+function declareLazyFFI(object, field, ...declareFFIArgs) {
+  Object.defineProperty(object, field, {
+    get: function() {
+      delete this[field];
+      let ffi = declareFFI(...declareFFIArgs);
+      if (ffi) {
+        return this[field] = ffi;
+      }
+      return undefined;
+    },
+    configurable: true,
+    enumerable: true
+  });
+}
+exports.declareLazyFFI = declareLazyFFI;
+
+/**
+ * Define a lazy getter to a js-ctypes function using ctypes method declare.
+ *
+ * @param {object} The object containing the function as a field.
+ * @param {string} The name of the field containing the function.
+ * @param {ctypes.library} lib The ctypes library holding the function.
+ * @param {string} symbol The name of the function, as defined in the
+ * library.
+ * @param {ctypes.abi} abi The abi to use, or |null| for default.
+ * @param {ctypes.CType} returnType The type of values returned by the function.
+ * @param {...ctypes.CType} argTypes The type of arguments to the function.
+ */
+function declareLazy(object, field, lib, ...declareArgs) {
+  Object.defineProperty(object, field, {
+    get: function() {
+      delete this[field];
+      try {
+        let ffi = lib.declare(...declareArgs);
+        return this[field] = ffi;
+      } catch (ex) {
+        // The symbol doesn't exist
+        return undefined;
+      }
+    },
+    configurable: true
+  });
+}
+exports.declareLazy = declareLazy;
+
+/**
+ * Utility function used to sanity check buffer and length arguments.  The
+ * buffer must be a Typed Array.
+ *
+ * @param {Typed array} candidate The buffer.
+ * @param {number} bytes The number of bytes that |candidate| should contain.
+ *
+ * @return number The bytes argument clamped to the length of the buffer.
+ */
+function normalizeBufferArgs(candidate, bytes) {
+  if (!candidate) {
+    throw new TypeError("Expecting a Typed Array");
+  }
+  if (!isTypedArray(candidate)) {
+    throw new TypeError("Expecting a Typed Array");
+  }
+  if (bytes == null) {
+    bytes = candidate.byteLength;
+  } else if (candidate.byteLength < bytes) {
+    throw new TypeError("Buffer is too short. I need at least " +
+                       bytes +
+                       " bytes but I have only " +
+                       candidate.byteLength +
+                        "bytes");
+  }
+  return bytes;
+};
+exports.normalizeBufferArgs = normalizeBufferArgs;
+
+///////////////////// OS interactions
+
+/**
+ * An OS error.
+ *
+ * This class is provided mostly for type-matching. If you need more
+ * details about an error, you should use the platform-specific error
+ * codes provided by subclasses of |OS.Shared.Error|.
+ *
+ * @param {string} operation The operation that failed.
+ * @param {string=} path The path of the file on which the operation failed,
+ * or nothing if there was no file involved in the failure.
+ *
+ * @constructor
+ */
+function OSError(operation, path = "") {
+  Error.call(this);
+  this.operation = operation;
+  this.path = path;
+}
+OSError.prototype = Object.create(Error.prototype);
+exports.OSError = OSError;
+
+
+///////////////////// Temporary boilerplate
+// Boilerplate, to simplify the transition to require()
+// Do not rely upon this symbol, it will disappear with
+// bug 883050.
+exports.OS = {
+  Constants: exports.Constants,
+  Shared: {
+    LOG: LOG,
+    clone: clone,
+    Type: Type,
+    HollowStructure: HollowStructure,
+    Error: OSError,
+    declareFFI: declareFFI,
+    projectValue: projectValue,
+    isTypedArray: isTypedArray,
+    defineLazyGetter: defineLazyGetter
+  }
+};
+
+Object.defineProperty(exports.OS.Shared, "DEBUG", {
+  get: function() {
+    return Config.DEBUG;
+  },
+  set: function(x) {
+    return Config.DEBUG = x;
+  }
+});
+Object.defineProperty(exports.OS.Shared, "TEST", {
+  get: function() {
+    return Config.TEST;
+  },
+  set: function(x) {
+    return Config.TEST = x;
+  }
+});
+
+
+///////////////////// Permanent boilerplate
+if (typeof Components != "undefined") {
+  this.EXPORTED_SYMBOLS = EXPORTED_SYMBOLS;
+  for (let symbol of EXPORTED_SYMBOLS) {
+    this[symbol] = exports[symbol];
+  }
+}
diff --git a/toolkit/components/osfile/modules/osfile_shared_front.jsm b/toolkit/components/osfile/modules/osfile_shared_front.jsm
new file mode 100644
index 000000000..a2971991d
--- /dev/null
+++ b/toolkit/components/osfile/modules/osfile_shared_front.jsm
@@ -0,0 +1,567 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * Code shared by OS.File front-ends.
+ *
+ * This code is meant to be included by another library. It is also meant to
+ * be executed only on a worker thread.
+ */
+
+if (typeof Components != "undefined") {
+  throw new Error("osfile_shared_front.jsm cannot be used from the main thread");
+}
+(function(exports) {
+
+var SharedAll =
+  require("resource://gre/modules/osfile/osfile_shared_allthreads.jsm");
+var Path = require("resource://gre/modules/osfile/ospath.jsm");
+var Lz4 =
+  require("resource://gre/modules/lz4.js");
+var LOG = SharedAll.LOG.bind(SharedAll, "Shared front-end");
+var clone = SharedAll.clone;
+
+/**
+ * Code shared by implementations of File.
+ *
+ * @param {*} fd An OS-specific file handle.
+ * @param {string} path File path of the file handle, used for error-reporting.
+ * @constructor
+ */
+var AbstractFile = function AbstractFile(fd, path) {
+  this._fd = fd;
+  if (!path) {
+    throw new TypeError("path is expected");
+  }
+  this._path = path;
+};
+
+AbstractFile.prototype = {
+  /**
+   * Return the file handle.
+   *
+   * @throw OS.File.Error if the file has been closed.
+   */
+  get fd() {
+    if (this._fd) {
+      return this._fd;
+    }
+    throw OS.File.Error.closed("accessing file", this._path);
+  },
+  /**
+   * Read bytes from this file to a new buffer.
+   *
+   * @param {number=} maybeBytes (deprecated, please use options.bytes)
+   * @param {JSON} options
+   * @return {Uint8Array} An array containing the bytes read.
+   */
+  read: function read(maybeBytes, options = {}) {
+    if (typeof maybeBytes === "object") {
+    // Caller has skipped `maybeBytes` and provided an options object.
+      options = clone(maybeBytes);
+      maybeBytes = null;
+    } else {
+      options = options || {};
+    }
+    let bytes = options.bytes || undefined;
+    if (bytes === undefined) {
+      bytes = maybeBytes == null ? this.stat().size : maybeBytes;
+    }
+    let buffer = new Uint8Array(bytes);
+    let pos = 0;
+    while (pos < bytes) {
+      let length = bytes - pos;
+      let view = new DataView(buffer.buffer, pos, length);
+      let chunkSize = this._read(view, length, options);
+      if (chunkSize == 0) {
+        break;
+      }
+      pos += chunkSize;
+    }
+    if (pos == bytes) {
+      return buffer;
+    } else {
+      return buffer.subarray(0, pos);
+    }
+  },
+
+  /**
+   * Write bytes from a buffer to this file.
+   *
+   * Note that, by default, this function may perform several I/O
+   * operations to ensure that the buffer is fully written.
+   *
+   * @param {Typed array} buffer The buffer in which the the bytes are
+   * stored. The buffer must be large enough to accomodate |bytes| bytes.
+   * @param {*=} options Optionally, an object that may contain the
+   * following fields:
+   * - {number} bytes The number of |bytes| to write from the buffer. If
+   * unspecified, this is |buffer.byteLength|.
+   *
+   * @return {number} The number of bytes actually written.
+   */
+  write: function write(buffer, options = {}) {
+    let bytes =
+      SharedAll.normalizeBufferArgs(buffer, ("bytes" in options) ? options.bytes : undefined);
+    let pos = 0;
+    while (pos < bytes) {
+      let length = bytes - pos;
+      let view = new DataView(buffer.buffer, buffer.byteOffset + pos, length);
+      let chunkSize = this._write(view, length, options);
+      pos += chunkSize;
+    }
+    return pos;
+  }
+};
+
+/**
+ * Creates and opens a file with a unique name. By default, generate a random HEX number and use it to create a unique new file name.
+ *
+ * @param {string} path The path to the file.
+ * @param {*=} options Additional options for file opening. This
+ * implementation interprets the following fields:
+ *
+ * - {number} humanReadable If |true|, create a new filename appending a decimal number. ie: filename-1.ext, filename-2.ext.
+ *  If |false| use HEX numbers ie: filename-A65BC0.ext
+ * - {number} maxReadableNumber Used to limit the amount of tries after a failed
+ *  file creation. Default is 20.
+ *
+ * @return {Object} contains A file object{file} and the path{path}.
+ * @throws {OS.File.Error} If the file could not be opened.
+ */
+AbstractFile.openUnique = function openUnique(path, options = {}) {
+  let mode = {
+    create : true
+  };
+
+  let dirName = Path.dirname(path);
+  let leafName = Path.basename(path);
+  let lastDotCharacter = leafName.lastIndexOf('.');
+  let fileName = leafName.substring(0, lastDotCharacter != -1 ? lastDotCharacter : leafName.length);
+  let suffix = (lastDotCharacter != -1 ? leafName.substring(lastDotCharacter) : "");
+  let uniquePath = "";
+  let maxAttempts = options.maxAttempts || 99;
+  let humanReadable = !!options.humanReadable;
+  const HEX_RADIX = 16;
+  // We produce HEX numbers between 0 and 2^24 - 1.
+  const MAX_HEX_NUMBER = 16777215;
+
+  try {
+    return {
+      path: path,
+      file: OS.File.open(path, mode)
+    };
+  } catch (ex if ex instanceof OS.File.Error && ex.becauseExists) {
+    for (let i = 0; i < maxAttempts; ++i) {
+      try {
+        if (humanReadable) {
+          uniquePath = Path.join(dirName, fileName + "-" + (i + 1) + suffix);
+        } else {
+          let hexNumber = Math.floor(Math.random() * MAX_HEX_NUMBER).toString(HEX_RADIX);
+          uniquePath = Path.join(dirName, fileName + "-" + hexNumber + suffix);
+        }
+        return {
+          path: uniquePath,
+          file: OS.File.open(uniquePath, mode)
+        };
+      } catch (ex if ex instanceof OS.File.Error && ex.becauseExists) {
+        // keep trying ...
+      }
+    }
+    throw OS.File.Error.exists("could not find an unused file name.", path);
+  }
+};
+
+/**
+ * Code shared by iterators.
+ */
+AbstractFile.AbstractIterator = function AbstractIterator() {
+};
+AbstractFile.AbstractIterator.prototype = {
+  /**
+   * Allow iterating with |for|
+   */
+  __iterator__: function __iterator__() {
+    return this;
+  },
+  /**
+   * Apply a function to all elements of the directory sequentially.
+   *
+   * @param {Function} cb This function will be applied to all entries
+   * of the directory. It receives as arguments
+   *  - the OS.File.Entry corresponding to the entry;
+   *  - the index of the entry in the enumeration;
+   *  - the iterator itself - calling |close| on the iterator stops
+   *   the loop.
+   */
+  forEach: function forEach(cb) {
+    let index = 0;
+    for (let entry in this) {
+      cb(entry, index++, this);
+    }
+  },
+  /**
+   * Return several entries at once.
+   *
+   * Entries are returned in the same order as a walk with |forEach| or
+   * |for(...)|.
+   *
+   * @param {number=} length If specified, the number of entries
+   * to return. If unspecified, return all remaining entries.
+   * @return {Array} An array containing the next |length| entries, or
+   * less if the iteration contains less than |length| entries left.
+   */
+  nextBatch: function nextBatch(length) {
+    let array = [];
+    let i = 0;
+    for (let entry in this) {
+      array.push(entry);
+      if (++i >= length) {
+        return array;
+      }
+    }
+    return array;
+  }
+};
+
+/**
+ * Utility function shared by implementations of |OS.File.open|:
+ * extract read/write/trunc/create/existing flags from a |mode|
+ * object.
+ *
+ * @param {*=} mode An object that may contain fields |read|,
+ * |write|, |truncate|, |create|, |existing|. These fields
+ * are interpreted only if true-ish.
+ * @return {{read:bool, write:bool, trunc:bool, create:bool,
+ * existing:bool}} an object recapitulating the options set
+ * by |mode|.
+ * @throws {TypeError} If |mode| contains other fields, or
+ * if it contains both |create| and |truncate|, or |create|
+ * and |existing|.
+ */
+AbstractFile.normalizeOpenMode = function normalizeOpenMode(mode) {
+  let result = {
+    read: false,
+    write: false,
+    trunc: false,
+    create: false,
+    existing: false,
+    append: true
+  };
+  for (let key in mode) {
+    let val = !!mode[key]; // bool cast.
+    switch (key) {
+    case "read":
+      result.read = val;
+      break;
+    case "write":
+      result.write = val;
+      break;
+    case "truncate": // fallthrough
+    case "trunc":
+      result.trunc = val;
+      result.write |= val;
+      break;
+    case "create":
+      result.create = val;
+      result.write |= val;
+      break;
+    case "existing": // fallthrough
+    case "exist":
+      result.existing = val;
+      break;
+    case "append":
+      result.append = val;
+      break;
+    default:
+      throw new TypeError("Mode " + key + " not understood");
+    }
+  }
+  // Reject opposite modes
+  if (result.existing && result.create) {
+    throw new TypeError("Cannot specify both existing:true and create:true");
+  }
+  if (result.trunc && result.create) {
+    throw new TypeError("Cannot specify both trunc:true and create:true");
+  }
+  // Handle read/write
+  if (!result.write) {
+    result.read = true;
+  }
+  return result;
+};
+
+/**
+ * Return the contents of a file.
+ *
+ * @param {string} path The path to the file.
+ * @param {number=} bytes Optionally, an upper bound to the number of bytes
+ * to read. DEPRECATED - please use options.bytes instead.
+ * @param {object=} options Optionally, an object with some of the following
+ * fields:
+ * - {number} bytes An upper bound to the number of bytes to read.
+ * - {string} compression If "lz4" and if the file is compressed using the lz4
+ *   compression algorithm, decompress the file contents on the fly.
+ *
+ * @return {Uint8Array} A buffer holding the bytes
+ * and the number of bytes read from the file.
+ */
+AbstractFile.read = function read(path, bytes, options = {}) {
+  if (bytes && typeof bytes == "object") {
+    options = bytes;
+    bytes = options.bytes || null;
+  }
+  if ("encoding" in options && typeof options.encoding != "string") {
+    throw new TypeError("Invalid type for option encoding");
+  }
+  if ("compression" in options && typeof options.compression != "string") {
+    throw new TypeError("Invalid type for option compression: " + options.compression);
+  }
+  if ("bytes" in options && typeof options.bytes != "number") {
+    throw new TypeError("Invalid type for option bytes");
+  }
+  let file = exports.OS.File.open(path);
+  try {
+    let buffer = file.read(bytes, options);
+    if ("compression" in options) {
+      if (options.compression == "lz4") {
+        options = Object.create(options);
+        options.path = path;
+        buffer = Lz4.decompressFileContent(buffer, options);
+      } else {
+        throw OS.File.Error.invalidArgument("Compression");
+      }
+    }
+    if (!("encoding" in options)) {
+      return buffer;
+    }
+    let decoder;
+    try {
+      decoder = new TextDecoder(options.encoding);
+    } catch (ex if ex instanceof RangeError) {
+      throw OS.File.Error.invalidArgument("Decode");
+    }
+    return decoder.decode(buffer);
+  } finally {
+    file.close();
+  }
+};
+
+/**
+ * Write a file, atomically.
+ *
+ * By opposition to a regular |write|, this operation ensures that,
+ * until the contents are fully written, the destination file is
+ * not modified.
+ *
+ * Limitation: In a few extreme cases (hardware failure during the
+ * write, user unplugging disk during the write, etc.), data may be
+ * corrupted. If your data is user-critical (e.g. preferences,
+ * application data, etc.), you may wish to consider adding options
+ * |tmpPath| and/or |flush| to reduce the likelihood of corruption, as
+ * detailed below. Note that no combination of options can be
+ * guaranteed to totally eliminate the risk of corruption.
+ *
+ * @param {string} path The path of the file to modify.
+ * @param {Typed Array | C pointer} buffer A buffer containing the bytes to write.
+ * @param {*=} options Optionally, an object determining the behavior
+ * of this function. This object may contain the following fields:
+ * - {number} bytes The number of bytes to write. If unspecified,
+ * |buffer.byteLength|. Required if |buffer| is a C pointer.
+ * - {string} tmpPath If |null| or unspecified, write all data directly
+ * to |path|. If specified, write all data to a temporary file called
+ * |tmpPath| and, once this write is complete, rename the file to
+ * replace |path|. Performing this additional operation is a little
+ * slower but also a little safer.
+ * - {bool} noOverwrite - If set, this function will fail if a file already
+ * exists at |path|.
+ * - {bool} flush - If |false| or unspecified, return immediately once the
+ * write is complete. If |true|, before writing, force the operating system
+ * to write its internal disk buffers to the disk. This is considerably slower
+ * (not just for the application but for the whole system) but also safer:
+ * if the system shuts down improperly (typically due to a kernel freeze
+ * or a power failure) or if the device is disconnected before the buffer
+ * is flushed, the file has more chances of not being corrupted.
+ * - {string} compression - If empty or unspecified, do not compress the file.
+ * If "lz4", compress the contents of the file atomically using lz4. For the
+ * time being, the container format is specific to Mozilla and cannot be read
+ * by means other than OS.File.read(..., { compression: "lz4"})
+ * - {string} backupTo - If specified, backup the destination file as |backupTo|.
+ * Note that this function renames the destination file before overwriting it.
+ * If the process or the operating system freezes or crashes
+ * during the short window between these operations,
+ * the destination file will have been moved to its backup.
+ *
+ * @return {number} The number of bytes actually written.
+ */
+AbstractFile.writeAtomic =
+     function writeAtomic(path, buffer, options = {}) {
+
+  // Verify that path is defined and of the correct type
+  if (typeof path != "string" || path == "") {
+    throw new TypeError("File path should be a (non-empty) string");
+  }
+  let noOverwrite = options.noOverwrite;
+  if (noOverwrite && OS.File.exists(path)) {
+    throw OS.File.Error.exists("writeAtomic", path);
+  }
+
+  if (typeof buffer == "string") {
+    // Normalize buffer to a C buffer by encoding it
+    let encoding = options.encoding || "utf-8";
+    buffer = new TextEncoder(encoding).encode(buffer);
+  }
+
+  if ("compression" in options && options.compression == "lz4") {
+    buffer = Lz4.compressFileContent(buffer, options);
+    options = Object.create(options);
+    options.bytes = buffer.byteLength;
+  }
+
+  let bytesWritten = 0;
+
+  if (!options.tmpPath) {
+    if (options.backupTo) {
+      try {
+        OS.File.move(path, options.backupTo, {noCopy: true});
+      } catch (ex if ex.becauseNoSuchFile) {
+        // The file doesn't exist, nothing to backup.
+      }
+    }
+    // Just write, without any renaming trick
+    let dest = OS.File.open(path, {write: true, truncate: true});
+    try {
+      bytesWritten = dest.write(buffer, options);
+      if (options.flush) {
+        dest.flush();
+      }
+    } finally {
+      dest.close();
+    }
+    return bytesWritten;
+  }
+
+  let tmpFile = OS.File.open(options.tmpPath, {write: true, truncate: true});
+  try {
+    bytesWritten = tmpFile.write(buffer, options);
+    if (options.flush) {
+      tmpFile.flush();
+    }
+  } catch (x) {
+    OS.File.remove(options.tmpPath);
+    throw x;
+  } finally {
+    tmpFile.close();
+  }
+
+  if (options.backupTo) {
+    try {
+      OS.File.move(path, options.backupTo, {noCopy: true});
+    } catch (ex if ex.becauseNoSuchFile) {
+      // The file doesn't exist, nothing to backup.
+    }
+  }
+
+  OS.File.move(options.tmpPath, path, {noCopy: true});
+  return bytesWritten;
+};
+
+/**
+ * This function is used by removeDir to avoid calling lstat for each
+ * files under the specified directory. External callers should not call
+ * this function directly.
+ */
+AbstractFile.removeRecursive = function(path, options = {}) {
+  let iterator = new OS.File.DirectoryIterator(path);
+  if (!iterator.exists()) {
+    if (!("ignoreAbsent" in options) || options.ignoreAbsent) {
+      return;
+    }
+  }
+
+  try {
+    for (let entry in iterator) {
+      if (entry.isDir) {
+        if (entry.isLink) {
+          // Unlike Unix symlinks, NTFS junctions or NTFS symlinks to
+          // directories are directories themselves. OS.File.remove()
+          // will not work for them.
+          OS.File.removeEmptyDir(entry.path, options);
+        } else {
+          // Normal directories.
+          AbstractFile.removeRecursive(entry.path, options);
+        }
+      } else {
+        // NTFS symlinks to files, Unix symlinks, or regular files.
+        OS.File.remove(entry.path, options);
+      }
+    }
+  } finally {
+    iterator.close();
+  }
+
+  OS.File.removeEmptyDir(path);
+};
+
+/**
+ * Create a directory and, optionally, its parent directories.
+ *
+ * @param {string} path The name of the directory.
+ * @param {*=} options Additional options.
+ *
+ * - {string} from If specified, the call to |makeDir| creates all the
+ * ancestors of |path| that are descendants of |from|. Note that |path|
+ * must be a descendant of |from|, and that |from| and its existing
+ * subdirectories present in |path|  must be user-writeable.
+ * Example:
+ *   makeDir(Path.join(profileDir, "foo", "bar"), { from: profileDir });
+ *  creates directories profileDir/foo, profileDir/foo/bar
+ * - {bool} ignoreExisting If |false|, throw an error if the directory
+ * already exists. |true| by default. Ignored if |from| is specified.
+ * - {number} unixMode Under Unix, if specified, a file creation mode,
+ * as per libc function |mkdir|. If unspecified, dirs are
+ * created with a default mode of 0700 (dir is private to
+ * the user, the user can read, write and execute). Ignored under Windows
+ * or if the file system does not support file creation modes.
+ * - {C pointer} winSecurity Under Windows, if specified, security
+ * attributes as per winapi function |CreateDirectory|. If
+ * unspecified, use the default security descriptor, inherited from
+ * the parent directory. Ignored under Unix or if the file system
+ * does not support security descriptors.
+ */
+AbstractFile.makeDir = function(path, options = {}) {
+  let from = options.from;
+  if (!from) {
+    OS.File._makeDir(path, options);
+    return;
+  }
+  if (!path.startsWith(from)) {
+    // Apparently, `from` is not a parent of `path`. However, we may
+    // have false negatives due to non-normalized paths, e.g.
+    // "foo//bar" is a parent of "foo/bar/sna".
+    path = Path.normalize(path);
+    from = Path.normalize(from);
+    if (!path.startsWith(from)) {
+      throw new Error("Incorrect use of option |from|: " + path + " is not a descendant of " + from);
+    }
+  }
+  let innerOptions = Object.create(options, {
+    ignoreExisting: {
+      value: true
+    }
+  });
+  // Compute the elements that appear in |path| but not in |from|.
+  let items = Path.split(path).components.slice(Path.split(from).components.length);
+  let current = from;
+  for (let item of items) {
+    current = Path.join(current, item);
+    OS.File._makeDir(current, innerOptions);
+  }
+};
+
+if (!exports.OS.Shared) {
+  exports.OS.Shared = {};
+}
+exports.OS.Shared.AbstractFile = AbstractFile;
+})(this);
diff --git a/toolkit/components/osfile/modules/osfile_unix_allthreads.jsm b/toolkit/components/osfile/modules/osfile_unix_allthreads.jsm
new file mode 100644
index 000000000..632f9c40b
--- /dev/null
+++ b/toolkit/components/osfile/modules/osfile_unix_allthreads.jsm
@@ -0,0 +1,375 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * This module defines the thread-agnostic components of the Unix version
+ * of OS.File. It depends on the thread-agnostic cross-platform components
+ * of OS.File.
+ *
+ * It serves the following purposes:
+ * - open libc;
+ * - define OS.Unix.Error;
+ * - define a few constants and types that need to be defined on all platforms.
+ *
+ * This module can be:
+ * - opened from the main thread as a jsm module;
+ * - opened from a chrome worker through require().
+ */
+
+"use strict";
+
+var SharedAll;
+if (typeof Components != "undefined") {
+  let Cu = Components.utils;
+  // Module is opened as a jsm module
+  Cu.import("resource://gre/modules/ctypes.jsm", this);
+
+  SharedAll = {};
+  Cu.import("resource://gre/modules/osfile/osfile_shared_allthreads.jsm", SharedAll);
+  this.exports = {};
+} else if (typeof module != "undefined" && typeof require != "undefined") {
+  // Module is loaded with require()
+  SharedAll = require("resource://gre/modules/osfile/osfile_shared_allthreads.jsm");
+} else {
+  throw new Error("Please open this module with Component.utils.import or with require()");
+}
+
+var LOG = SharedAll.LOG.bind(SharedAll, "Unix", "allthreads");
+var Const = SharedAll.Constants.libc;
+
+// Open libc
+var libc = new SharedAll.Library("libc",
+                                 "libc.so", "libSystem.B.dylib", "a.out");
+exports.libc = libc;
+
+// Define declareFFI
+var declareFFI = SharedAll.declareFFI.bind(null, libc);
+exports.declareFFI = declareFFI;
+
+// Define lazy binding
+var LazyBindings = {};
+libc.declareLazy(LazyBindings, "strerror",
+                 "strerror", ctypes.default_abi,
+                 /*return*/ ctypes.char.ptr,
+                 /*errnum*/ ctypes.int);
+
+/**
+ * A File-related error.
+ *
+ * To obtain a human-readable error message, use method |toString|.
+ * To determine the cause of the error, use the various |becauseX|
+ * getters. To determine the operation that failed, use field
+ * |operation|.
+ *
+ * Additionally, this implementation offers a field
+ * |unixErrno|, which holds the OS-specific error
+ * constant. If you need this level of detail, you may match the value
+ * of this field against the error constants of |OS.Constants.libc|.
+ *
+ * @param {string=} operation The operation that failed. If unspecified,
+ * the name of the calling function is taken to be the operation that
+ * failed.
+ * @param {number=} lastError The OS-specific constant detailing the
+ * reason of the error. If unspecified, this is fetched from the system
+ * status.
+ * @param {string=} path The file path that manipulated. If unspecified,
+ * assign the empty string.
+ *
+ * @constructor
+ * @extends {OS.Shared.Error}
+ */
+var OSError = function OSError(operation = "unknown operation",
+                               errno = ctypes.errno, path = "") {
+  SharedAll.OSError.call(this, operation, path);
+  this.unixErrno = errno;
+};
+OSError.prototype = Object.create(SharedAll.OSError.prototype);
+OSError.prototype.toString = function toString() {
+  return "Unix error " + this.unixErrno +
+    " during operation " + this.operation +
+    (this.path? " on file " + this.path : "") +
+    " (" + LazyBindings.strerror(this.unixErrno).readString() + ")";
+};
+OSError.prototype.toMsg = function toMsg() {
+  return OSError.toMsg(this);
+};
+
+/**
+ * |true| if the error was raised because a file or directory
+ * already exists, |false| otherwise.
+ */
+Object.defineProperty(OSError.prototype, "becauseExists", {
+  get: function becauseExists() {
+    return this.unixErrno == Const.EEXIST;
+  }
+});
+/**
+ * |true| if the error was raised because a file or directory
+ * does not exist, |false| otherwise.
+ */
+Object.defineProperty(OSError.prototype, "becauseNoSuchFile", {
+  get: function becauseNoSuchFile() {
+    return this.unixErrno == Const.ENOENT;
+  }
+});
+
+/**
+ * |true| if the error was raised because a directory is not empty
+ * does not exist, |false| otherwise.
+ */
+ Object.defineProperty(OSError.prototype, "becauseNotEmpty", {
+   get: function becauseNotEmpty() {
+     return this.unixErrno == Const.ENOTEMPTY;
+   }
+ });
+/**
+ * |true| if the error was raised because a file or directory
+ * is closed, |false| otherwise.
+ */
+Object.defineProperty(OSError.prototype, "becauseClosed", {
+  get: function becauseClosed() {
+    return this.unixErrno == Const.EBADF;
+  }
+});
+/**
+ * |true| if the error was raised because permission is denied to
+ * access a file or directory, |false| otherwise.
+ */
+Object.defineProperty(OSError.prototype, "becauseAccessDenied", {
+  get: function becauseAccessDenied() {
+    return this.unixErrno == Const.EACCES;
+  }
+});
+/**
+ * |true| if the error was raised because some invalid argument was passed,
+ * |false| otherwise.
+ */
+Object.defineProperty(OSError.prototype, "becauseInvalidArgument", {
+  get: function becauseInvalidArgument() {
+    return this.unixErrno == Const.EINVAL;
+  }
+});
+
+/**
+ * Serialize an instance of OSError to something that can be
+ * transmitted across threads (not necessarily a string).
+ */
+OSError.toMsg = function toMsg(error) {
+  return {
+    exn: "OS.File.Error",
+    fileName: error.moduleName,
+    lineNumber: error.lineNumber,
+    stack: error.moduleStack,
+    operation: error.operation,
+    unixErrno: error.unixErrno,
+    path: error.path
+  };
+};
+
+/**
+ * Deserialize a message back to an instance of OSError
+ */
+OSError.fromMsg = function fromMsg(msg) {
+  let error = new OSError(msg.operation, msg.unixErrno, msg.path);
+  error.stack = msg.stack;
+  error.fileName = msg.fileName;
+  error.lineNumber = msg.lineNumber;
+  return error;
+};
+exports.Error = OSError;
+
+/**
+ * Code shared by implementations of File.Info on Unix
+ *
+ * @constructor
+*/
+var AbstractInfo = function AbstractInfo(path, isDir, isSymLink, size, lastAccessDate,
+                                         lastModificationDate, unixLastStatusChangeDate,
+                                         unixOwner, unixGroup, unixMode) {
+  this._path = path;
+  this._isDir = isDir;
+  this._isSymlLink = isSymLink;
+  this._size = size;
+  this._lastAccessDate = lastAccessDate;
+  this._lastModificationDate = lastModificationDate;
+  this._unixLastStatusChangeDate = unixLastStatusChangeDate;
+  this._unixOwner = unixOwner;
+  this._unixGroup = unixGroup;
+  this._unixMode = unixMode;
+};
+
+AbstractInfo.prototype = {
+  /**
+   * The path of the file, used for error-reporting.
+   *
+   * @type {string}
+   */
+  get path() {
+    return this._path;
+  },
+  /**
+   * |true| if this file is a directory, |false| otherwise
+   */
+  get isDir() {
+    return this._isDir;
+  },
+  /**
+   * |true| if this file is a symbolink link, |false| otherwise
+   */
+  get isSymLink() {
+    return this._isSymlLink;
+  },
+  /**
+   * The size of the file, in bytes.
+   *
+   * Note that the result may be |NaN| if the size of the file cannot be
+   * represented in JavaScript.
+   *
+   * @type {number}
+   */
+  get size() {
+    return this._size;
+  },
+  /**
+   * The date of last access to this file.
+   *
+   * Note that the definition of last access may depend on the
+   * underlying operating system and file system.
+   *
+   * @type {Date}
+   */
+  get lastAccessDate() {
+    return this._lastAccessDate;
+  },
+  /**
+   * Return the date of last modification of this file.
+   */
+  get lastModificationDate() {
+    return this._lastModificationDate;
+  },
+  /**
+   * Return the date at which the status of this file was last modified
+   * (this is the date of the latest write/renaming/mode change/...
+   * of the file)
+   */
+  get unixLastStatusChangeDate() {
+    return this._unixLastStatusChangeDate;
+  },
+  /*
+   * Return the Unix owner of this file
+   */
+  get unixOwner() {
+    return this._unixOwner;
+  },
+  /*
+   * Return the Unix group of this file
+   */
+  get unixGroup() {
+    return this._unixGroup;
+  },
+  /*
+   * Return the Unix group of this file
+   */
+  get unixMode() {
+    return this._unixMode;
+  }
+};
+exports.AbstractInfo = AbstractInfo;
+
+/**
+ * Code shared by implementations of File.DirectoryIterator.Entry on Unix
+ *
+ * @constructor
+*/
+var AbstractEntry = function AbstractEntry(isDir, isSymLink, name, path) {
+  this._isDir = isDir;
+  this._isSymlLink = isSymLink;
+  this._name = name;
+  this._path = path;
+};
+
+AbstractEntry.prototype = {
+  /**
+   * |true| if the entry is a directory, |false| otherwise
+   */
+  get isDir() {
+    return this._isDir;
+  },
+  /**
+   * |true| if the entry is a directory, |false| otherwise
+   */
+  get isSymLink() {
+    return this._isSymlLink;
+  },
+  /**
+   * The name of the entry
+   * @type {string}
+   */
+  get name() {
+    return this._name;
+  },
+  /**
+   * The full path to the entry
+   */
+  get path() {
+    return this._path;
+  }
+};
+exports.AbstractEntry = AbstractEntry;
+
+// Special constants that need to be defined on all platforms
+
+exports.POS_START = Const.SEEK_SET;
+exports.POS_CURRENT = Const.SEEK_CUR;
+exports.POS_END = Const.SEEK_END;
+
+// Special types that need to be defined for communication
+// between threads
+var Type = Object.create(SharedAll.Type);
+exports.Type = Type;
+
+/**
+ * Native paths
+ *
+ * Under Unix, expressed as C strings
+ */
+Type.path = Type.cstring.withName("[in] path");
+Type.out_path = Type.out_cstring.withName("[out] path");
+
+// Special constructors that need to be defined on all threads
+OSError.closed = function closed(operation, path) {
+  return new OSError(operation, Const.EBADF, path);
+};
+
+OSError.exists = function exists(operation, path) {
+  return new OSError(operation, Const.EEXIST, path);
+};
+
+OSError.noSuchFile = function noSuchFile(operation, path) {
+  return new OSError(operation, Const.ENOENT, path);
+};
+
+OSError.invalidArgument = function invalidArgument(operation) {
+  return new OSError(operation, Const.EINVAL);
+};
+
+var EXPORTED_SYMBOLS = [
+  "declareFFI",
+  "libc",
+  "Error",
+  "AbstractInfo",
+  "AbstractEntry",
+  "Type",
+  "POS_START",
+  "POS_CURRENT",
+  "POS_END"
+];
+
+//////////// Boilerplate
+if (typeof Components != "undefined") {
+  this.EXPORTED_SYMBOLS = EXPORTED_SYMBOLS;
+  for (let symbol of EXPORTED_SYMBOLS) {
+    this[symbol] = exports[symbol];
+  }
+}
diff --git a/toolkit/components/osfile/modules/osfile_unix_back.jsm b/toolkit/components/osfile/modules/osfile_unix_back.jsm
new file mode 100644
index 000000000..a028dda7d
--- /dev/null
+++ b/toolkit/components/osfile/modules/osfile_unix_back.jsm
@@ -0,0 +1,735 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+{
+  if (typeof Components != "undefined") {
+    // We do not wish osfile_unix_back.jsm to be used directly as a main thread
+    // module yet. When time comes, it will be loaded by a combination of
+    // a main thread front-end/worker thread implementation that makes sure
+    // that we are not executing synchronous IO code in the main thread.
+
+    throw new Error("osfile_unix_back.jsm cannot be used from the main thread yet");
+  }
+  (function(exports) {
+     "use strict";
+     if (exports.OS && exports.OS.Unix && exports.OS.Unix.File) {
+       return; // Avoid double initialization
+     }
+
+     let SharedAll =
+       require("resource://gre/modules/osfile/osfile_shared_allthreads.jsm");
+     let SysAll =
+       require("resource://gre/modules/osfile/osfile_unix_allthreads.jsm");
+     let LOG = SharedAll.LOG.bind(SharedAll, "Unix", "back");
+     let libc = SysAll.libc;
+     let Const = SharedAll.Constants.libc;
+
+     /**
+      * Initialize the Unix module.
+      *
+      * @param {function=} declareFFI
+      */
+     // FIXME: Both |init| and |aDeclareFFI| are deprecated, we should remove them
+     let init = function init(aDeclareFFI) {
+       let declareFFI;
+       if (aDeclareFFI) {
+         declareFFI = aDeclareFFI.bind(null, libc);
+       } else {
+         declareFFI = SysAll.declareFFI;
+       }
+       let declareLazyFFI = SharedAll.declareLazyFFI;
+
+       // Initialize types that require additional OS-specific
+       // support - either finalization or matching against
+       // OS-specific constants.
+       let Type = Object.create(SysAll.Type);
+       let SysFile = exports.OS.Unix.File = { Type: Type };
+
+       /**
+        * A file descriptor.
+        */
+       Type.fd = Type.int.withName("fd");
+       Type.fd.importFromC = function importFromC(fd_int) {
+         return ctypes.CDataFinalizer(fd_int, SysFile._close);
+       };
+
+
+       /**
+        * A C integer holding -1 in case of error or a file descriptor
+        * in case of success.
+        */
+       Type.negativeone_or_fd = Type.fd.withName("negativeone_or_fd");
+       Type.negativeone_or_fd.importFromC =
+         function importFromC(fd_int) {
+           if (fd_int == -1) {
+             return -1;
+           }
+           return ctypes.CDataFinalizer(fd_int, SysFile._close);
+         };
+
+       /**
+        * A C integer holding -1 in case of error or a meaningless value
+        * in case of success.
+        */
+       Type.negativeone_or_nothing =
+         Type.int.withName("negativeone_or_nothing");
+
+       /**
+        * A C integer holding -1 in case of error or a positive integer
+        * in case of success.
+        */
+       Type.negativeone_or_ssize_t =
+         Type.ssize_t.withName("negativeone_or_ssize_t");
+
+       /**
+        * Various libc integer types
+        */
+       Type.mode_t =
+         Type.intn_t(Const.OSFILE_SIZEOF_MODE_T).withName("mode_t");
+       Type.uid_t =
+         Type.intn_t(Const.OSFILE_SIZEOF_UID_T).withName("uid_t");
+       Type.gid_t =
+         Type.intn_t(Const.OSFILE_SIZEOF_GID_T).withName("gid_t");
+
+       /**
+        * Type |time_t|
+        */
+       Type.time_t =
+         Type.intn_t(Const.OSFILE_SIZEOF_TIME_T).withName("time_t");
+
+       // Structure |dirent|
+       // Building this type is rather complicated, as its layout varies between
+       // variants of Unix. For this reason, we rely on a number of constants
+       // (computed in C from the C data structures) that give us the layout.
+       // The structure we compute looks like
+       //  { int8_t[...] before_d_type; // ignored content
+       //    int8_t      d_type       ;
+       //    int8_t[...] before_d_name; // ignored content
+       //    char[...]   d_name;
+       //    };
+       {
+         let d_name_extra_size = 0;
+         if (Const.OSFILE_SIZEOF_DIRENT_D_NAME < 8) {
+           // d_name is defined like "char d_name[1];" on some platforms
+           // (e.g. Solaris), we need to give it more size for our structure.
+           d_name_extra_size = 256;
+         }
+
+         let dirent = new SharedAll.HollowStructure("dirent",
+           Const.OSFILE_SIZEOF_DIRENT + d_name_extra_size);
+         if (Const.OSFILE_OFFSETOF_DIRENT_D_TYPE != undefined) {
+           // |dirent| doesn't have d_type on some platforms (e.g. Solaris).
+           dirent.add_field_at(Const.OSFILE_OFFSETOF_DIRENT_D_TYPE,
+             "d_type", ctypes.uint8_t);
+         }
+         dirent.add_field_at(Const.OSFILE_OFFSETOF_DIRENT_D_NAME,
+           "d_name", ctypes.ArrayType(ctypes.char,
+             Const.OSFILE_SIZEOF_DIRENT_D_NAME + d_name_extra_size));
+
+         // We now have built |dirent|.
+         Type.dirent = dirent.getType();
+       }
+       Type.null_or_dirent_ptr =
+         new SharedAll.Type("null_of_dirent",
+                  Type.dirent.out_ptr.implementation);
+
+       // Structure |stat|
+       // Same technique
+       {
+         let stat = new SharedAll.HollowStructure("stat",
+           Const.OSFILE_SIZEOF_STAT);
+         stat.add_field_at(Const.OSFILE_OFFSETOF_STAT_ST_MODE,
+                        "st_mode", Type.mode_t.implementation);
+         stat.add_field_at(Const.OSFILE_OFFSETOF_STAT_ST_UID,
+                          "st_uid", Type.uid_t.implementation);
+         stat.add_field_at(Const.OSFILE_OFFSETOF_STAT_ST_GID,
+                          "st_gid", Type.gid_t.implementation);
+
+         // Here, things get complicated with different data structures.
+         // Some platforms have |time_t st_atime| and some platforms have
+         // |timespec st_atimespec|. However, since |timespec| starts with
+         // a |time_t|, followed by nanoseconds, we just cheat and pretend
+         // that everybody has |time_t st_atime|, possibly followed by padding
+         stat.add_field_at(Const.OSFILE_OFFSETOF_STAT_ST_ATIME,
+                          "st_atime", Type.time_t.implementation);
+         stat.add_field_at(Const.OSFILE_OFFSETOF_STAT_ST_MTIME,
+                          "st_mtime", Type.time_t.implementation);
+         stat.add_field_at(Const.OSFILE_OFFSETOF_STAT_ST_CTIME,
+                          "st_ctime", Type.time_t.implementation);
+
+         // To complicate further, MacOS and some BSDs have a field |birthtime|
+         if ("OSFILE_OFFSETOF_STAT_ST_BIRTHTIME" in Const) {
+           stat.add_field_at(Const.OSFILE_OFFSETOF_STAT_ST_BIRTHTIME,
+                             "st_birthtime", Type.time_t.implementation);
+         }
+
+         stat.add_field_at(Const.OSFILE_OFFSETOF_STAT_ST_SIZE,
+                        "st_size", Type.off_t.implementation);
+         Type.stat = stat.getType();
+       }
+
+       // Structure |DIR|
+       if ("OSFILE_SIZEOF_DIR" in Const) {
+         // On platforms for which we need to access the fields of DIR
+         // directly (e.g. because certain functions are implemented
+         // as macros), we need to define DIR as a hollow structure.
+         let DIR = new SharedAll.HollowStructure(
+           "DIR",
+           Const.OSFILE_SIZEOF_DIR);
+
+         DIR.add_field_at(
+           Const.OSFILE_OFFSETOF_DIR_DD_FD,
+           "dd_fd",
+           Type.fd.implementation);
+
+         Type.DIR = DIR.getType();
+       } else {
+         // On other platforms, we keep DIR as a blackbox
+         Type.DIR =
+           new SharedAll.Type("DIR",
+             ctypes.StructType("DIR"));
+       }
+
+       Type.null_or_DIR_ptr =
+         Type.DIR.out_ptr.withName("null_or_DIR*");
+       Type.null_or_DIR_ptr.importFromC = function importFromC(dir) {
+         if (dir == null || dir.isNull()) {
+           return null;
+         }
+         return ctypes.CDataFinalizer(dir, SysFile._close_dir);
+       };
+
+       // Structure |timeval|
+       {
+         let timeval = new SharedAll.HollowStructure(
+           "timeval",
+           Const.OSFILE_SIZEOF_TIMEVAL);
+         timeval.add_field_at(
+           Const.OSFILE_OFFSETOF_TIMEVAL_TV_SEC,
+           "tv_sec",
+           Type.long.implementation);
+         timeval.add_field_at(
+           Const.OSFILE_OFFSETOF_TIMEVAL_TV_USEC,
+           "tv_usec",
+           Type.long.implementation);
+         Type.timeval = timeval.getType();
+         Type.timevals = new SharedAll.Type("two timevals",
+           ctypes.ArrayType(Type.timeval.implementation, 2));
+       }
+
+       // Types fsblkcnt_t and fsfilcnt_t, used by structure |statvfs|
+       Type.fsblkcnt_t =
+         Type.uintn_t(Const.OSFILE_SIZEOF_FSBLKCNT_T).withName("fsblkcnt_t");
+
+       // Structure |statvfs|
+       // Use an hollow structure
+       {
+         let statvfs = new SharedAll.HollowStructure("statvfs",
+           Const.OSFILE_SIZEOF_STATVFS);
+
+         statvfs.add_field_at(Const.OSFILE_OFFSETOF_STATVFS_F_BSIZE,
+                        "f_bsize", Type.unsigned_long.implementation);
+         statvfs.add_field_at(Const.OSFILE_OFFSETOF_STATVFS_F_BAVAIL,
+                        "f_bavail", Type.fsblkcnt_t.implementation);
+
+         Type.statvfs = statvfs.getType();
+       }
+
+       // Declare libc functions as functions of |OS.Unix.File|
+
+       // Finalizer-related functions
+       libc.declareLazy(SysFile, "_close",
+                        "close", ctypes.default_abi,
+                        /*return */ctypes.int,
+                        /*fd*/     ctypes.int);
+
+       SysFile.close = function close(fd) {
+         // Detach the finalizer and call |_close|.
+         return fd.dispose();
+       };
+
+       libc.declareLazy(SysFile, "_close_dir",
+                        "closedir", ctypes.default_abi,
+                        /*return */ctypes.int,
+                        /*dirp*/   Type.DIR.in_ptr.implementation);
+
+       SysFile.closedir = function closedir(fd) {
+         // Detach the finalizer and call |_close_dir|.
+         return fd.dispose();
+       };
+
+       {
+         // Symbol free() is special.
+         // We override the definition of free() on several platforms.
+         let default_lib = new SharedAll.Library("default_lib",
+                                                 "a.out");
+
+         // On platforms for which we override free(), nspr defines
+         // a special library name "a.out" that will resolve to the
+         // correct implementation free().
+         // If it turns out we don't have an a.out library or a.out
+         // doesn't contain free, use the ordinary libc free.
+
+         default_lib.declareLazyWithFallback(libc, SysFile, "free",
+           "free", ctypes.default_abi,
+           /*return*/ ctypes.void_t,
+           /*ptr*/    ctypes.voidptr_t);
+       }
+
+
+       // Other functions
+       libc.declareLazyFFI(SysFile,  "access",
+                           "access", ctypes.default_abi,
+                           /*return*/ Type.negativeone_or_nothing,
+                           /*path*/   Type.path,
+                           /*mode*/   Type.int);
+
+       libc.declareLazyFFI(SysFile,  "chdir",
+                           "chdir", ctypes.default_abi,
+                           /*return*/ Type.negativeone_or_nothing,
+                           /*path*/   Type.path);
+
+       libc.declareLazyFFI(SysFile,  "chmod",
+                           "chmod", ctypes.default_abi,
+                           /*return*/ Type.negativeone_or_nothing,
+                           /*path*/   Type.path,
+                           /*mode*/   Type.mode_t);
+
+       libc.declareLazyFFI(SysFile,  "chown",
+                           "chown", ctypes.default_abi,
+                           /*return*/ Type.negativeone_or_nothing,
+                           /*path*/   Type.path,
+                           /*uid*/    Type.uid_t,
+                           /*gid*/    Type.gid_t);
+
+       libc.declareLazyFFI(SysFile,  "copyfile",
+                           "copyfile", ctypes.default_abi,
+                           /*return*/ Type.negativeone_or_nothing,
+                           /*source*/ Type.path,
+                           /*dest*/   Type.path,
+                           /*state*/  Type.void_t.in_ptr, // Ignored atm
+                           /*flags*/  Type.uint32_t);
+
+       libc.declareLazyFFI(SysFile,  "dup",
+                           "dup", ctypes.default_abi,
+                           /*return*/ Type.negativeone_or_fd,
+                           /*fd*/     Type.fd);
+
+       if ("OSFILE_SIZEOF_DIR" in Const) {
+         // On platforms for which |dirfd| is a macro
+         SysFile.dirfd =
+           function dirfd(DIRp) {
+             return Type.DIR.in_ptr.implementation(DIRp).contents.dd_fd;
+           };
+       } else {
+         // On platforms for which |dirfd| is a function
+         libc.declareLazyFFI(SysFile,  "dirfd",
+                             "dirfd", ctypes.default_abi,
+                             /*return*/ Type.negativeone_or_fd,
+                             /*dir*/    Type.DIR.in_ptr);
+       }
+
+       libc.declareLazyFFI(SysFile,  "chdir",
+                           "chdir", ctypes.default_abi,
+                           /*return*/ Type.negativeone_or_nothing,
+                           /*path*/   Type.path);
+
+       libc.declareLazyFFI(SysFile,  "fchdir",
+                           "fchdir", ctypes.default_abi,
+                           /*return*/ Type.negativeone_or_nothing,
+                           /*fd*/     Type.fd);
+
+       libc.declareLazyFFI(SysFile,  "fchmod",
+                           "fchmod", ctypes.default_abi,
+                           /*return*/ Type.negativeone_or_nothing,
+                           /*fd*/     Type.fd,
+                           /*mode*/   Type.mode_t);
+
+       libc.declareLazyFFI(SysFile,  "fchown",
+                           "fchown", ctypes.default_abi,
+                           /*return*/ Type.negativeone_or_nothing,
+                           /*fd*/     Type.fd,
+                           /*uid_t*/  Type.uid_t,
+                           /*gid_t*/  Type.gid_t);
+
+       libc.declareLazyFFI(SysFile,  "fsync",
+                           "fsync", ctypes.default_abi,
+                           /*return*/ Type.negativeone_or_nothing,
+                           /*fd*/     Type.fd);
+
+       libc.declareLazyFFI(SysFile,  "getcwd",
+                           "getcwd", ctypes.default_abi,
+                           /*return*/ Type.out_path,
+                           /*buf*/    Type.out_path,
+                           /*size*/   Type.size_t);
+
+       libc.declareLazyFFI(SysFile,  "getwd",
+                           "getwd", ctypes.default_abi,
+                           /*return*/ Type.out_path,
+                           /*buf*/    Type.out_path);
+
+       // Two variants of |getwd| which allocate the memory
+       // dynamically.
+
+       // Linux/Android version
+       libc.declareLazyFFI(SysFile,  "get_current_dir_name",
+                           "get_current_dir_name", ctypes.default_abi,
+                           /*return*/ Type.out_path.releaseWithLazy(() =>
+                             SysFile.free
+                           ));
+
+       // MacOS/BSD version (will return NULL on Linux/Android)
+       libc.declareLazyFFI(SysFile,  "getwd_auto",
+                           "getwd", ctypes.default_abi,
+                           /*return*/ Type.out_path.releaseWithLazy(() =>
+                             SysFile.free
+                           ),
+                           /*buf*/    Type.void_t.out_ptr);
+
+       libc.declareLazyFFI(SysFile,  "fdatasync",
+                           "fdatasync", ctypes.default_abi,
+                           /*return*/ Type.negativeone_or_nothing,
+                           /*fd*/     Type.fd); // Note: MacOS/BSD-specific
+
+       libc.declareLazyFFI(SysFile,  "ftruncate",
+                           "ftruncate", ctypes.default_abi,
+                           /*return*/ Type.negativeone_or_nothing,
+                           /*fd*/     Type.fd,
+                           /*length*/ Type.off_t);
+
+
+       libc.declareLazyFFI(SysFile,  "lchown",
+                           "lchown", ctypes.default_abi,
+                           /*return*/ Type.negativeone_or_nothing,
+                           /*path*/   Type.path,
+                           /*uid_t*/  Type.uid_t,
+                           /*gid_t*/  Type.gid_t);
+
+       libc.declareLazyFFI(SysFile,  "link",
+                           "link", ctypes.default_abi,
+                           /*return*/ Type.negativeone_or_nothing,
+                           /*source*/ Type.path,
+                           /*dest*/   Type.path);
+
+       libc.declareLazyFFI(SysFile,  "lseek",
+                           "lseek", ctypes.default_abi,
+                           /*return*/ Type.off_t,
+                           /*fd*/     Type.fd,
+                           /*offset*/ Type.off_t,
+                           /*whence*/ Type.int);
+
+       libc.declareLazyFFI(SysFile,  "mkdir",
+                           "mkdir", ctypes.default_abi,
+                           /*return*/ Type.int,
+                           /*path*/ Type.path,
+                           /*mode*/ Type.int);
+
+       libc.declareLazyFFI(SysFile,  "mkstemp",
+                           "mkstemp", ctypes.default_abi,
+                           /*return*/   Type.fd,
+                           /*template*/ Type.out_path);
+
+       libc.declareLazyFFI(SysFile,  "open",
+                           "open", ctypes.default_abi,
+                           /*return*/ Type.negativeone_or_fd,
+                           /*path*/   Type.path,
+                           /*oflags*/ Type.int,
+                           /*mode*/   Type.int);
+
+       if (OS.Constants.Sys.Name == "NetBSD") {
+          libc.declareLazyFFI(SysFile,  "opendir",
+                              "__opendir30", ctypes.default_abi,
+                   /*return*/ Type.null_or_DIR_ptr,
+                   /*path*/   Type.path);
+       } else {
+          libc.declareLazyFFI(SysFile,  "opendir",
+                              "opendir", ctypes.default_abi,
+                    /*return*/ Type.null_or_DIR_ptr,
+                    /*path*/   Type.path);
+       }
+
+       libc.declareLazyFFI(SysFile,  "pread",
+                               "pread", ctypes.default_abi,
+                    /*return*/ Type.negativeone_or_ssize_t,
+                    /*fd*/     Type.fd,
+                    /*buf*/    Type.void_t.out_ptr,
+                    /*nbytes*/ Type.size_t,
+                    /*offset*/ Type.off_t);
+
+       libc.declareLazyFFI(SysFile,  "pwrite",
+                               "pwrite", ctypes.default_abi,
+                    /*return*/ Type.negativeone_or_ssize_t,
+                    /*fd*/     Type.fd,
+                    /*buf*/    Type.void_t.in_ptr,
+                    /*nbytes*/ Type.size_t,
+                    /*offset*/ Type.off_t);
+
+       libc.declareLazyFFI(SysFile,  "read",
+                              "read", ctypes.default_abi,
+                    /*return*/Type.negativeone_or_ssize_t,
+                    /*fd*/    Type.fd,
+                    /*buf*/   Type.void_t.out_ptr,
+                    /*nbytes*/Type.size_t);
+
+       libc.declareLazyFFI(SysFile,  "posix_fadvise",
+                               "posix_fadvise", ctypes.default_abi,
+                    /*return*/ Type.int,
+                    /*fd*/     Type.fd,
+                    /*offset*/ Type.off_t,
+                    /*len*/    Type.off_t,
+                    /*advise*/ Type.int);
+
+       if (Const._DARWIN_FEATURE_64_BIT_INODE) {
+         // Special case for MacOS X 10.5+
+         // Symbol name "readdir" still exists but is used for a
+         // deprecated function that does not match the
+         // constants of |Const|.
+         libc.declareLazyFFI(SysFile,  "readdir",
+                                "readdir$INODE64", ctypes.default_abi,
+                     /*return*/ Type.null_or_dirent_ptr,
+                      /*dir*/   Type.DIR.in_ptr); // For MacOS X
+       } else if (OS.Constants.Sys.Name == "NetBSD") {
+         libc.declareLazyFFI(SysFile,  "readdir",
+                                "__readdir30", ctypes.default_abi,
+                      /*return*/Type.null_or_dirent_ptr,
+                      /*dir*/   Type.DIR.in_ptr); // Other Unices
+       } else {
+         libc.declareLazyFFI(SysFile,  "readdir",
+                                "readdir", ctypes.default_abi,
+                      /*return*/Type.null_or_dirent_ptr,
+                      /*dir*/   Type.DIR.in_ptr); // Other Unices
+       }
+
+       libc.declareLazyFFI(SysFile,  "rename",
+                               "rename", ctypes.default_abi,
+                    /*return*/ Type.negativeone_or_nothing,
+                    /*old*/    Type.path,
+                    /*new*/    Type.path);
+
+       libc.declareLazyFFI(SysFile,  "rmdir",
+                               "rmdir", ctypes.default_abi,
+                    /*return*/ Type.int,
+                    /*path*/   Type.path);
+
+       libc.declareLazyFFI(SysFile,  "splice",
+                               "splice", ctypes.default_abi,
+                    /*return*/ Type.long,
+                    /*fd_in*/  Type.fd,
+                    /*off_in*/ Type.off_t.in_ptr,
+                    /*fd_out*/ Type.fd,
+                    /*off_out*/Type.off_t.in_ptr,
+                    /*len*/    Type.size_t,
+                    /*flags*/  Type.unsigned_int); // Linux/Android-specific
+
+       libc.declareLazyFFI(SysFile,  "statfs",
+                               "statfs", ctypes.default_abi,
+                    /*return*/ Type.negativeone_or_nothing,
+                    /*path*/   Type.path,
+                    /*buf*/    Type.statvfs.out_ptr); // Android,B2G
+
+       libc.declareLazyFFI(SysFile,  "statvfs",
+                               "statvfs", ctypes.default_abi,
+                    /*return*/ Type.negativeone_or_nothing,
+                    /*path*/   Type.path,
+                    /*buf*/    Type.statvfs.out_ptr); // Other platforms
+
+       libc.declareLazyFFI(SysFile,  "symlink",
+                               "symlink", ctypes.default_abi,
+                    /*return*/ Type.negativeone_or_nothing,
+                    /*source*/ Type.path,
+                    /*dest*/   Type.path);
+
+       libc.declareLazyFFI(SysFile,  "truncate",
+                              "truncate", ctypes.default_abi,
+                    /*return*/Type.negativeone_or_nothing,
+                    /*path*/  Type.path,
+                    /*length*/ Type.off_t);
+
+       libc.declareLazyFFI(SysFile,  "unlink",
+                           "unlink", ctypes.default_abi,
+                    /*return*/ Type.negativeone_or_nothing,
+                    /*path*/ Type.path);
+
+       libc.declareLazyFFI(SysFile,  "write",
+                           "write", ctypes.default_abi,
+                    /*return*/ Type.negativeone_or_ssize_t,
+                    /*fd*/     Type.fd,
+                    /*buf*/    Type.void_t.in_ptr,
+                    /*nbytes*/ Type.size_t);
+
+       // Weird cases that require special treatment
+
+       // OSes use a variety of hacks to differentiate between
+       // 32-bits and 64-bits versions of |stat|, |lstat|, |fstat|.
+       if (Const._DARWIN_FEATURE_64_BIT_INODE) {
+         // MacOS X 64-bits
+         libc.declareLazyFFI(SysFile,  "stat",
+                             "stat$INODE64", ctypes.default_abi,
+                      /*return*/ Type.negativeone_or_nothing,
+                      /*path*/   Type.path,
+                      /*buf*/    Type.stat.out_ptr
+                     );
+         libc.declareLazyFFI(SysFile,  "lstat",
+                             "lstat$INODE64", ctypes.default_abi,
+                      /*return*/ Type.negativeone_or_nothing,
+                      /*path*/   Type.path,
+                      /*buf*/    Type.stat.out_ptr
+                     );
+         libc.declareLazyFFI(SysFile,  "fstat",
+                             "fstat$INODE64", ctypes.default_abi,
+                      /*return*/ Type.negativeone_or_nothing,
+                      /*path*/   Type.fd,
+                      /*buf*/    Type.stat.out_ptr
+                     );
+       } else if (Const._STAT_VER != undefined) {
+         const ver = Const._STAT_VER;
+         let xstat_name, lxstat_name, fxstat_name;
+         if (OS.Constants.Sys.Name == "SunOS") {
+           // Solaris
+           xstat_name = "_xstat";
+           lxstat_name = "_lxstat";
+           fxstat_name = "_fxstat";
+         } else {
+           // Linux, all widths
+           xstat_name = "__xstat";
+           lxstat_name = "__lxstat";
+           fxstat_name = "__fxstat";
+         }
+
+         let Stat = {};
+         libc.declareLazyFFI(Stat,  "xstat",
+                             xstat_name, ctypes.default_abi,
+                      /*return*/    Type.negativeone_or_nothing,
+                      /*_stat_ver*/ Type.int,
+                      /*path*/      Type.path,
+                      /*buf*/       Type.stat.out_ptr);
+         libc.declareLazyFFI(Stat,  "lxstat",
+                             lxstat_name, ctypes.default_abi,
+                      /*return*/    Type.negativeone_or_nothing,
+                      /*_stat_ver*/ Type.int,
+                      /*path*/      Type.path,
+                      /*buf*/       Type.stat.out_ptr);
+         libc.declareLazyFFI(Stat, "fxstat",
+                             fxstat_name, ctypes.default_abi,
+                      /*return*/    Type.negativeone_or_nothing,
+                      /*_stat_ver*/ Type.int,
+                      /*fd*/        Type.fd,
+                      /*buf*/       Type.stat.out_ptr);
+
+
+         SysFile.stat = function stat(path, buf) {
+           return Stat.xstat(ver, path, buf);
+         };
+
+         SysFile.lstat = function lstat(path, buf) {
+           return Stat.lxstat(ver, path, buf);
+         };
+
+         SysFile.fstat = function fstat(fd, buf) {
+           return Stat.fxstat(ver, fd, buf);
+         };
+       } else if (OS.Constants.Sys.Name == "NetBSD") {
+         // NetBSD 5.0 and newer
+         libc.declareLazyFFI(SysFile,  "stat",
+                             "__stat50", ctypes.default_abi,
+                      /*return*/ Type.negativeone_or_nothing,
+                      /*path*/   Type.path,
+                      /*buf*/    Type.stat.out_ptr
+                     );
+         libc.declareLazyFFI(SysFile,  "lstat",
+                             "__lstat50", ctypes.default_abi,
+                      /*return*/ Type.negativeone_or_nothing,
+                      /*path*/   Type.path,
+                      /*buf*/    Type.stat.out_ptr
+                     );
+         libc.declareLazyFFI(SysFile,  "fstat",
+                             "__fstat50", ctypes.default_abi,
+                      /*return*/ Type.negativeone_or_nothing,
+                      /*fd*/     Type.fd,
+                      /*buf*/    Type.stat.out_ptr
+                     );
+       } else {
+         // Mac OS X 32-bits, other Unix
+         libc.declareLazyFFI(SysFile,  "stat",
+                             "stat", ctypes.default_abi,
+                      /*return*/ Type.negativeone_or_nothing,
+                      /*path*/   Type.path,
+                      /*buf*/    Type.stat.out_ptr
+                     );
+         libc.declareLazyFFI(SysFile,  "lstat",
+                             "lstat", ctypes.default_abi,
+                      /*return*/ Type.negativeone_or_nothing,
+                      /*path*/   Type.path,
+                      /*buf*/    Type.stat.out_ptr
+                     );
+         libc.declareLazyFFI(SysFile,  "fstat",
+                             "fstat", ctypes.default_abi,
+                      /*return*/ Type.negativeone_or_nothing,
+                      /*fd*/     Type.fd,
+                      /*buf*/    Type.stat.out_ptr
+                     );
+       }
+
+       // We cannot make a C array of CDataFinalizer, so
+       // pipe cannot be directly defined as a C function.
+
+       let Pipe = {};
+       libc.declareLazyFFI(Pipe, "_pipe",
+         "pipe", ctypes.default_abi,
+         /*return*/ Type.negativeone_or_nothing,
+         /*fds*/    new SharedAll.Type("two file descriptors",
+             ctypes.ArrayType(ctypes.int, 2)));
+
+       // A shared per-thread buffer used to communicate with |pipe|
+       let _pipebuf = new (ctypes.ArrayType(ctypes.int, 2))();
+
+       SysFile.pipe = function pipe(array) {
+         let result = Pipe._pipe(_pipebuf);
+         if (result == -1) {
+           return result;
+         }
+         array[0] = ctypes.CDataFinalizer(_pipebuf[0], SysFile._close);
+         array[1] = ctypes.CDataFinalizer(_pipebuf[1], SysFile._close);
+         return result;
+       };
+
+       if (OS.Constants.Sys.Name == "NetBSD") {
+           libc.declareLazyFFI(SysFile, "utimes",
+                               "__utimes50", ctypes.default_abi,
+                      /*return*/     Type.negativeone_or_nothing,
+                      /*path*/       Type.path,
+                      /*timeval[2]*/ Type.timevals.out_ptr
+                      );
+       } else {
+           libc.declareLazyFFI(SysFile, "utimes",
+                               "utimes", ctypes.default_abi,
+                      /*return*/     Type.negativeone_or_nothing,
+                      /*path*/       Type.path,
+                      /*timeval[2]*/ Type.timevals.out_ptr
+                      );
+       }
+       if (OS.Constants.Sys.Name == "NetBSD") {
+           libc.declareLazyFFI(SysFile, "futimes",
+                               "__futimes50", ctypes.default_abi,
+                      /*return*/     Type.negativeone_or_nothing,
+                      /*fd*/         Type.fd,
+                      /*timeval[2]*/ Type.timevals.out_ptr
+                      );
+       } else {
+           libc.declareLazyFFI(SysFile, "futimes",
+                               "futimes", ctypes.default_abi,
+                      /*return*/     Type.negativeone_or_nothing,
+                      /*fd*/         Type.fd,
+                      /*timeval[2]*/ Type.timevals.out_ptr
+                      );
+       }
+     };
+
+     exports.OS.Unix = {
+       File: {
+         _init: init
+       }
+     };
+   })(this);
+}
diff --git a/toolkit/components/osfile/modules/osfile_unix_front.jsm b/toolkit/components/osfile/modules/osfile_unix_front.jsm
new file mode 100644
index 000000000..19a27ae1a
--- /dev/null
+++ b/toolkit/components/osfile/modules/osfile_unix_front.jsm
@@ -0,0 +1,1193 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * Synchronous front-end for the JavaScript OS.File library.
+ * Unix implementation.
+ *
+ * This front-end is meant to be imported by a worker thread.
+ */
+
+{
+  if (typeof Components != "undefined") {
+    // We do not wish osfile_unix_front.jsm to be used directly as a main thread
+    // module yet.
+
+    throw new Error("osfile_unix_front.jsm cannot be used from the main thread yet");
+  }
+  (function(exports) {
+     "use strict";
+
+     // exports.OS.Unix is created by osfile_unix_back.jsm
+     if (exports.OS && exports.OS.File) {
+       return; // Avoid double-initialization
+     }
+
+     let SharedAll = require("resource://gre/modules/osfile/osfile_shared_allthreads.jsm");
+     let Path = require("resource://gre/modules/osfile/ospath.jsm");
+     let SysAll = require("resource://gre/modules/osfile/osfile_unix_allthreads.jsm");
+     exports.OS.Unix.File._init();
+     let LOG = SharedAll.LOG.bind(SharedAll, "Unix front-end");
+     let Const = SharedAll.Constants.libc;
+     let UnixFile = exports.OS.Unix.File;
+     let Type = UnixFile.Type;
+
+     /**
+      * Representation of a file.
+      *
+      * You generally do not need to call this constructor yourself. Rather,
+      * to open a file, use function |OS.File.open|.
+      *
+      * @param fd A OS-specific file descriptor.
+      * @param {string} path File path of the file handle, used for error-reporting.
+      * @constructor
+      */
+     let File = function File(fd, path) {
+       exports.OS.Shared.AbstractFile.call(this, fd, path);
+       this._closeResult = null;
+     };
+     File.prototype = Object.create(exports.OS.Shared.AbstractFile.prototype);
+
+     /**
+      * Close the file.
+      *
+      * This method has no effect if the file is already closed. However,
+      * if the first call to |close| has thrown an error, further calls
+      * will throw the same error.
+      *
+      * @throws File.Error If closing the file revealed an error that could
+      * not be reported earlier.
+      */
+     File.prototype.close = function close() {
+       if (this._fd) {
+         let fd = this._fd;
+         this._fd = null;
+        // Call |close(fd)|, detach finalizer if any
+         // (|fd| may not be a CDataFinalizer if it has been
+         // instantiated from a controller thread).
+         let result = UnixFile._close(fd);
+         if (typeof fd == "object" && "forget" in fd) {
+           fd.forget();
+         }
+         if (result == -1) {
+           this._closeResult = new File.Error("close", ctypes.errno, this._path);
+         }
+       }
+       if (this._closeResult) {
+         throw this._closeResult;
+       }
+       return;
+     };
+
+     /**
+      * Read some bytes from a file.
+      *
+      * @param {C pointer} buffer A buffer for holding the data
+      * once it is read.
+      * @param {number} nbytes The number of bytes to read. It must not
+      * exceed the size of |buffer| in bytes but it may exceed the number
+      * of bytes unread in the file.
+      * @param {*=} options Additional options for reading. Ignored in
+      * this implementation.
+      *
+      * @return {number} The number of bytes effectively read. If zero,
+      * the end of the file has been reached.
+      * @throws {OS.File.Error} In case of I/O error.
+      */
+     File.prototype._read = function _read(buffer, nbytes, options = {}) {
+      // Populate the page cache with data from a file so the subsequent reads
+      // from that file will not block on disk I/O.
+       if (typeof(UnixFile.posix_fadvise) === 'function' &&
+           (options.sequential || !("sequential" in options))) {
+         UnixFile.posix_fadvise(this.fd, 0, nbytes,
+          OS.Constants.libc.POSIX_FADV_SEQUENTIAL);
+       }
+       return throw_on_negative("read",
+         UnixFile.read(this.fd, buffer, nbytes),
+         this._path
+       );
+     };
+
+     /**
+      * Write some bytes to a file.
+      *
+      * @param {Typed array} buffer A buffer holding the data that must be
+      * written.
+      * @param {number} nbytes The number of bytes to write. It must not
+      * exceed the size of |buffer| in bytes.
+      * @param {*=} options Additional options for writing. Ignored in
+      * this implementation.
+      *
+      * @return {number} The number of bytes effectively written.
+      * @throws {OS.File.Error} In case of I/O error.
+      */
+     File.prototype._write = function _write(buffer, nbytes, options = {}) {
+       return throw_on_negative("write",
+         UnixFile.write(this.fd, buffer, nbytes),
+         this._path
+       );
+     };
+
+     /**
+      * Return the current position in the file.
+      */
+     File.prototype.getPosition = function getPosition(pos) {
+         return this.setPosition(0, File.POS_CURRENT);
+     };
+
+     /**
+      * Change the current position in the file.
+      *
+      * @param {number} pos The new position. Whether this position
+      * is considered from the current position, from the start of
+      * the file or from the end of the file is determined by
+      * argument |whence|.  Note that |pos| may exceed the length of
+      * the file.
+      * @param {number=} whence The reference position. If omitted
+      * or |OS.File.POS_START|, |pos| is relative to the start of the
+      * file.  If |OS.File.POS_CURRENT|, |pos| is relative to the
+      * current position in the file. If |OS.File.POS_END|, |pos| is
+      * relative to the end of the file.
+      *
+      * @return The new position in the file.
+      */
+     File.prototype.setPosition = function setPosition(pos, whence) {
+       if (whence === undefined) {
+         whence = Const.SEEK_SET;
+       }
+       return throw_on_negative("setPosition",
+         UnixFile.lseek(this.fd, pos, whence),
+         this._path
+       );
+     };
+
+     /**
+      * Fetch the information on the file.
+      *
+      * @return File.Info The information on |this| file.
+      */
+     File.prototype.stat = function stat() {
+       throw_on_negative("stat", UnixFile.fstat(this.fd, gStatDataPtr),
+                         this._path);
+       return new File.Info(gStatData, this._path);
+     };
+
+     /**
+      * Set the file's access permissions.
+      *
+      * This operation is likely to fail if applied to a file that was
+      * not created by the currently running program (more precisely,
+      * if it was created by a program running under a different OS-level
+      * user account).  It may also fail, or silently do nothing, if the
+      * filesystem containing the file does not support access permissions.
+      *
+      * @param {*=} options Object specifying the requested permissions:
+      *
+      * - {number} unixMode The POSIX file mode to set on the file.  If omitted,
+      *  the POSIX file mode is reset to the default used by |OS.file.open|.  If
+      *  specified, the permissions will respect the process umask as if they
+      *  had been specified as arguments of |OS.File.open|, unless the
+      *  |unixHonorUmask| parameter tells otherwise.
+      * - {bool} unixHonorUmask If omitted or true, any |unixMode| value is
+      *  modified by the process umask, as |OS.File.open| would have done.  If
+      *  false, the exact value of |unixMode| will be applied.
+      */
+     File.prototype.setPermissions = function setPermissions(options = {}) {
+       throw_on_negative("setPermissions",
+                         UnixFile.fchmod(this.fd, unixMode(options)),
+                         this._path);
+     };
+
+     /**
+      * Set the last access and modification date of the file.
+      * The time stamp resolution is 1 second at best, but might be worse
+      * depending on the platform.
+      *
+      * WARNING: This method is not implemented on Android/B2G. On Android/B2G,
+      * you should use File.setDates instead.
+      *
+      * @param {Date,number=} accessDate The last access date. If numeric,
+      * milliseconds since epoch. If omitted or null, then the current date
+      * will be used.
+      * @param {Date,number=} modificationDate The last modification date. If
+      * numeric, milliseconds since epoch. If omitted or null, then the current
+      * date will be used.
+      *
+      * @throws {TypeError} In case of invalid parameters.
+      * @throws {OS.File.Error} In case of I/O error.
+      */
+     if (SharedAll.Constants.Sys.Name != "Android") {
+       File.prototype.setDates = function(accessDate, modificationDate) {
+         let {value, ptr} = datesToTimevals(accessDate, modificationDate);
+         throw_on_negative("setDates",
+           UnixFile.futimes(this.fd, ptr),
+           this._path);
+       };
+     }
+
+     /**
+      * Flushes the file's buffers and causes all buffered data
+      * to be written.
+      * Disk flushes are very expensive and therefore should be used carefully,
+      * sparingly and only in scenarios where it is vital that data survives
+      * system crashes. Even though the function will be executed off the
+      * main-thread, it might still affect the overall performance of any
+      * running application.
+      *
+      * @throws {OS.File.Error} In case of I/O error.
+      */
+     File.prototype.flush = function flush() {
+       throw_on_negative("flush", UnixFile.fsync(this.fd), this._path);
+     };
+
+     // The default unix mode for opening (0600)
+     const DEFAULT_UNIX_MODE = 384;
+
+     /**
+      * Open a file
+      *
+      * @param {string} path The path to the file.
+      * @param {*=} mode The opening mode for the file, as
+      * an object that may contain the following fields:
+      *
+      * - {bool} truncate If |true|, the file will be opened
+      *  for writing. If the file does not exist, it will be
+      *  created. If the file exists, its contents will be
+      *  erased. Cannot be specified with |create|.
+      * - {bool} create If |true|, the file will be opened
+      *  for writing. If the file exists, this function fails.
+      *  If the file does not exist, it will be created. Cannot
+      *  be specified with |truncate| or |existing|.
+      * - {bool} existing. If the file does not exist, this function
+      *  fails. Cannot be specified with |create|.
+      * - {bool} read If |true|, the file will be opened for
+      *  reading. The file may also be opened for writing, depending
+      *  on the other fields of |mode|.
+      * - {bool} write If |true|, the file will be opened for
+      *  writing. The file may also be opened for reading, depending
+      *  on the other fields of |mode|.
+      * - {bool} append If |true|, the file will be opened for appending,
+      *  meaning the equivalent of |.setPosition(0, POS_END)| is executed
+      *  before each write. The default is |true|, i.e. opening a file for
+      *  appending. Specify |append: false| to open the file in regular mode.
+      *
+      * If neither |truncate|, |create| or |write| is specified, the file
+      * is opened for reading.
+      *
+      * Note that |false|, |null| or |undefined| flags are simply ignored.
+      *
+      * @param {*=} options Additional options for file opening. This
+      * implementation interprets the following fields:
+      *
+      * - {number} unixFlags If specified, file opening flags, as
+      *  per libc function |open|. Replaces |mode|.
+      * - {number} unixMode If specified, a file creation mode,
+      *  as per libc function |open|. If unspecified, files are
+      *  created with a default mode of 0600 (file is private to the
+      *  user, the user can read and write).
+      *
+      * @return {File} A file object.
+      * @throws {OS.File.Error} If the file could not be opened.
+      */
+     File.open = function Unix_open(path, mode, options = {}) {
+       // We don't need to filter for the umask because "open" does this for us.
+       let omode = options.unixMode !== undefined ?
+                     options.unixMode : DEFAULT_UNIX_MODE;
+       let flags;
+       if (options.unixFlags !== undefined) {
+         flags = options.unixFlags;
+       } else {
+         mode = OS.Shared.AbstractFile.normalizeOpenMode(mode);
+         // Handle read/write
+         if (!mode.write) {
+           flags = Const.O_RDONLY;
+         } else if (mode.read) {
+           flags = Const.O_RDWR;
+         } else {
+           flags = Const.O_WRONLY;
+         }
+         // Finally, handle create/existing/trunc
+         if (mode.trunc) {
+           if (mode.existing) {
+             flags |= Const.O_TRUNC;
+           } else {
+             flags |= Const.O_CREAT | Const.O_TRUNC;
+           }
+         } else if (mode.create) {
+           flags |= Const.O_CREAT | Const.O_EXCL;
+         } else if (mode.read && !mode.write) {
+           // flags are sufficient
+         } else if (!mode.existing) {
+           flags |= Const.O_CREAT;
+         }
+         if (mode.append) {
+           flags |= Const.O_APPEND;
+         }
+       }
+       return error_or_file(UnixFile.open(path, flags, omode), path);
+     };
+
+     /**
+      * Checks if a file exists
+      *
+      * @param {string} path The path to the file.
+      *
+      * @return {bool} true if the file exists, false otherwise.
+      */
+     File.exists = function Unix_exists(path) {
+       if (UnixFile.access(path, Const.F_OK) == -1) {
+         return false;
+       } else {
+         return true;
+       }
+     };
+
+     /**
+      * Remove an existing file.
+      *
+      * @param {string} path The name of the file.
+      * @param {*=} options Additional options.
+      *   - {bool} ignoreAbsent If |false|, throw an error if the file does
+      *     not exist. |true| by default.
+      *
+      * @throws {OS.File.Error} In case of I/O error.
+      */
+     File.remove = function remove(path, options = {}) {
+       let result = UnixFile.unlink(path);
+       if (result == -1) {
+         if ((!("ignoreAbsent" in options) || options.ignoreAbsent) &&
+             ctypes.errno == Const.ENOENT) {
+           return;
+         }
+         throw new File.Error("remove", ctypes.errno, path);
+       }
+     };
+
+     /**
+      * Remove an empty directory.
+      *
+      * @param {string} path The name of the directory to remove.
+      * @param {*=} options Additional options.
+      *   - {bool} ignoreAbsent If |false|, throw an error if the directory
+      *     does not exist. |true| by default
+      */
+     File.removeEmptyDir = function removeEmptyDir(path, options = {}) {
+       let result = UnixFile.rmdir(path);
+       if (result == -1) {
+         if ((!("ignoreAbsent" in options) || options.ignoreAbsent) &&
+             ctypes.errno == Const.ENOENT) {
+           return;
+         }
+         throw new File.Error("removeEmptyDir", ctypes.errno, path);
+       }
+     };
+
+     /**
+      * Gets the number of bytes available on disk to the current user.
+      *
+      * @param {string} sourcePath Platform-specific path to a directory on
+      * the disk to query for free available bytes.
+      *
+      * @return {number} The number of bytes available for the current user.
+      * @throws {OS.File.Error} In case of any error.
+      */
+     File.getAvailableFreeSpace = function Unix_getAvailableFreeSpace(sourcePath) {
+       let fileSystemInfo = new Type.statvfs.implementation();
+       let fileSystemInfoPtr = fileSystemInfo.address();
+
+       throw_on_negative("statvfs",  (UnixFile.statvfs || UnixFile.statfs)(sourcePath, fileSystemInfoPtr));
+
+       let bytes = new Type.uint64_t.implementation(
+                        fileSystemInfo.f_bsize * fileSystemInfo.f_bavail);
+
+       return bytes.value;
+     };
+
+     /**
+      * Default mode for opening directories.
+      */
+     const DEFAULT_UNIX_MODE_DIR = Const.S_IRWXU;
+
+     /**
+      * Create a directory.
+      *
+      * @param {string} path The name of the directory.
+      * @param {*=} options Additional options. This
+      * implementation interprets the following fields:
+      *
+      * - {number} unixMode If specified, a file creation mode,
+      * as per libc function |mkdir|. If unspecified, dirs are
+      * created with a default mode of 0700 (dir is private to
+      * the user, the user can read, write and execute).
+      * - {bool} ignoreExisting If |false|, throw error if the directory
+      * already exists. |true| by default
+      * - {string} from If specified, the call to |makeDir| creates all the
+      * ancestors of |path| that are descendants of |from|. Note that |from|
+      * and its existing descendants must be user-writeable and that |path|
+      * must be a descendant of |from|.
+      * Example:
+      *   makeDir(Path.join(profileDir, "foo", "bar"), { from: profileDir });
+      *  creates directories profileDir/foo, profileDir/foo/bar
+       */
+     File._makeDir = function makeDir(path, options = {}) {
+       let omode = options.unixMode !== undefined ? options.unixMode : DEFAULT_UNIX_MODE_DIR;
+       let result = UnixFile.mkdir(path, omode);
+       if (result == -1) {
+         if ((!("ignoreExisting" in options) || options.ignoreExisting) &&
+             (ctypes.errno == Const.EEXIST || ctypes.errno == Const.EISDIR)) {
+           return;
+         }
+         throw new File.Error("makeDir", ctypes.errno, path);
+       }
+     };
+
+     /**
+      * Copy a file to a destination.
+      *
+      * @param {string} sourcePath The platform-specific path at which
+      * the file may currently be found.
+      * @param {string} destPath The platform-specific path at which the
+      * file should be copied.
+      * @param {*=} options An object which may contain the following fields:
+      *
+      * @option {bool} noOverwrite - If set, this function will fail if
+      * a file already exists at |destPath|. Otherwise, if this file exists,
+      * it will be erased silently.
+      *
+      * @throws {OS.File.Error} In case of any error.
+      *
+      * General note: The behavior of this function is defined only when
+      * it is called on a single file. If it is called on a directory, the
+      * behavior is undefined and may not be the same across all platforms.
+      *
+      * General note: The behavior of this function with respect to metadata
+      * is unspecified. Metadata may or may not be copied with the file. The
+      * behavior may not be the same across all platforms.
+      */
+     File.copy = null;
+
+     /**
+      * Move a file to a destination.
+      *
+      * @param {string} sourcePath The platform-specific path at which
+      * the file may currently be found.
+      * @param {string} destPath The platform-specific path at which the
+      * file should be moved.
+      * @param {*=} options An object which may contain the following fields:
+      *
+      * @option {bool} noOverwrite - If set, this function will fail if
+      * a file already exists at |destPath|. Otherwise, if this file exists,
+      * it will be erased silently.
+      * @option {bool} noCopy - If set, this function will fail if the
+      * operation is more sophisticated than a simple renaming, i.e. if
+      * |sourcePath| and |destPath| are not situated on the same device.
+      *
+      * @throws {OS.File.Error} In case of any error.
+      *
+      * General note: The behavior of this function is defined only when
+      * it is called on a single file. If it is called on a directory, the
+      * behavior is undefined and may not be the same across all platforms.
+      *
+      * General note: The behavior of this function with respect to metadata
+      * is unspecified. Metadata may or may not be moved with the file. The
+      * behavior may not be the same across all platforms.
+      */
+     File.move = null;
+
+     if (UnixFile.copyfile) {
+       // This implementation uses |copyfile(3)|, from the BSD library.
+       // Adding copying of hierarchies and/or attributes is just a flag
+       // away.
+       File.copy = function copyfile(sourcePath, destPath, options = {}) {
+         let flags = Const.COPYFILE_DATA;
+         if (options.noOverwrite) {
+           flags |= Const.COPYFILE_EXCL;
+         }
+         throw_on_negative("copy",
+           UnixFile.copyfile(sourcePath, destPath, null, flags),
+           sourcePath
+         );
+       };
+     } else {
+       // If the OS does not implement file copying for us, we need to
+       // implement it ourselves. For this purpose, we need to define
+       // a pumping function.
+
+       /**
+        * Copy bytes from one file to another one.
+        *
+        * @param {File} source The file containing the data to be copied. It
+        * should be opened for reading.
+        * @param {File} dest The file to which the data should be written. It
+        * should be opened for writing.
+        * @param {*=} options An object which may contain the following fields:
+        *
+        * @option {number} nbytes The maximal number of bytes to
+        * copy. If unspecified, copy everything from the current
+        * position.
+        * @option {number} bufSize A hint regarding the size of the
+        * buffer to use for copying. The implementation may decide to
+        * ignore this hint.
+        * @option {bool} unixUserland Will force the copy operation to be
+        * caried out in user land, instead of using optimized syscalls such
+        * as splice(2).
+        *
+        * @throws {OS.File.Error} In case of error.
+        */
+       let pump;
+
+       // A buffer used by |pump_userland|
+       let pump_buffer = null;
+
+       // An implementation of |pump| using |read|/|write|
+       let pump_userland = function pump_userland(source, dest, options = {}) {
+         let bufSize = options.bufSize > 0 ? options.bufSize : 4096;
+         let nbytes = options.nbytes > 0 ? options.nbytes : Infinity;
+         if (!pump_buffer || pump_buffer.length < bufSize) {
+           pump_buffer = new (ctypes.ArrayType(ctypes.char))(bufSize);
+         }
+         let read = source._read.bind(source);
+         let write = dest._write.bind(dest);
+         // Perform actual copy
+         let total_read = 0;
+         while (true) {
+           let chunk_size = Math.min(nbytes, bufSize);
+           let bytes_just_read = read(pump_buffer, bufSize);
+           if (bytes_just_read == 0) {
+             return total_read;
+           }
+           total_read += bytes_just_read;
+           let bytes_written = 0;
+           do {
+             bytes_written += write(
+               pump_buffer.addressOfElement(bytes_written),
+               bytes_just_read - bytes_written
+             );
+           } while (bytes_written < bytes_just_read);
+           nbytes -= bytes_written;
+           if (nbytes <= 0) {
+             return total_read;
+           }
+         }
+       };
+
+       // Fortunately, under Linux, that pumping function can be optimized.
+       if (UnixFile.splice) {
+         const BUFSIZE = 1 << 17;
+
+         // An implementation of |pump| using |splice| (for Linux/Android)
+         pump = function pump_splice(source, dest, options = {}) {
+           let nbytes = options.nbytes > 0 ? options.nbytes : Infinity;
+           let pipe = [];
+           throw_on_negative("pump", UnixFile.pipe(pipe));
+           let pipe_read = pipe[0];
+           let pipe_write = pipe[1];
+           let source_fd = source.fd;
+           let dest_fd = dest.fd;
+           let total_read = 0;
+           let total_written = 0;
+           try {
+             while (true) {
+               let chunk_size = Math.min(nbytes, BUFSIZE);
+               let bytes_read = throw_on_negative("pump",
+                 UnixFile.splice(source_fd, null,
+                 pipe_write, null, chunk_size, 0)
+               );
+               if (!bytes_read) {
+                 break;
+               }
+               total_read += bytes_read;
+               let bytes_written = throw_on_negative(
+                 "pump",
+                 UnixFile.splice(pipe_read, null,
+                   dest_fd, null, bytes_read,
+                     (bytes_read == chunk_size)?Const.SPLICE_F_MORE:0
+               ));
+               if (!bytes_written) {
+                 // This should never happen
+                 throw new Error("Internal error: pipe disconnected");
+               }
+               total_written += bytes_written;
+               nbytes -= bytes_read;
+               if (!nbytes) {
+                 break;
+               }
+             }
+             return total_written;
+           } catch (x) {
+             if (x.unixErrno == Const.EINVAL) {
+               // We *might* be on a file system that does not support splice.
+               // Try again with a fallback pump.
+               if (total_read) {
+                 source.setPosition(-total_read, File.POS_CURRENT);
+               }
+               if (total_written) {
+                 dest.setPosition(-total_written, File.POS_CURRENT);
+               }
+               return pump_userland(source, dest, options);
+             }
+             throw x;
+           } finally {
+             pipe_read.dispose();
+             pipe_write.dispose();
+           }
+         };
+       } else {
+         // Fallback implementation of pump for other Unix platforms.
+         pump = pump_userland;
+       }
+
+       // Implement |copy| using |pump|.
+       // This implementation would require some work before being able to
+       // copy directories
+       File.copy = function copy(sourcePath, destPath, options = {}) {
+         let source, dest;
+         let result;
+         try {
+           source = File.open(sourcePath);
+           // Need to open the output file with |append:false|, or else |splice|
+           // won't work.
+           if (options.noOverwrite) {
+             dest = File.open(destPath, {create:true, append:false});
+           } else {
+             dest = File.open(destPath, {trunc:true, append:false});
+           }
+           if (options.unixUserland) {
+             result = pump_userland(source, dest, options);
+           } else {
+             result = pump(source, dest, options);
+           }
+         } catch (x) {
+           if (dest) {
+             dest.close();
+           }
+           if (source) {
+             source.close();
+           }
+           throw x;
+         }
+       };
+     } // End of definition of copy
+
+     // Implement |move| using |rename| (wherever possible) or |copy|
+     // (if files are on distinct devices).
+     File.move = function move(sourcePath, destPath, options = {}) {
+       // An implementation using |rename| whenever possible or
+       // |File.pump| when required, for other Unices.
+       // It can move directories on one file system, not
+       // across file systems
+
+       // If necessary, fail if the destination file exists
+       if (options.noOverwrite) {
+         let fd = UnixFile.open(destPath, Const.O_RDONLY, 0);
+         if (fd != -1) {
+           fd.dispose();
+           // The file exists and we have access
+           throw new File.Error("move", Const.EEXIST, sourcePath);
+         } else if (ctypes.errno == Const.EACCESS) {
+           // The file exists and we don't have access
+           throw new File.Error("move", Const.EEXIST, sourcePath);
+         }
+       }
+
+       // If we can, rename the file
+       let result = UnixFile.rename(sourcePath, destPath);
+       if (result != -1)
+         return;
+
+       // If the error is not EXDEV ("not on the same device"),
+       // or if the error is EXDEV and we have passed an option
+       // that prevents us from crossing devices, throw the
+       // error.
+       if (ctypes.errno != Const.EXDEV || options.noCopy) {
+         throw new File.Error("move", ctypes.errno, sourcePath);
+       }
+
+       // Otherwise, copy and remove.
+       File.copy(sourcePath, destPath, options);
+       // FIXME: Clean-up in case of copy error?
+       File.remove(sourcePath);
+     };
+
+     File.unixSymLink = function unixSymLink(sourcePath, destPath) {
+       throw_on_negative("symlink", UnixFile.symlink(sourcePath, destPath),
+           sourcePath);
+     };
+
+     /**
+      * Iterate on one directory.
+      *
+      * This iterator will not enter subdirectories.
+      *
+      * @param {string} path The directory upon which to iterate.
+      * @param {*=} options Ignored in this implementation.
+      *
+      * @throws {File.Error} If |path| does not represent a directory or
+      * if the directory cannot be iterated.
+      * @constructor
+      */
+     File.DirectoryIterator = function DirectoryIterator(path, options) {
+       exports.OS.Shared.AbstractFile.AbstractIterator.call(this);
+       this._path = path;
+       this._dir = UnixFile.opendir(this._path);
+       if (this._dir == null) {
+         let error = ctypes.errno;
+         if (error != Const.ENOENT) {
+           throw new File.Error("DirectoryIterator", error, path);
+         }
+         this._exists = false;
+         this._closed = true;
+       } else {
+         this._exists = true;
+         this._closed = false;
+       }
+     };
+     File.DirectoryIterator.prototype = Object.create(exports.OS.Shared.AbstractFile.AbstractIterator.prototype);
+
+     /**
+      * Return the next entry in the directory, if any such entry is
+      * available.
+      *
+      * Skip special directories "." and "..".
+      *
+      * @return {File.Entry} The next entry in the directory.
+      * @throws {StopIteration} Once all files in the directory have been
+      * encountered.
+      */
+     File.DirectoryIterator.prototype.next = function next() {
+       if (!this._exists) {
+         throw File.Error.noSuchFile("DirectoryIterator.prototype.next", this._path);
+       }
+       if (this._closed) {
+         throw StopIteration;
+       }
+       for (let entry = UnixFile.readdir(this._dir);
+            entry != null && !entry.isNull();
+            entry = UnixFile.readdir(this._dir)) {
+         let contents = entry.contents;
+         let name = contents.d_name.readString();
+         if (name == "." || name == "..") {
+           continue;
+         }
+
+         let isDir, isSymLink;
+         if (!("d_type" in contents)) {
+           // |dirent| doesn't have d_type on some platforms (e.g. Solaris).
+           let path = Path.join(this._path, name);
+           throw_on_negative("lstat", UnixFile.lstat(path, gStatDataPtr), this._path);
+           isDir = (gStatData.st_mode & Const.S_IFMT) == Const.S_IFDIR;
+           isSymLink = (gStatData.st_mode & Const.S_IFMT) == Const.S_IFLNK;
+         } else {
+           isDir = contents.d_type == Const.DT_DIR;
+           isSymLink = contents.d_type == Const.DT_LNK;
+         }
+
+         return new File.DirectoryIterator.Entry(isDir, isSymLink, name, this._path);
+       }
+       this.close();
+       throw StopIteration;
+     };
+
+     /**
+      * Close the iterator and recover all resources.
+      * You should call this once you have finished iterating on a directory.
+      */
+     File.DirectoryIterator.prototype.close = function close() {
+       if (this._closed) return;
+       this._closed = true;
+       UnixFile.closedir(this._dir);
+       this._dir = null;
+     };
+
+    /**
+     * Determine whether the directory exists.
+     *
+     * @return {boolean}
+     */
+     File.DirectoryIterator.prototype.exists = function exists() {
+       return this._exists;
+     };
+
+     /**
+      * Return directory as |File|
+      */
+     File.DirectoryIterator.prototype.unixAsFile = function unixAsFile() {
+       if (!this._dir) throw File.Error.closed("unixAsFile", this._path);
+       return error_or_file(UnixFile.dirfd(this._dir), this._path);
+     };
+
+     /**
+      * An entry in a directory.
+      */
+     File.DirectoryIterator.Entry = function Entry(isDir, isSymLink, name, parent) {
+       // Copy the relevant part of |unix_entry| to ensure that
+       // our data is not overwritten prematurely.
+       this._parent = parent;
+       let path = Path.join(this._parent, name);
+
+       SysAll.AbstractEntry.call(this, isDir, isSymLink, name, path);
+     };
+     File.DirectoryIterator.Entry.prototype = Object.create(SysAll.AbstractEntry.prototype);
+
+     /**
+      * Return a version of an instance of
+      * File.DirectoryIterator.Entry that can be sent from a worker
+      * thread to the main thread. Note that deserialization is
+      * asymmetric and returns an object with a different
+      * implementation.
+      */
+     File.DirectoryIterator.Entry.toMsg = function toMsg(value) {
+       if (!value instanceof File.DirectoryIterator.Entry) {
+         throw new TypeError("parameter of " +
+           "File.DirectoryIterator.Entry.toMsg must be a " +
+           "File.DirectoryIterator.Entry");
+       }
+       let serialized = {};
+       for (let key in File.DirectoryIterator.Entry.prototype) {
+         serialized[key] = value[key];
+       }
+       return serialized;
+     };
+
+     let gStatData = new Type.stat.implementation();
+     let gStatDataPtr = gStatData.address();
+
+     let MODE_MASK = 4095 /*= 07777*/;
+     File.Info = function Info(stat, path) {
+       let isDir = (stat.st_mode & Const.S_IFMT) == Const.S_IFDIR;
+       let isSymLink = (stat.st_mode & Const.S_IFMT) == Const.S_IFLNK;
+       let size = Type.off_t.importFromC(stat.st_size);
+
+       let lastAccessDate = new Date(stat.st_atime * 1000);
+       let lastModificationDate = new Date(stat.st_mtime * 1000);
+       let unixLastStatusChangeDate = new Date(stat.st_ctime * 1000);
+
+       let unixOwner = Type.uid_t.importFromC(stat.st_uid);
+       let unixGroup = Type.gid_t.importFromC(stat.st_gid);
+       let unixMode = Type.mode_t.importFromC(stat.st_mode & MODE_MASK);
+
+       SysAll.AbstractInfo.call(this, path, isDir, isSymLink, size,
+           lastAccessDate, lastModificationDate, unixLastStatusChangeDate,
+           unixOwner, unixGroup, unixMode);
+
+       // Some platforms (e.g. MacOS X, some BSDs) store a file creation date
+       if ("OSFILE_OFFSETOF_STAT_ST_BIRTHTIME" in Const) {
+         let date = new Date(stat.st_birthtime * 1000);
+
+        /**
+         * The date of creation of this file.
+         *
+         * Note that the date returned by this method is not always
+         * reliable. Not all file systems are able to provide this
+         * information.
+         *
+         * @type {Date}
+         */
+         this.macBirthDate = date;
+       }
+     };
+     File.Info.prototype = Object.create(SysAll.AbstractInfo.prototype);
+
+     // Deprecated, use macBirthDate/winBirthDate instead
+     Object.defineProperty(File.Info.prototype, "creationDate", {
+      get: function creationDate() {
+        // On the Macintosh, returns the birth date if available.
+        // On other Unix, as the birth date is not available,
+        // returns the epoch.
+        return this.macBirthDate || new Date(0);
+      }
+     });
+
+     /**
+      * Return a version of an instance of File.Info that can be sent
+      * from a worker thread to the main thread. Note that deserialization
+      * is asymmetric and returns an object with a different implementation.
+      */
+     File.Info.toMsg = function toMsg(stat) {
+       if (!stat instanceof File.Info) {
+         throw new TypeError("parameter of File.Info.toMsg must be a File.Info");
+       }
+       let serialized = {};
+       for (let key in File.Info.prototype) {
+         serialized[key] = stat[key];
+       }
+       return serialized;
+     };
+
+     /**
+      * Fetch the information on a file.
+      *
+      * @param {string} path The full name of the file to open.
+      * @param {*=} options Additional options. In this implementation:
+      *
+      * - {bool} unixNoFollowingLinks If set and |true|, if |path|
+      * represents a symbolic link, the call will return the information
+      * of the link itself, rather than that of the target file.
+      *
+      * @return {File.Information}
+      */
+     File.stat = function stat(path, options = {}) {
+       if (options.unixNoFollowingLinks) {
+         throw_on_negative("stat", UnixFile.lstat(path, gStatDataPtr), path);
+       } else {
+         throw_on_negative("stat", UnixFile.stat(path, gStatDataPtr), path);
+       }
+       return new File.Info(gStatData, path);
+     };
+
+     /**
+      * Set the file's access permissions.
+      *
+      * This operation is likely to fail if applied to a file that was
+      * not created by the currently running program (more precisely,
+      * if it was created by a program running under a different OS-level
+      * user account).  It may also fail, or silently do nothing, if the
+      * filesystem containing the file does not support access permissions.
+      *
+      * @param {string} path The name of the file to reset the permissions of.
+      * @param {*=} options Object specifying the requested permissions:
+      *
+      * - {number} unixMode The POSIX file mode to set on the file.  If omitted,
+      *  the POSIX file mode is reset to the default used by |OS.file.open|.  If
+      *  specified, the permissions will respect the process umask as if they
+      *  had been specified as arguments of |OS.File.open|, unless the
+      *  |unixHonorUmask| parameter tells otherwise.
+      * - {bool} unixHonorUmask If omitted or true, any |unixMode| value is
+      *  modified by the process umask, as |OS.File.open| would have done.  If
+      *  false, the exact value of |unixMode| will be applied.
+      */
+     File.setPermissions = function setPermissions(path, options = {}) {
+       throw_on_negative("setPermissions",
+                         UnixFile.chmod(path, unixMode(options)),
+                         path);
+     };
+
+     /**
+      * Convert an access date and a modification date to an array
+      * of two |timeval|.
+      */
+     function datesToTimevals(accessDate, modificationDate) {
+       accessDate = normalizeDate("File.setDates", accessDate);
+       modificationDate = normalizeDate("File.setDates", modificationDate);
+
+       let timevals = new Type.timevals.implementation();
+       let timevalsPtr = timevals.address();
+
+       timevals[0].tv_sec = (accessDate / 1000) | 0;
+       timevals[0].tv_usec = 0;
+       timevals[1].tv_sec = (modificationDate / 1000) | 0;
+       timevals[1].tv_usec = 0;
+
+       return { value: timevals, ptr: timevalsPtr };
+     }
+
+     /**
+      * Set the last access and modification date of the file.
+      * The time stamp resolution is 1 second at best, but might be worse
+      * depending on the platform.
+      *
+      * @param {string} path The full name of the file to set the dates for.
+      * @param {Date,number=} accessDate The last access date. If numeric,
+      * milliseconds since epoch. If omitted or null, then the current date
+      * will be used.
+      * @param {Date,number=} modificationDate The last modification date. If
+      * numeric, milliseconds since epoch. If omitted or null, then the current
+      * date will be used.
+      *
+      * @throws {TypeError} In case of invalid paramters.
+      * @throws {OS.File.Error} In case of I/O error.
+      */
+     File.setDates = function setDates(path, accessDate, modificationDate) {
+       let {value, ptr} = datesToTimevals(accessDate, modificationDate);
+       throw_on_negative("setDates",
+                         UnixFile.utimes(path, ptr),
+                         path);
+     };
+
+     File.read = exports.OS.Shared.AbstractFile.read;
+     File.writeAtomic = exports.OS.Shared.AbstractFile.writeAtomic;
+     File.openUnique = exports.OS.Shared.AbstractFile.openUnique;
+     File.makeDir = exports.OS.Shared.AbstractFile.makeDir;
+
+     /**
+      * Remove an existing directory and its contents.
+      *
+      * @param {string} path The name of the directory.
+      * @param {*=} options Additional options.
+      *   - {bool} ignoreAbsent If |false|, throw an error if the directory doesn't
+      *     exist. |true| by default.
+      *   - {boolean} ignorePermissions If |true|, remove the file even when lacking write
+      *     permission.
+      *
+      * @throws {OS.File.Error} In case of I/O error, in particular if |path| is
+      *         not a directory.
+      *
+      * Note: This function will remove a symlink even if it points a directory.
+      */
+     File.removeDir = function(path, options = {}) {
+       let isSymLink;
+       try {
+         let info = File.stat(path, {unixNoFollowingLinks: true});
+         isSymLink = info.isSymLink;
+       } catch (e) {
+         if ((!("ignoreAbsent" in options) || options.ignoreAbsent) &&
+             ctypes.errno == Const.ENOENT) {
+           return;
+         }
+         throw e;
+       }
+       if (isSymLink) {
+         // A Unix symlink itself is not a directory even if it points
+         // a directory.
+         File.remove(path, options);
+         return;
+       }
+       exports.OS.Shared.AbstractFile.removeRecursive(path, options);
+     };
+
+     /**
+      * Get the current directory by getCurrentDirectory.
+      */
+     File.getCurrentDirectory = function getCurrentDirectory() {
+       let path, buf;
+       if (UnixFile.get_current_dir_name) {
+	 path = UnixFile.get_current_dir_name();
+       } else if (UnixFile.getwd_auto) {
+         path = UnixFile.getwd_auto(null);
+       } else {
+	 for (let length = Const.PATH_MAX; !path; length *= 2) {
+	   buf = new (ctypes.char.array(length));
+	   path = UnixFile.getcwd(buf, length);
+	 };
+       }
+       throw_on_null("getCurrentDirectory", path);
+       return path.readString();
+     };
+
+     /**
+      * Set the current directory by setCurrentDirectory.
+      */
+     File.setCurrentDirectory = function setCurrentDirectory(path) {
+       throw_on_negative("setCurrentDirectory",
+         UnixFile.chdir(path),
+         path
+       );
+     };
+
+     /**
+      * Get/set the current directory.
+      */
+     Object.defineProperty(File, "curDir", {
+         set: function(path) {
+           this.setCurrentDirectory(path);
+         },
+         get: function() {
+           return this.getCurrentDirectory();
+         }
+       }
+     );
+
+     // Utility functions
+
+     /**
+      * Turn the result of |open| into an Error or a File
+      * @param {number} maybe The result of the |open| operation that may
+      * represent either an error or a success. If -1, this function raises
+      * an error holding ctypes.errno, otherwise it returns the opened file.
+      * @param {string=} path The path of the file.
+      */
+     function error_or_file(maybe, path) {
+       if (maybe == -1) {
+         throw new File.Error("open", ctypes.errno, path);
+       }
+       return new File(maybe, path);
+     }
+
+     /**
+      * Utility function to sort errors represented as "-1" from successes.
+      *
+      * @param {string=} operation The name of the operation. If unspecified,
+      * the name of the caller function.
+      * @param {number} result The result of the operation that may
+      * represent either an error or a success. If -1, this function raises
+      * an error holding ctypes.errno, otherwise it returns |result|.
+      * @param {string=} path The path of the file.
+      */
+     function throw_on_negative(operation, result, path) {
+       if (result < 0) {
+         throw new File.Error(operation, ctypes.errno, path);
+       }
+       return result;
+     }
+
+     /**
+      * Utility function to sort errors represented as |null| from successes.
+      *
+      * @param {string=} operation The name of the operation. If unspecified,
+      * the name of the caller function.
+      * @param {pointer} result The result of the operation that may
+      * represent either an error or a success. If |null|, this function raises
+      * an error holding ctypes.errno, otherwise it returns |result|.
+      * @param {string=} path The path of the file.
+      */
+     function throw_on_null(operation, result, path) {
+       if (result == null || (result.isNull && result.isNull())) {
+         throw new File.Error(operation, ctypes.errno, path);
+       }
+       return result;
+     }
+
+     /**
+      * Normalize and verify a Date or numeric date value.
+      *
+      * @param {string} fn Function name of the calling function.
+      * @param {Date,number} date The date to normalize. If omitted or null,
+      * then the current date will be used.
+      *
+      * @throws {TypeError} Invalid date provided.
+      *
+      * @return {number} Sanitized, numeric date in milliseconds since epoch.
+      */
+     function normalizeDate(fn, date) {
+       if (typeof date !== "number" && !date) {
+         // |date| was Omitted or null.
+         date = Date.now();
+       } else if (typeof date.getTime === "function") {
+         // Input might be a date or date-like object.
+         date = date.getTime();
+       }
+
+       if (typeof date !== "number" || Number.isNaN(date)) {
+         throw new TypeError("|date| parameter of " + fn + " must be a " +
+                             "|Date| instance or number");
+       }
+       return date;
+     };
+
+     /**
+      * Helper used by both versions of setPermissions.
+      */
+     function unixMode(options) {
+       let mode = options.unixMode !== undefined ?
+                    options.unixMode : DEFAULT_UNIX_MODE;
+       let unixHonorUmask = true;
+       if ("unixHonorUmask" in options) {
+         unixHonorUmask = options.unixHonorUmask;
+       }
+       if (unixHonorUmask) {
+         mode &= ~SharedAll.Constants.Sys.umask;
+       }
+       return mode;
+     }
+
+     File.Unix = exports.OS.Unix.File;
+     File.Error = SysAll.Error;
+     exports.OS.File = File;
+     exports.OS.Shared.Type = Type;
+
+     Object.defineProperty(File, "POS_START", { value: SysAll.POS_START });
+     Object.defineProperty(File, "POS_CURRENT", { value: SysAll.POS_CURRENT });
+     Object.defineProperty(File, "POS_END", { value: SysAll.POS_END });
+   })(this);
+}
diff --git a/toolkit/components/osfile/modules/osfile_win_allthreads.jsm b/toolkit/components/osfile/modules/osfile_win_allthreads.jsm
new file mode 100644
index 000000000..b059d4e12
--- /dev/null
+++ b/toolkit/components/osfile/modules/osfile_win_allthreads.jsm
@@ -0,0 +1,425 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * This module defines the thread-agnostic components of the Win version
+ * of OS.File. It depends on the thread-agnostic cross-platform components
+ * of OS.File.
+ *
+ * It serves the following purposes:
+ * - open kernel32;
+ * - define OS.Shared.Win.Error;
+ * - define a few constants and types that need to be defined on all platforms.
+ *
+ * This module can be:
+ * - opened from the main thread as a jsm module;
+ * - opened from a chrome worker through require().
+ */
+
+"use strict";
+
+var SharedAll;
+if (typeof Components != "undefined") {
+  let Cu = Components.utils;
+  // Module is opened as a jsm module
+  Cu.import("resource://gre/modules/ctypes.jsm", this);
+
+  SharedAll = {};
+  Cu.import("resource://gre/modules/osfile/osfile_shared_allthreads.jsm", SharedAll);
+  this.exports = {};
+} else if (typeof module != "undefined" && typeof require != "undefined") {
+  // Module is loaded with require()
+  SharedAll = require("resource://gre/modules/osfile/osfile_shared_allthreads.jsm");
+} else {
+  throw new Error("Please open this module with Component.utils.import or with require()");
+}
+
+var LOG = SharedAll.LOG.bind(SharedAll, "Win", "allthreads");
+var Const = SharedAll.Constants.Win;
+
+// Open libc
+var libc = new SharedAll.Library("libc", "kernel32.dll");
+exports.libc = libc;
+
+// Define declareFFI
+var declareFFI = SharedAll.declareFFI.bind(null, libc);
+exports.declareFFI = declareFFI;
+
+var Scope = {};
+
+// Define Error
+libc.declareLazy(Scope, "FormatMessage",
+                 "FormatMessageW", ctypes.winapi_abi,
+                 /*return*/ ctypes.uint32_t,
+                 /*flags*/  ctypes.uint32_t,
+                 /*source*/ ctypes.voidptr_t,
+                 /*msgid*/  ctypes.uint32_t,
+                 /*langid*/ ctypes.uint32_t,
+                 /*buf*/    ctypes.char16_t.ptr,
+                 /*size*/   ctypes.uint32_t,
+                 /*Arguments*/ctypes.voidptr_t);
+
+/**
+ * A File-related error.
+ *
+ * To obtain a human-readable error message, use method |toString|.
+ * To determine the cause of the error, use the various |becauseX|
+ * getters. To determine the operation that failed, use field
+ * |operation|.
+ *
+ * Additionally, this implementation offers a field
+ * |winLastError|, which holds the OS-specific error
+ * constant. If you need this level of detail, you may match the value
+ * of this field against the error constants of |OS.Constants.Win|.
+ *
+ * @param {string=} operation The operation that failed. If unspecified,
+ * the name of the calling function is taken to be the operation that
+ * failed.
+ * @param {number=} lastError The OS-specific constant detailing the
+ * reason of the error. If unspecified, this is fetched from the system
+ * status.
+ * @param {string=} path The file path that manipulated. If unspecified,
+ * assign the empty string.
+ *
+ * @constructor
+ * @extends {OS.Shared.Error}
+ */
+var OSError = function OSError(operation = "unknown operation",
+                               lastError = ctypes.winLastError, path = "") {
+  operation = operation;
+  SharedAll.OSError.call(this, operation, path);
+  this.winLastError = lastError;
+};
+OSError.prototype = Object.create(SharedAll.OSError.prototype);
+OSError.prototype.toString = function toString() {
+  let buf = new (ctypes.ArrayType(ctypes.char16_t, 1024))();
+  let result = Scope.FormatMessage(
+    Const.FORMAT_MESSAGE_FROM_SYSTEM |
+    Const.FORMAT_MESSAGE_IGNORE_INSERTS,
+    null,
+    /* The error number */ this.winLastError,
+    /* Default language */ 0,
+    /* Output buffer*/     buf,
+    /* Minimum size of buffer */ 1024,
+    /* Format args*/       null
+  );
+  if (!result) {
+    buf = "additional error " +
+      ctypes.winLastError +
+      " while fetching system error message";
+  }
+  return "Win error " + this.winLastError + " during operation "
+    + this.operation + (this.path? " on file " + this.path : "") +
+    " (" + buf.readString() + ")";
+};
+OSError.prototype.toMsg = function toMsg() {
+  return OSError.toMsg(this);
+};
+
+/**
+ * |true| if the error was raised because a file or directory
+ * already exists, |false| otherwise.
+ */
+Object.defineProperty(OSError.prototype, "becauseExists", {
+  get: function becauseExists() {
+    return this.winLastError == Const.ERROR_FILE_EXISTS ||
+      this.winLastError == Const.ERROR_ALREADY_EXISTS;
+  }
+});
+/**
+ * |true| if the error was raised because a file or directory
+ * does not exist, |false| otherwise.
+ */
+Object.defineProperty(OSError.prototype, "becauseNoSuchFile", {
+  get: function becauseNoSuchFile() {
+    return this.winLastError == Const.ERROR_FILE_NOT_FOUND ||
+      this.winLastError == Const.ERROR_PATH_NOT_FOUND;
+  }
+});
+/**
+ * |true| if the error was raised because a directory is not empty
+ * does not exist, |false| otherwise.
+ */
+Object.defineProperty(OSError.prototype, "becauseNotEmpty", {
+  get: function becauseNotEmpty() {
+    return this.winLastError == Const.ERROR_DIR_NOT_EMPTY;
+  }
+});
+/**
+ * |true| if the error was raised because a file or directory
+ * is closed, |false| otherwise.
+ */
+Object.defineProperty(OSError.prototype, "becauseClosed", {
+  get: function becauseClosed() {
+    return this.winLastError == Const.ERROR_INVALID_HANDLE;
+  }
+});
+/**
+ * |true| if the error was raised because permission is denied to
+ * access a file or directory, |false| otherwise.
+ */
+Object.defineProperty(OSError.prototype, "becauseAccessDenied", {
+  get: function becauseAccessDenied() {
+    return this.winLastError == Const.ERROR_ACCESS_DENIED;
+  }
+});
+/**
+ * |true| if the error was raised because some invalid argument was passed,
+ * |false| otherwise.
+ */
+Object.defineProperty(OSError.prototype, "becauseInvalidArgument", {
+  get: function becauseInvalidArgument() {
+    return this.winLastError == Const.ERROR_NOT_SUPPORTED ||
+           this.winLastError == Const.ERROR_BAD_ARGUMENTS;
+  }
+});
+
+/**
+ * Serialize an instance of OSError to something that can be
+ * transmitted across threads (not necessarily a string).
+ */
+OSError.toMsg = function toMsg(error) {
+  return {
+    exn: "OS.File.Error",
+    fileName: error.moduleName,
+    lineNumber: error.lineNumber,
+    stack: error.moduleStack,
+    operation: error.operation,
+    winLastError: error.winLastError,
+    path: error.path
+  };
+};
+
+/**
+ * Deserialize a message back to an instance of OSError
+ */
+OSError.fromMsg = function fromMsg(msg) {
+  let error = new OSError(msg.operation, msg.winLastError, msg.path);
+  error.stack = msg.stack;
+  error.fileName = msg.fileName;
+  error.lineNumber = msg.lineNumber;
+  return error;
+};
+exports.Error = OSError;
+
+/**
+ * Code shared by implementation of File.Info on Windows
+ *
+ * @constructor
+ */
+var AbstractInfo = function AbstractInfo(path, isDir, isSymLink, size,
+                                         winBirthDate,
+                                         lastAccessDate, lastWriteDate,
+                                         winAttributes) {
+  this._path = path;
+  this._isDir = isDir;
+  this._isSymLink = isSymLink;
+  this._size = size;
+  this._winBirthDate = winBirthDate;
+  this._lastAccessDate = lastAccessDate;
+  this._lastModificationDate = lastWriteDate;
+  this._winAttributes = winAttributes;
+};
+
+AbstractInfo.prototype = {
+  /**
+   * The path of the file, used for error-reporting.
+   *
+   * @type {string}
+   */
+  get path() {
+    return this._path;
+  },
+  /**
+   * |true| if this file is a directory, |false| otherwise
+   */
+  get isDir() {
+    return this._isDir;
+  },
+  /**
+   * |true| if this file is a symbolic link, |false| otherwise
+   */
+  get isSymLink() {
+    return this._isSymLink;
+  },
+  /**
+   * The size of the file, in bytes.
+   *
+   * Note that the result may be |NaN| if the size of the file cannot be
+   * represented in JavaScript.
+   *
+   * @type {number}
+   */
+  get size() {
+    return this._size;
+  },
+  // Deprecated
+  get creationDate() {
+    return this._winBirthDate;
+  },
+  /**
+   * The date of creation of this file.
+   *
+   * @type {Date}
+   */
+  get winBirthDate() {
+    return this._winBirthDate;
+  },
+  /**
+   * The date of last access to this file.
+   *
+   * Note that the definition of last access may depend on the underlying
+   * operating system and file system.
+   *
+   * @type {Date}
+   */
+  get lastAccessDate() {
+    return this._lastAccessDate;
+  },
+  /**
+   * The date of last modification of this file.
+   *
+   * Note that the definition of last access may depend on the underlying
+   * operating system and file system.
+   *
+   * @type {Date}
+   */
+  get lastModificationDate() {
+    return this._lastModificationDate;
+  },
+  /**
+   * The Object with following boolean properties of this file.
+   * {readOnly, system, hidden}
+   *
+   * @type {object}
+   */
+  get winAttributes() {
+    return this._winAttributes;
+  }
+};
+exports.AbstractInfo = AbstractInfo;
+
+/**
+ * Code shared by implementation of File.DirectoryIterator.Entry on Windows
+ *
+ * @constructor
+ */
+var AbstractEntry = function AbstractEntry(isDir, isSymLink, name,
+                                           winCreationDate, winLastWriteDate,
+                                           winLastAccessDate, path) {
+  this._isDir = isDir;
+  this._isSymLink = isSymLink;
+  this._name = name;
+  this._winCreationDate = winCreationDate;
+  this._winLastWriteDate = winLastWriteDate;
+  this._winLastAccessDate = winLastAccessDate;
+  this._path = path;
+};
+
+AbstractEntry.prototype = {
+  /**
+   * |true| if the entry is a directory, |false| otherwise
+   */
+  get isDir() {
+    return this._isDir;
+  },
+  /**
+   * |true| if the entry is a symbolic link, |false| otherwise
+   */
+  get isSymLink() {
+    return this._isSymLink;
+  },
+  /**
+   * The name of the entry.
+   * @type {string}
+   */
+  get name() {
+    return this._name;
+  },
+  /**
+   * The creation time of this file.
+   * @type {Date}
+   */
+  get winCreationDate() {
+    return this._winCreationDate;
+  },
+  /**
+   * The last modification time of this file.
+   * @type {Date}
+   */
+  get winLastWriteDate() {
+    return this._winLastWriteDate;
+  },
+  /**
+   * The last access time of this file.
+   * @type {Date}
+   */
+  get winLastAccessDate() {
+    return this._winLastAccessDate;
+  },
+  /**
+   * The full path of the entry
+   * @type {string}
+   */
+  get path() {
+    return this._path;
+  }
+};
+exports.AbstractEntry = AbstractEntry;
+
+// Special constants that need to be defined on all platforms
+
+exports.POS_START = Const.FILE_BEGIN;
+exports.POS_CURRENT = Const.FILE_CURRENT;
+exports.POS_END = Const.FILE_END;
+
+// Special types that need to be defined for communication
+// between threads
+var Type = Object.create(SharedAll.Type);
+exports.Type = Type;
+
+/**
+ * Native paths
+ *
+ * Under Windows, expressed as wide strings
+ */
+Type.path = Type.wstring.withName("[in] path");
+Type.out_path = Type.out_wstring.withName("[out] path");
+
+// Special constructors that need to be defined on all threads
+OSError.closed = function closed(operation, path) {
+  return new OSError(operation, Const.ERROR_INVALID_HANDLE, path);
+};
+
+OSError.exists = function exists(operation, path) {
+  return new OSError(operation, Const.ERROR_FILE_EXISTS, path);
+};
+
+OSError.noSuchFile = function noSuchFile(operation, path) {
+  return new OSError(operation, Const.ERROR_FILE_NOT_FOUND, path);
+};
+
+OSError.invalidArgument = function invalidArgument(operation) {
+  return new OSError(operation, Const.ERROR_NOT_SUPPORTED);
+};
+
+var EXPORTED_SYMBOLS = [
+  "declareFFI",
+  "libc",
+  "Error",
+  "AbstractInfo",
+  "AbstractEntry",
+  "Type",
+  "POS_START",
+  "POS_CURRENT",
+  "POS_END"
+];
+
+//////////// Boilerplate
+if (typeof Components != "undefined") {
+  this.EXPORTED_SYMBOLS = EXPORTED_SYMBOLS;
+  for (let symbol of EXPORTED_SYMBOLS) {
+    this[symbol] = exports[symbol];
+  }
+}
diff --git a/toolkit/components/osfile/modules/osfile_win_back.jsm b/toolkit/components/osfile/modules/osfile_win_back.jsm
new file mode 100644
index 000000000..21172b6bf
--- /dev/null
+++ b/toolkit/components/osfile/modules/osfile_win_back.jsm
@@ -0,0 +1,437 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * This file can be used in the following contexts:
+ *
+ *  1. included from a non-osfile worker thread using importScript
+ *   (it serves to define a synchronous API for that worker thread)
+ *   (bug 707681)
+ *
+ *  2. included from the main thread using Components.utils.import
+ *   (it serves to define the asynchronous API, whose implementation
+ *    resides in the worker thread)
+ *   (bug 729057)
+ *
+ * 3. included from the osfile worker thread using importScript
+ *   (it serves to define the implementation of the asynchronous API)
+ *   (bug 729057)
+ */
+
+{
+  if (typeof Components != "undefined") {
+    // We do not wish osfile_win.jsm to be used directly as a main thread
+    // module yet. When time comes, it will be loaded by a combination of
+    // a main thread front-end/worker thread implementation that makes sure
+    // that we are not executing synchronous IO code in the main thread.
+
+    throw new Error("osfile_win.jsm cannot be used from the main thread yet");
+  }
+
+  (function(exports) {
+     "use strict";
+     if (exports.OS && exports.OS.Win && exports.OS.Win.File) {
+       return; // Avoid double initialization
+     }
+
+     let SharedAll = require("resource://gre/modules/osfile/osfile_shared_allthreads.jsm");
+     let SysAll = require("resource://gre/modules/osfile/osfile_win_allthreads.jsm");
+     let LOG = SharedAll.LOG.bind(SharedAll, "Unix", "back");
+     let libc = SysAll.libc;
+     let advapi32 = new SharedAll.Library("advapi32", "advapi32.dll");
+     let Const = SharedAll.Constants.Win;
+
+     /**
+      * Initialize the Windows module.
+      *
+      * @param {function=} declareFFI
+      */
+     // FIXME: Both |init| and |aDeclareFFI| are deprecated, we should remove them
+     let init = function init(aDeclareFFI) {
+       let declareFFI;
+       if (aDeclareFFI) {
+         declareFFI = aDeclareFFI.bind(null, libc);
+       } else {
+         declareFFI = SysAll.declareFFI;
+       }
+       let declareLazyFFI = SharedAll.declareLazyFFI;
+
+       // Initialize types that require additional OS-specific
+       // support - either finalization or matching against
+       // OS-specific constants.
+       let Type = Object.create(SysAll.Type);
+       let SysFile = exports.OS.Win.File = { Type: Type };
+
+       // Initialize types
+
+       /**
+        * A C integer holding INVALID_HANDLE_VALUE in case of error or
+        * a file descriptor in case of success.
+        */
+       Type.HANDLE =
+         Type.voidptr_t.withName("HANDLE");
+       Type.HANDLE.importFromC = function importFromC(maybe) {
+         if (Type.int.cast(maybe).value == INVALID_HANDLE) {
+           // Ensure that API clients can effectively compare against
+           // Const.INVALID_HANDLE_VALUE. Without this cast,
+           // == would always return |false|.
+           return INVALID_HANDLE;
+         }
+         return ctypes.CDataFinalizer(maybe, this.finalizeHANDLE);
+       };
+       Type.HANDLE.finalizeHANDLE = function placeholder() {
+         throw new Error("finalizeHANDLE should be implemented");
+       };
+       let INVALID_HANDLE = Const.INVALID_HANDLE_VALUE;
+
+       Type.file_HANDLE = Type.HANDLE.withName("file HANDLE");
+       SharedAll.defineLazyGetter(Type.file_HANDLE,
+         "finalizeHANDLE",
+         function() {
+           return SysFile._CloseHandle;
+         });
+
+       Type.find_HANDLE = Type.HANDLE.withName("find HANDLE");
+       SharedAll.defineLazyGetter(Type.find_HANDLE,
+         "finalizeHANDLE",
+         function() {
+           return SysFile._FindClose;
+         });
+
+       Type.DWORD = Type.uint32_t.withName("DWORD");
+
+       /* A special type used to represent flags passed as DWORDs to a function.
+        * In JavaScript, bitwise manipulation of numbers, such as or-ing flags,
+        * can produce negative numbers. Since DWORD is unsigned, these negative
+        * numbers simply cannot be converted to DWORD. For this reason, whenever
+        * bit manipulation is called for, you should rather use DWORD_FLAGS,
+        * which is represented as a signed integer, hence has the correct
+        * semantics.
+        */
+       Type.DWORD_FLAGS = Type.int32_t.withName("DWORD_FLAGS");
+
+       /**
+        * A C integer holding 0 in case of error or a positive integer
+        * in case of success.
+        */
+       Type.zero_or_DWORD =
+         Type.DWORD.withName("zero_or_DWORD");
+
+       /**
+        * A C integer holding 0 in case of error, any other value in
+        * case of success.
+        */
+       Type.zero_or_nothing =
+         Type.int.withName("zero_or_nothing");
+
+       /**
+        * A C integer holding flags related to NTFS security.
+        */
+       Type.SECURITY_ATTRIBUTES =
+         Type.void_t.withName("SECURITY_ATTRIBUTES");
+
+       /**
+        * A C integer holding pointers related to NTFS security.
+        */
+       Type.PSID =
+         Type.voidptr_t.withName("PSID");
+
+       Type.PACL =
+         Type.voidptr_t.withName("PACL");
+
+       Type.PSECURITY_DESCRIPTOR =
+         Type.voidptr_t.withName("PSECURITY_DESCRIPTOR");
+
+       /**
+        * A C integer holding Win32 local memory handle.
+        */
+       Type.HLOCAL =
+         Type.voidptr_t.withName("HLOCAL");
+
+       Type.FILETIME =
+         new SharedAll.Type("FILETIME",
+                  ctypes.StructType("FILETIME", [
+                  { lo: Type.DWORD.implementation },
+                  { hi: Type.DWORD.implementation }]));
+
+       Type.FindData =
+         new SharedAll.Type("FIND_DATA",
+                  ctypes.StructType("FIND_DATA", [
+                    { dwFileAttributes: ctypes.uint32_t },
+                    { ftCreationTime:   Type.FILETIME.implementation },
+                    { ftLastAccessTime: Type.FILETIME.implementation },
+                    { ftLastWriteTime:  Type.FILETIME.implementation },
+                    { nFileSizeHigh:    Type.DWORD.implementation },
+                    { nFileSizeLow:     Type.DWORD.implementation },
+                    { dwReserved0:      Type.DWORD.implementation },
+                    { dwReserved1:      Type.DWORD.implementation },
+                    { cFileName:        ctypes.ArrayType(ctypes.char16_t, Const.MAX_PATH) },
+                    { cAlternateFileName: ctypes.ArrayType(ctypes.char16_t, 14) }
+                      ]));
+
+       Type.FILE_INFORMATION =
+         new SharedAll.Type("FILE_INFORMATION",
+                  ctypes.StructType("FILE_INFORMATION", [
+                    { dwFileAttributes: ctypes.uint32_t },
+                    { ftCreationTime:   Type.FILETIME.implementation },
+                    { ftLastAccessTime: Type.FILETIME.implementation },
+                    { ftLastWriteTime:  Type.FILETIME.implementation },
+                    { dwVolumeSerialNumber: ctypes.uint32_t },
+                    { nFileSizeHigh:    Type.DWORD.implementation },
+                    { nFileSizeLow:     Type.DWORD.implementation },
+                    { nNumberOfLinks:   ctypes.uint32_t },
+                    { nFileIndex: ctypes.uint64_t }
+                   ]));
+
+       Type.SystemTime =
+         new SharedAll.Type("SystemTime",
+                  ctypes.StructType("SystemTime", [
+                  { wYear:      ctypes.int16_t },
+                  { wMonth:     ctypes.int16_t },
+                  { wDayOfWeek: ctypes.int16_t },
+                  { wDay:       ctypes.int16_t },
+                  { wHour:      ctypes.int16_t },
+                  { wMinute:    ctypes.int16_t },
+                  { wSecond:    ctypes.int16_t },
+                  { wMilliSeconds: ctypes.int16_t }
+                  ]));
+
+       // Special case: these functions are used by the
+       // finalizer
+       libc.declareLazy(SysFile, "_CloseHandle",
+                        "CloseHandle", ctypes.winapi_abi,
+                        /*return */ctypes.bool,
+                        /*handle*/ ctypes.voidptr_t);
+
+       SysFile.CloseHandle = function(fd) {
+         if (fd == INVALID_HANDLE) {
+           return true;
+         } else {
+           return fd.dispose(); // Returns the value of |CloseHandle|.
+         }
+       };
+
+       libc.declareLazy(SysFile, "_FindClose",
+                        "FindClose", ctypes.winapi_abi,
+                        /*return */ctypes.bool,
+                        /*handle*/ ctypes.voidptr_t);
+
+       SysFile.FindClose = function(handle) {
+         if (handle == INVALID_HANDLE) {
+           return true;
+         } else {
+           return handle.dispose(); // Returns the value of |FindClose|.
+         }
+       };
+
+       // Declare libc functions as functions of |OS.Win.File|
+
+       libc.declareLazyFFI(SysFile, "CopyFile",
+         "CopyFileW", ctypes.winapi_abi,
+                    /*return*/ Type.zero_or_nothing,
+                    /*sourcePath*/ Type.path,
+                    /*destPath*/   Type.path,
+                    /*bailIfExist*/Type.bool);
+
+       libc.declareLazyFFI(SysFile, "CreateDirectory",
+         "CreateDirectoryW", ctypes.winapi_abi,
+                    /*return*/ Type.zero_or_nothing,
+                    /*name*/   Type.char16_t.in_ptr,
+                    /*security*/Type.SECURITY_ATTRIBUTES.in_ptr);
+
+       libc.declareLazyFFI(SysFile, "CreateFile",
+         "CreateFileW", ctypes.winapi_abi,
+                    /*return*/  Type.file_HANDLE,
+                    /*name*/    Type.path,
+                    /*access*/  Type.DWORD_FLAGS,
+                    /*share*/   Type.DWORD_FLAGS,
+                    /*security*/Type.SECURITY_ATTRIBUTES.in_ptr,
+                    /*creation*/Type.DWORD_FLAGS,
+                    /*flags*/   Type.DWORD_FLAGS,
+                    /*template*/Type.HANDLE);
+
+       libc.declareLazyFFI(SysFile, "DeleteFile",
+         "DeleteFileW", ctypes.winapi_abi,
+                    /*return*/ Type.zero_or_nothing,
+                    /*path*/   Type.path);
+
+       libc.declareLazyFFI(SysFile, "FileTimeToSystemTime",
+         "FileTimeToSystemTime", ctypes.winapi_abi,
+                    /*return*/ Type.zero_or_nothing,
+                    /*filetime*/Type.FILETIME.in_ptr,
+                    /*systime*/ Type.SystemTime.out_ptr);
+
+       libc.declareLazyFFI(SysFile, "SystemTimeToFileTime",
+         "SystemTimeToFileTime", ctypes.winapi_abi,
+                    /*return*/   Type.zero_or_nothing,
+                    /*systime*/  Type.SystemTime.in_ptr,
+                    /*filetime*/ Type.FILETIME.out_ptr);
+
+       libc.declareLazyFFI(SysFile, "FindFirstFile",
+         "FindFirstFileW", ctypes.winapi_abi,
+                    /*return*/ Type.find_HANDLE,
+                    /*pattern*/Type.path,
+                    /*data*/   Type.FindData.out_ptr);
+
+       libc.declareLazyFFI(SysFile, "FindNextFile",
+         "FindNextFileW", ctypes.winapi_abi,
+                    /*return*/ Type.zero_or_nothing,
+                    /*prev*/   Type.find_HANDLE,
+                    /*data*/   Type.FindData.out_ptr);
+
+       libc.declareLazyFFI(SysFile, "FormatMessage",
+         "FormatMessageW", ctypes.winapi_abi,
+                    /*return*/ Type.DWORD,
+                    /*flags*/  Type.DWORD_FLAGS,
+                    /*source*/ Type.void_t.in_ptr,
+                    /*msgid*/  Type.DWORD_FLAGS,
+                    /*langid*/ Type.DWORD_FLAGS,
+                    /*buf*/    Type.out_wstring,
+                    /*size*/   Type.DWORD,
+                    /*Arguments*/Type.void_t.in_ptr
+                   );
+
+       libc.declareLazyFFI(SysFile, "GetCurrentDirectory",
+         "GetCurrentDirectoryW", ctypes.winapi_abi,
+                    /*return*/ Type.zero_or_DWORD,
+                    /*length*/ Type.DWORD,
+                    /*buf*/    Type.out_path
+                   );
+
+       libc.declareLazyFFI(SysFile, "GetFullPathName",
+         "GetFullPathNameW", ctypes.winapi_abi,
+                    /*return*/   Type.zero_or_DWORD,
+                    /*fileName*/ Type.path,
+                    /*length*/   Type.DWORD,
+                    /*buf*/      Type.out_path,
+                    /*filePart*/ Type.DWORD
+                   );
+
+       libc.declareLazyFFI(SysFile, "GetDiskFreeSpaceEx",
+         "GetDiskFreeSpaceExW", ctypes.winapi_abi,
+                    /*return*/ Type.zero_or_nothing,
+                    /*directoryName*/ Type.path,
+                    /*freeBytesForUser*/ Type.uint64_t.out_ptr,
+                    /*totalBytesForUser*/ Type.uint64_t.out_ptr,
+                    /*freeTotalBytesOnDrive*/ Type.uint64_t.out_ptr);
+
+       libc.declareLazyFFI(SysFile, "GetFileInformationByHandle",
+         "GetFileInformationByHandle", ctypes.winapi_abi,
+                    /*return*/ Type.zero_or_nothing,
+                    /*handle*/ Type.HANDLE,
+                    /*info*/   Type.FILE_INFORMATION.out_ptr);
+
+       libc.declareLazyFFI(SysFile, "MoveFileEx",
+         "MoveFileExW", ctypes.winapi_abi,
+                    /*return*/   Type.zero_or_nothing,
+                    /*sourcePath*/ Type.path,
+                    /*destPath*/ Type.path,
+                    /*flags*/    Type.DWORD
+                   );
+
+       libc.declareLazyFFI(SysFile, "ReadFile",
+         "ReadFile", ctypes.winapi_abi,
+                    /*return*/ Type.zero_or_nothing,
+                    /*file*/   Type.HANDLE,
+                    /*buffer*/ Type.voidptr_t,
+                    /*nbytes*/ Type.DWORD,
+                    /*nbytes_read*/Type.DWORD.out_ptr,
+                    /*overlapped*/Type.void_t.inout_ptr // FIXME: Implement?
+         );
+
+       libc.declareLazyFFI(SysFile, "RemoveDirectory",
+         "RemoveDirectoryW", ctypes.winapi_abi,
+                    /*return*/ Type.zero_or_nothing,
+                    /*path*/   Type.path);
+
+       libc.declareLazyFFI(SysFile, "SetCurrentDirectory",
+         "SetCurrentDirectoryW", ctypes.winapi_abi,
+                    /*return*/ Type.zero_or_nothing,
+                    /*path*/   Type.path
+                   );
+
+       libc.declareLazyFFI(SysFile, "SetEndOfFile",
+         "SetEndOfFile", ctypes.winapi_abi,
+                    /*return*/ Type.zero_or_nothing,
+                    /*file*/   Type.HANDLE);
+
+       libc.declareLazyFFI(SysFile, "SetFilePointer",
+         "SetFilePointer", ctypes.winapi_abi,
+                    /*return*/ Type.DWORD,
+                    /*file*/   Type.HANDLE,
+                    /*distlow*/Type.long,
+                    /*disthi*/ Type.long.in_ptr,
+                    /*method*/ Type.DWORD);
+
+       libc.declareLazyFFI(SysFile, "SetFileTime",
+         "SetFileTime",  ctypes.winapi_abi,
+                    /*return*/   Type.zero_or_nothing,
+                    /*file*/     Type.HANDLE,
+                    /*creation*/ Type.FILETIME.in_ptr,
+                    /*access*/   Type.FILETIME.in_ptr,
+                    /*write*/    Type.FILETIME.in_ptr);
+
+
+       libc.declareLazyFFI(SysFile, "WriteFile",
+         "WriteFile", ctypes.winapi_abi,
+                    /*return*/ Type.zero_or_nothing,
+                    /*file*/   Type.HANDLE,
+                    /*buffer*/ Type.voidptr_t,
+                    /*nbytes*/ Type.DWORD,
+                    /*nbytes_wr*/Type.DWORD.out_ptr,
+                    /*overlapped*/Type.void_t.inout_ptr // FIXME: Implement?
+         );
+
+        libc.declareLazyFFI(SysFile, "FlushFileBuffers",
+          "FlushFileBuffers", ctypes.winapi_abi,
+                     /*return*/ Type.zero_or_nothing,
+                     /*file*/   Type.HANDLE);
+
+        libc.declareLazyFFI(SysFile, "GetFileAttributes",
+          "GetFileAttributesW", ctypes.winapi_abi,
+                     /*return*/   Type.DWORD_FLAGS,
+                     /*fileName*/ Type.path);
+
+        libc.declareLazyFFI(SysFile, "SetFileAttributes",
+          "SetFileAttributesW", ctypes.winapi_abi,
+                     /*return*/         Type.zero_or_nothing,
+                     /*fileName*/       Type.path,
+                     /*fileAttributes*/ Type.DWORD_FLAGS);
+
+        advapi32.declareLazyFFI(SysFile, "GetNamedSecurityInfo",
+          "GetNamedSecurityInfoW", ctypes.winapi_abi,
+                     /*return*/       Type.DWORD,
+                     /*objectName*/   Type.path,
+                     /*objectType*/   Type.DWORD,
+                     /*securityInfo*/ Type.DWORD,
+                     /*sidOwner*/     Type.PSID.out_ptr,
+                     /*sidGroup*/     Type.PSID.out_ptr,
+                     /*dacl*/         Type.PACL.out_ptr,
+                     /*sacl*/         Type.PACL.out_ptr,
+                     /*securityDesc*/ Type.PSECURITY_DESCRIPTOR.out_ptr);
+
+        advapi32.declareLazyFFI(SysFile, "SetNamedSecurityInfo",
+          "SetNamedSecurityInfoW", ctypes.winapi_abi,
+                     /*return*/       Type.DWORD,
+                     /*objectName*/   Type.path,
+                     /*objectType*/   Type.DWORD,
+                     /*securityInfo*/ Type.DWORD,
+                     /*sidOwner*/     Type.PSID,
+                     /*sidGroup*/     Type.PSID,
+                     /*dacl*/         Type.PACL,
+                     /*sacl*/         Type.PACL);
+
+        libc.declareLazyFFI(SysFile, "LocalFree",
+          "LocalFree", ctypes.winapi_abi,
+                     /*return*/       Type.HLOCAL,
+                     /*mem*/          Type.HLOCAL);
+     };
+
+     exports.OS.Win = {
+       File: {
+           _init:  init
+       }
+     };
+   })(this);
+}
diff --git a/toolkit/components/osfile/modules/osfile_win_front.jsm b/toolkit/components/osfile/modules/osfile_win_front.jsm
new file mode 100644
index 000000000..387dd08b5
--- /dev/null
+++ b/toolkit/components/osfile/modules/osfile_win_front.jsm
@@ -0,0 +1,1266 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * Synchronous front-end for the JavaScript OS.File library.
+ * Windows implementation.
+ *
+ * This front-end is meant to be imported by a worker thread.
+ */
+
+{
+  if (typeof Components != "undefined") {
+    // We do not wish osfile_win_front.jsm to be used directly as a main thread
+    // module yet.
+    throw new Error("osfile_win_front.jsm cannot be used from the main thread yet");
+  }
+
+  (function(exports) {
+     "use strict";
+
+
+      // exports.OS.Win is created by osfile_win_back.jsm
+     if (exports.OS && exports.OS.File) {
+        return; // Avoid double-initialization
+     }
+
+     let SharedAll = require("resource://gre/modules/osfile/osfile_shared_allthreads.jsm");
+     let Path = require("resource://gre/modules/osfile/ospath.jsm");
+     let SysAll = require("resource://gre/modules/osfile/osfile_win_allthreads.jsm");
+     exports.OS.Win.File._init();
+     let Const = exports.OS.Constants.Win;
+     let WinFile = exports.OS.Win.File;
+     let Type = WinFile.Type;
+
+     // Mutable thread-global data
+     // In the Windows implementation, methods |read| and |write|
+     // require passing a pointer to an uint32 to determine how many
+     // bytes have been read/written. In C, this is a benigne operation,
+     // but in js-ctypes, this has a cost. Rather than re-allocating a
+     // C uint32 and a C uint32* for each |read|/|write|, we take advantage
+     // of the fact that the state is thread-private -- hence that two
+     // |read|/|write| operations cannot take place at the same time --
+     // and we use the following global mutable values:
+     let gBytesRead = new ctypes.uint32_t(0);
+     let gBytesReadPtr = gBytesRead.address();
+     let gBytesWritten = new ctypes.uint32_t(0);
+     let gBytesWrittenPtr = gBytesWritten.address();
+
+     // Same story for GetFileInformationByHandle
+     let gFileInfo = new Type.FILE_INFORMATION.implementation();
+     let gFileInfoPtr = gFileInfo.address();
+
+     /**
+      * Representation of a file.
+      *
+      * You generally do not need to call this constructor yourself. Rather,
+      * to open a file, use function |OS.File.open|.
+      *
+      * @param fd A OS-specific file descriptor.
+      * @param {string} path File path of the file handle, used for error-reporting.
+      * @constructor
+      */
+     let File = function File(fd, path) {
+       exports.OS.Shared.AbstractFile.call(this, fd, path);
+       this._closeResult = null;
+     };
+     File.prototype = Object.create(exports.OS.Shared.AbstractFile.prototype);
+
+     /**
+      * Close the file.
+      *
+      * This method has no effect if the file is already closed. However,
+      * if the first call to |close| has thrown an error, further calls
+      * will throw the same error.
+      *
+      * @throws File.Error If closing the file revealed an error that could
+      * not be reported earlier.
+      */
+     File.prototype.close = function close() {
+       if (this._fd) {
+         let fd = this._fd;
+         this._fd = null;
+         // Call |close(fd)|, detach finalizer if any
+         // (|fd| may not be a CDataFinalizer if it has been
+         // instantiated from a controller thread).
+         let result = WinFile._CloseHandle(fd);
+         if (typeof fd == "object" && "forget" in fd) {
+           fd.forget();
+         }
+         if (result == -1) {
+           this._closeResult = new File.Error("close", ctypes.winLastError, this._path);
+         }
+       }
+       if (this._closeResult) {
+         throw this._closeResult;
+       }
+       return;
+     };
+
+     /**
+      * Read some bytes from a file.
+      *
+      * @param {C pointer} buffer A buffer for holding the data
+      * once it is read.
+      * @param {number} nbytes The number of bytes to read. It must not
+      * exceed the size of |buffer| in bytes but it may exceed the number
+      * of bytes unread in the file.
+      * @param {*=} options Additional options for reading. Ignored in
+      * this implementation.
+      *
+      * @return {number} The number of bytes effectively read. If zero,
+      * the end of the file has been reached.
+      * @throws {OS.File.Error} In case of I/O error.
+      */
+     File.prototype._read = function _read(buffer, nbytes, options) {
+       // |gBytesReadPtr| is a pointer to |gBytesRead|.
+       throw_on_zero("read",
+         WinFile.ReadFile(this.fd, buffer, nbytes, gBytesReadPtr, null),
+         this._path
+       );
+       return gBytesRead.value;
+     };
+
+     /**
+      * Write some bytes to a file.
+      *
+      * @param {Typed array} buffer A buffer holding the data that must be
+      * written.
+      * @param {number} nbytes The number of bytes to write. It must not
+      * exceed the size of |buffer| in bytes.
+      * @param {*=} options Additional options for writing. Ignored in
+      * this implementation.
+      *
+      * @return {number} The number of bytes effectively written.
+      * @throws {OS.File.Error} In case of I/O error.
+      */
+     File.prototype._write = function _write(buffer, nbytes, options) {
+       if (this._appendMode) {
+         // Need to manually seek on Windows, as O_APPEND is not supported.
+         // This is, of course, a race, but there is no real way around this.
+         this.setPosition(0, File.POS_END);
+       }
+       // |gBytesWrittenPtr| is a pointer to |gBytesWritten|.
+       throw_on_zero("write",
+         WinFile.WriteFile(this.fd, buffer, nbytes, gBytesWrittenPtr, null),
+         this._path
+       );
+       return gBytesWritten.value;
+     };
+
+     /**
+      * Return the current position in the file.
+      */
+     File.prototype.getPosition = function getPosition(pos) {
+       return this.setPosition(0, File.POS_CURRENT);
+     };
+
+     /**
+      * Change the current position in the file.
+      *
+      * @param {number} pos The new position. Whether this position
+      * is considered from the current position, from the start of
+      * the file or from the end of the file is determined by
+      * argument |whence|.  Note that |pos| may exceed the length of
+      * the file.
+      * @param {number=} whence The reference position. If omitted
+      * or |OS.File.POS_START|, |pos| is relative to the start of the
+      * file.  If |OS.File.POS_CURRENT|, |pos| is relative to the
+      * current position in the file. If |OS.File.POS_END|, |pos| is
+      * relative to the end of the file.
+      *
+      * @return The new position in the file.
+      */
+     File.prototype.setPosition = function setPosition(pos, whence) {
+       if (whence === undefined) {
+         whence = Const.FILE_BEGIN;
+       }
+       let pos64 = ctypes.Int64(pos);
+       // Per MSDN, while |lDistanceToMove| (low) is declared as int32_t, when
+       // providing |lDistanceToMoveHigh| as well, it should countain the
+       // bottom 32 bits of the 64-bit integer. Hence the following |posLo|
+       // cast is OK.
+       let posLo = new ctypes.uint32_t(ctypes.Int64.lo(pos64));
+       posLo = ctypes.cast(posLo, ctypes.int32_t);
+       let posHi = new ctypes.int32_t(ctypes.Int64.hi(pos64));
+       let result = WinFile.SetFilePointer(
+         this.fd, posLo.value, posHi.address(), whence);
+       // INVALID_SET_FILE_POINTER might be still a valid result, as it
+       // represents the lower 32 bit of the int64 result. MSDN says to check
+       // both, INVALID_SET_FILE_POINTER and a non-zero winLastError.
+       if (result == Const.INVALID_SET_FILE_POINTER && ctypes.winLastError) {
+         throw new File.Error("setPosition", ctypes.winLastError, this._path);
+       }
+       pos64 = ctypes.Int64.join(posHi.value, result);
+       return Type.int64_t.project(pos64);
+     };
+
+     /**
+      * Fetch the information on the file.
+      *
+      * @return File.Info The information on |this| file.
+      */
+     File.prototype.stat = function stat() {
+       throw_on_zero("stat",
+         WinFile.GetFileInformationByHandle(this.fd, gFileInfoPtr),
+         this._path);
+       return new File.Info(gFileInfo, this._path);
+     };
+
+     /**
+      * Set the last access and modification date of the file.
+      * The time stamp resolution is 1 second at best, but might be worse
+      * depending on the platform.
+      *
+      * @param {Date,number=} accessDate The last access date. If numeric,
+      * milliseconds since epoch. If omitted or null, then the current date
+      * will be used.
+      * @param {Date,number=} modificationDate The last modification date. If
+      * numeric, milliseconds since epoch. If omitted or null, then the current
+      * date will be used.
+      *
+      * @throws {TypeError} In case of invalid parameters.
+      * @throws {OS.File.Error} In case of I/O error.
+      */
+     File.prototype.setDates = function setDates(accessDate, modificationDate) {
+       accessDate = Date_to_FILETIME("File.prototype.setDates", accessDate, this._path);
+       modificationDate = Date_to_FILETIME("File.prototype.setDates",
+                                           modificationDate,
+                                           this._path);
+       throw_on_zero("setDates",
+                     WinFile.SetFileTime(this.fd, null, accessDate.address(),
+                                         modificationDate.address()),
+                     this._path);
+     };
+
+     /**
+      * Set the file's access permission bits.
+      */
+     File.prototype.setPermissions = function setPermissions(options = {}) {
+       if (!("winAttributes" in options)) {
+         return;
+       }
+       let oldAttributes = WinFile.GetFileAttributes(this._path);
+       if (oldAttributes == Const.INVALID_FILE_ATTRIBUTES) {
+         throw new File.Error("setPermissions", ctypes.winLastError, this._path);
+       }
+       let newAttributes = toFileAttributes(options.winAttributes, oldAttributes);
+       throw_on_zero("setPermissions",
+                     WinFile.SetFileAttributes(this._path, newAttributes),
+                     this._path);
+     };
+
+     /**
+      * Flushes the file's buffers and causes all buffered data
+      * to be written.
+      * Disk flushes are very expensive and therefore should be used carefully,
+      * sparingly and only in scenarios where it is vital that data survives
+      * system crashes. Even though the function will be executed off the
+      * main-thread, it might still affect the overall performance of any
+      * running application.
+      *
+      * @throws {OS.File.Error} In case of I/O error.
+      */
+     File.prototype.flush = function flush() {
+       throw_on_zero("flush", WinFile.FlushFileBuffers(this.fd), this._path);
+     };
+
+     // The default sharing mode for opening files: files are not
+     // locked against being reopened for reading/writing or against
+     // being deleted by the same process or another process.
+     // This is consistent with the default Unix policy.
+     const DEFAULT_SHARE = Const.FILE_SHARE_READ |
+       Const.FILE_SHARE_WRITE | Const.FILE_SHARE_DELETE;
+
+     // The default flags for opening files.
+     const DEFAULT_FLAGS = Const.FILE_ATTRIBUTE_NORMAL;
+
+     /**
+      * Open a file
+      *
+      * @param {string} path The path to the file.
+      * @param {*=} mode The opening mode for the file, as
+      * an object that may contain the following fields:
+      *
+      * - {bool} truncate If |true|, the file will be opened
+      *  for writing. If the file does not exist, it will be
+      *  created. If the file exists, its contents will be
+      *  erased. Cannot be specified with |create|.
+      * - {bool} create If |true|, the file will be opened
+      *  for writing. If the file exists, this function fails.
+      *  If the file does not exist, it will be created. Cannot
+      *  be specified with |truncate| or |existing|.
+      * - {bool} existing. If the file does not exist, this function
+      *  fails. Cannot be specified with |create|.
+      * - {bool} read If |true|, the file will be opened for
+      *  reading. The file may also be opened for writing, depending
+      *  on the other fields of |mode|.
+      * - {bool} write If |true|, the file will be opened for
+      *  writing. The file may also be opened for reading, depending
+      *  on the other fields of |mode|.
+      * - {bool} append If |true|, the file will be opened for appending,
+      *  meaning the equivalent of |.setPosition(0, POS_END)| is executed
+      *  before each write. The default is |true|, i.e. opening a file for
+      *  appending. Specify |append: false| to open the file in regular mode.
+      *
+      * If neither |truncate|, |create| or |write| is specified, the file
+      * is opened for reading.
+      *
+      * Note that |false|, |null| or |undefined| flags are simply ignored.
+      *
+      * @param {*=} options Additional options for file opening. This
+      * implementation interprets the following fields:
+      *
+      * - {number} winShare If specified, a share mode, as per
+      * Windows function |CreateFile|. You can build it from
+      * constants |OS.Constants.Win.FILE_SHARE_*|. If unspecified,
+      * the file uses the default sharing policy: it can be opened
+      * for reading and/or writing and it can be removed by other
+      * processes and by the same process.
+      * - {number} winSecurity If specified, Windows security
+      * attributes, as per Windows function |CreateFile|. If unspecified,
+      * no security attributes.
+      * - {number} winAccess If specified, Windows access mode, as
+      * per Windows function |CreateFile|. This also requires option
+      * |winDisposition| and this replaces argument |mode|. If unspecified,
+      * uses the string |mode|.
+      * - {number} winDisposition If specified, Windows disposition mode,
+      * as per Windows function |CreateFile|. This also requires option
+      * |winAccess| and this replaces argument |mode|. If unspecified,
+      * uses the string |mode|.
+      *
+      * @return {File} A file object.
+      * @throws {OS.File.Error} If the file could not be opened.
+      */
+     File.open = function Win_open(path, mode = {}, options = {}) {
+       let share = options.winShare !== undefined ? options.winShare : DEFAULT_SHARE;
+       let security = options.winSecurity || null;
+       let flags = options.winFlags !== undefined ? options.winFlags : DEFAULT_FLAGS;
+       let template = options.winTemplate ? options.winTemplate._fd : null;
+       let access;
+       let disposition;
+
+       mode = OS.Shared.AbstractFile.normalizeOpenMode(mode);
+
+       // The following option isn't a generic implementation of access to paths
+       // of arbitrary lengths. It allows for the specific case of writing to an
+       // Alternate Data Stream on a file whose path length is already close to
+       // MAX_PATH. This implementation is safe with a full path as input, if
+       // the first part of the path comes from local configuration and the
+       // file without the ADS was successfully opened before, so we know the
+       // path is valid.
+       if (options.winAllowLengthBeyondMaxPathWithCaveats) {
+         // Use the \\?\ syntax to allow lengths beyond MAX_PATH. This limited
+         // implementation only supports a DOS local path or UNC path as input.
+         let isUNC = path.length >= 2 && (path[0] == "\\" || path[0] == "/") &&
+                                         (path[1] == "\\" || path[1] == "/");
+         let pathToUse = "\\\\?\\" + (isUNC ? "UNC\\" + path.slice(2) : path);
+         // Use GetFullPathName to normalize slashes into backslashes. This is
+         // required because CreateFile won't do this for the \\?\ syntax.
+         let buffer_size = 512;
+         let array = new (ctypes.ArrayType(ctypes.char16_t, buffer_size))();
+         let expected_size = throw_on_zero("open",
+           WinFile.GetFullPathName(pathToUse, buffer_size, array, 0)
+         );
+         if (expected_size > buffer_size) {
+           // We don't need to allow an arbitrary path length for now.
+           throw new File.Error("open", ctypes.winLastError, path);
+         }
+         path = array.readString();
+       }
+
+       if ("winAccess" in options && "winDisposition" in options) {
+         access = options.winAccess;
+         disposition = options.winDisposition;
+       } else if (("winAccess" in options && !("winDisposition" in options))
+                 ||(!("winAccess" in options) && "winDisposition" in options)) {
+         throw new TypeError("OS.File.open requires either both options " +
+           "winAccess and winDisposition or neither");
+       } else {
+         if (mode.read) {
+           access |= Const.GENERIC_READ;
+         }
+         if (mode.write) {
+           access |= Const.GENERIC_WRITE;
+         }
+         // Finally, handle create/existing/trunc
+         if (mode.trunc) {
+           if (mode.existing) {
+             // It seems that Const.TRUNCATE_EXISTING is broken
+             // in presence of links (source, anyone?). We need
+             // to open normally, then perform truncation manually.
+             disposition = Const.OPEN_EXISTING;
+           } else {
+             disposition = Const.CREATE_ALWAYS;
+           }
+         } else if (mode.create) {
+           disposition = Const.CREATE_NEW;
+         } else if (mode.read && !mode.write) {
+           disposition = Const.OPEN_EXISTING;
+         } else if (mode.existing) {
+           disposition = Const.OPEN_EXISTING;
+         } else {
+           disposition = Const.OPEN_ALWAYS;
+         }
+       }
+
+       let file = error_or_file(WinFile.CreateFile(path,
+         access, share, security, disposition, flags, template), path);
+
+       file._appendMode = !!mode.append;
+
+       if (!(mode.trunc && mode.existing)) {
+         return file;
+       }
+       // Now, perform manual truncation
+       file.setPosition(0, File.POS_START);
+       throw_on_zero("open",
+         WinFile.SetEndOfFile(file.fd),
+         path);
+       return file;
+     };
+
+     /**
+      * Checks if a file or directory exists
+      *
+      * @param {string} path The path to the file.
+      *
+      * @return {bool} true if the file exists, false otherwise.
+      */
+     File.exists = function Win_exists(path) {
+       try {
+         let file = File.open(path, FILE_STAT_MODE, FILE_STAT_OPTIONS);
+         file.close();
+         return true;
+       } catch (x) {
+         return false;
+       }
+     };
+
+     /**
+      * Remove an existing file.
+      *
+      * @param {string} path The name of the file.
+      * @param {*=} options Additional options.
+      *   - {bool} ignoreAbsent If |false|, throw an error if the file does
+      *     not exist. |true| by default.
+      *
+      * @throws {OS.File.Error} In case of I/O error.
+      */
+     File.remove = function remove(path, options = {}) {
+       if (WinFile.DeleteFile(path)) {
+         return;
+       }
+
+       if (ctypes.winLastError == Const.ERROR_FILE_NOT_FOUND ||
+           ctypes.winLastError == Const.ERROR_PATH_NOT_FOUND) {
+         if ((!("ignoreAbsent" in options) || options.ignoreAbsent)) {
+           return;
+         }
+       } else if (ctypes.winLastError == Const.ERROR_ACCESS_DENIED) {
+         // Save winLastError before another ctypes call.
+         let lastError = ctypes.winLastError;
+         let attributes = WinFile.GetFileAttributes(path);
+         if (attributes != Const.INVALID_FILE_ATTRIBUTES) {
+           if (!(attributes & Const.FILE_ATTRIBUTE_READONLY)) {
+             throw new File.Error("remove", lastError, path);
+           }
+           let newAttributes = attributes & ~Const.FILE_ATTRIBUTE_READONLY;
+           if (WinFile.SetFileAttributes(path, newAttributes) &&
+               WinFile.DeleteFile(path)) {
+             return;
+           }
+         }
+       }
+
+       throw new File.Error("remove", ctypes.winLastError, path);
+     };
+
+     /**
+      * Remove an empty directory.
+      *
+      * @param {string} path The name of the directory to remove.
+      * @param {*=} options Additional options.
+      *   - {bool} ignoreAbsent If |false|, throw an error if the directory
+      *     does not exist. |true| by default
+      */
+     File.removeEmptyDir = function removeEmptyDir(path, options = {}) {
+       let result = WinFile.RemoveDirectory(path);
+       if (!result) {
+         if ((!("ignoreAbsent" in options) || options.ignoreAbsent) &&
+             ctypes.winLastError == Const.ERROR_FILE_NOT_FOUND) {
+           return;
+         }
+         throw new File.Error("removeEmptyDir", ctypes.winLastError, path);
+       }
+     };
+
+     /**
+      * Create a directory and, optionally, its parent directories.
+      *
+      * @param {string} path The name of the directory.
+      * @param {*=} options Additional options. This
+      * implementation interprets the following fields:
+      *
+      * - {C pointer} winSecurity If specified, security attributes
+      * as per winapi function |CreateDirectory|. If unspecified,
+      * use the default security descriptor, inherited from the
+      * parent directory.
+      * - {bool} ignoreExisting If |false|, throw an error if the directory
+      * already exists. |true| by default
+      * - {string} from If specified, the call to |makeDir| creates all the
+      * ancestors of |path| that are descendants of |from|. Note that |from|
+      * and its existing descendants must be user-writeable and that |path|
+      * must be a descendant of |from|.
+      * Example:
+      *   makeDir(Path.join(profileDir, "foo", "bar"), { from: profileDir });
+      *  creates directories profileDir/foo, profileDir/foo/bar
+       */
+     File._makeDir = function makeDir(path, options = {}) {
+       let security = options.winSecurity || null;
+       let result = WinFile.CreateDirectory(path, security);
+
+       if (result) {
+         return;
+       }
+
+       if (("ignoreExisting" in options) && !options.ignoreExisting) {
+         throw new File.Error("makeDir", ctypes.winLastError, path);
+       }
+
+       if (ctypes.winLastError == Const.ERROR_ALREADY_EXISTS) {
+         return;
+       }
+
+       // If the user has no access, but it's a root directory, no error should be thrown
+       let splitPath = OS.Path.split(path);
+       // Removing last component if it's empty
+       // An empty last component is caused by trailing slashes in path
+       // This is always the case with root directories
+       if( splitPath.components[splitPath.components.length - 1].length === 0 ) {
+         splitPath.components.pop();
+       }
+       // One component consisting of a drive letter implies a directory root.
+       if (ctypes.winLastError == Const.ERROR_ACCESS_DENIED &&
+           splitPath.winDrive &&
+           splitPath.components.length === 1 ) {
+         return;
+       }
+
+       throw new File.Error("makeDir", ctypes.winLastError, path);
+     };
+
+     /**
+      * Copy a file to a destination.
+      *
+      * @param {string} sourcePath The platform-specific path at which
+      * the file may currently be found.
+      * @param {string} destPath The platform-specific path at which the
+      * file should be copied.
+      * @param {*=} options An object which may contain the following fields:
+      *
+      * @option {bool} noOverwrite - If true, this function will fail if
+      * a file already exists at |destPath|. Otherwise, if this file exists,
+      * it will be erased silently.
+      *
+      * @throws {OS.File.Error} In case of any error.
+      *
+      * General note: The behavior of this function is defined only when
+      * it is called on a single file. If it is called on a directory, the
+      * behavior is undefined and may not be the same across all platforms.
+      *
+      * General note: The behavior of this function with respect to metadata
+      * is unspecified. Metadata may or may not be copied with the file. The
+      * behavior may not be the same across all platforms.
+     */
+     File.copy = function copy(sourcePath, destPath, options = {}) {
+       throw_on_zero("copy",
+         WinFile.CopyFile(sourcePath, destPath, options.noOverwrite || false),
+         sourcePath
+       );
+     };
+
+     /**
+      * Move a file to a destination.
+      *
+      * @param {string} sourcePath The platform-specific path at which
+      * the file may currently be found.
+      * @param {string} destPath The platform-specific path at which the
+      * file should be moved.
+      * @param {*=} options An object which may contain the following fields:
+      *
+      * @option {bool} noOverwrite - If set, this function will fail if
+      * a file already exists at |destPath|. Otherwise, if this file exists,
+      * it will be erased silently.
+      * @option {bool} noCopy - If set, this function will fail if the
+      * operation is more sophisticated than a simple renaming, i.e. if
+      * |sourcePath| and |destPath| are not situated on the same drive.
+      *
+      * @throws {OS.File.Error} In case of any error.
+      *
+      * General note: The behavior of this function is defined only when
+      * it is called on a single file. If it is called on a directory, the
+      * behavior is undefined and may not be the same across all platforms.
+      *
+      * General note: The behavior of this function with respect to metadata
+      * is unspecified. Metadata may or may not be moved with the file. The
+      * behavior may not be the same across all platforms.
+      */
+     File.move = function move(sourcePath, destPath, options = {}) {
+       let flags = 0;
+       if (!options.noCopy) {
+         flags = Const.MOVEFILE_COPY_ALLOWED;
+       }
+       if (!options.noOverwrite) {
+         flags = flags | Const.MOVEFILE_REPLACE_EXISTING;
+       }
+       throw_on_zero("move",
+         WinFile.MoveFileEx(sourcePath, destPath, flags),
+         sourcePath
+       );
+
+       // Inherit NTFS permissions from the destination directory
+       // if possible.
+       if (Path.dirname(sourcePath) === Path.dirname(destPath)) {
+         // Skip if the move operation was the simple rename,
+         return;
+       }
+       // The function may fail for various reasons (e.g. not all
+       // filesystems support NTFS permissions or the user may not
+       // have the enough rights to read/write permissions).
+       // However we can safely ignore errors. The file was already
+       // moved. Setting permissions is not mandatory.
+       let dacl = new ctypes.voidptr_t();
+       let sd = new ctypes.voidptr_t();
+       WinFile.GetNamedSecurityInfo(destPath, Const.SE_FILE_OBJECT,
+                                    Const.DACL_SECURITY_INFORMATION,
+                                    null /*sidOwner*/, null /*sidGroup*/,
+                                    dacl.address(), null /*sacl*/,
+                                    sd.address());
+       // dacl will be set only if the function succeeds.
+       if (!dacl.isNull()) {
+         WinFile.SetNamedSecurityInfo(destPath, Const.SE_FILE_OBJECT,
+                                      Const.DACL_SECURITY_INFORMATION |
+                                      Const.UNPROTECTED_DACL_SECURITY_INFORMATION,
+                                      null /*sidOwner*/, null /*sidGroup*/,
+                                      dacl, null /*sacl*/);
+       }
+       // sd will be set only if the function succeeds.
+       if (!sd.isNull()) {
+           WinFile.LocalFree(Type.HLOCAL.cast(sd));
+       }
+     };
+
+     /**
+      * Gets the number of bytes available on disk to the current user.
+      *
+      * @param {string} sourcePath Platform-specific path to a directory on 
+      * the disk to query for free available bytes.
+      *
+      * @return {number} The number of bytes available for the current user.
+      * @throws {OS.File.Error} In case of any error.
+      */
+     File.getAvailableFreeSpace = function Win_getAvailableFreeSpace(sourcePath) {
+       let freeBytesAvailableToUser = new Type.uint64_t.implementation(0);
+       let freeBytesAvailableToUserPtr = freeBytesAvailableToUser.address();
+
+       throw_on_zero("getAvailableFreeSpace",
+         WinFile.GetDiskFreeSpaceEx(sourcePath, freeBytesAvailableToUserPtr, null, null)
+       );
+
+       return freeBytesAvailableToUser.value;
+     };
+
+     /**
+      * A global value used to receive data during time conversions.
+      */
+     let gSystemTime = new Type.SystemTime.implementation();
+     let gSystemTimePtr = gSystemTime.address();
+
+     /**
+      * Utility function: convert a FILETIME to a JavaScript Date.
+      */
+     let FILETIME_to_Date = function FILETIME_to_Date(fileTime, path) {
+       if (fileTime == null) {
+         throw new TypeError("Expecting a non-null filetime");
+       }
+       throw_on_zero("FILETIME_to_Date",
+                     WinFile.FileTimeToSystemTime(fileTime.address(),
+                                                  gSystemTimePtr),
+                     path);
+       // Windows counts hours, minutes, seconds from UTC,
+       // JS counts from local time, so we need to go through UTC.
+       let utc = Date.UTC(gSystemTime.wYear,
+                          gSystemTime.wMonth - 1
+                          /*Windows counts months from 1, JS from 0*/,
+                          gSystemTime.wDay, gSystemTime.wHour,
+                          gSystemTime.wMinute, gSystemTime.wSecond,
+                          gSystemTime.wMilliSeconds);
+       return new Date(utc);
+     };
+
+     /**
+      * Utility function: convert Javascript Date to FileTime.
+      *
+      * @param {string} fn Name of the calling function.
+      * @param {Date,number} date The date to be converted. If omitted or null,
+      * then the current date will be used. If numeric, assumed to be the date
+      * in milliseconds since epoch.
+      */
+     let Date_to_FILETIME = function Date_to_FILETIME(fn, date, path) {
+       if (typeof date === "number") {
+         date = new Date(date);
+       } else if (!date) {
+         date = new Date();
+       } else if (typeof date.getUTCFullYear !== "function") {
+         throw new TypeError("|date| parameter of " + fn + " must be a " +
+                             "|Date| instance or number");
+       }
+       gSystemTime.wYear = date.getUTCFullYear();
+       // Windows counts months from 1, JS from 0.
+       gSystemTime.wMonth = date.getUTCMonth() + 1;
+       gSystemTime.wDay = date.getUTCDate();
+       gSystemTime.wHour = date.getUTCHours();
+       gSystemTime.wMinute = date.getUTCMinutes();
+       gSystemTime.wSecond = date.getUTCSeconds();
+       gSystemTime.wMilliseconds = date.getUTCMilliseconds();
+       let result = new OS.Shared.Type.FILETIME.implementation();
+       throw_on_zero("Date_to_FILETIME",
+                     WinFile.SystemTimeToFileTime(gSystemTimePtr,
+                                                  result.address()),
+                     path);
+       return result;
+     };
+
+     /**
+      * Iterate on one directory.
+      *
+      * This iterator will not enter subdirectories.
+      *
+      * @param {string} path The directory upon which to iterate.
+      * @param {*=} options An object that may contain the following field:
+      * @option {string} winPattern Windows file name pattern; if set,
+      * only files matching this pattern are returned.
+      *
+      * @throws {File.Error} If |path| does not represent a directory or
+      * if the directory cannot be iterated.
+      * @constructor
+      */
+     File.DirectoryIterator = function DirectoryIterator(path, options) {
+       exports.OS.Shared.AbstractFile.AbstractIterator.call(this);
+       if (options && options.winPattern) {
+         this._pattern = path + "\\" + options.winPattern;
+       } else {
+         this._pattern = path + "\\*";
+       }
+       this._path = path;
+
+       // Pre-open the first item.
+       this._first = true;
+       this._findData = new Type.FindData.implementation();
+       this._findDataPtr = this._findData.address();
+       this._handle = WinFile.FindFirstFile(this._pattern, this._findDataPtr);
+       if (this._handle == Const.INVALID_HANDLE_VALUE) {
+         let error = ctypes.winLastError;
+         this._findData = null;
+         this._findDataPtr = null;
+         if (error == Const.ERROR_FILE_NOT_FOUND) {
+           // Directory is empty, let's behave as if it were closed
+           SharedAll.LOG("Directory is empty");
+           this._closed = true;
+           this._exists = true;
+         } else if (error == Const.ERROR_PATH_NOT_FOUND) {
+           // Directory does not exist, let's throw if we attempt to walk it
+           SharedAll.LOG("Directory does not exist");
+           this._closed = true;
+           this._exists = false;
+         } else {
+           throw new File.Error("DirectoryIterator", error, this._path);
+         }
+       } else {
+         this._closed = false;
+         this._exists = true;
+       }
+     };
+
+     File.DirectoryIterator.prototype = Object.create(exports.OS.Shared.AbstractFile.AbstractIterator.prototype);
+
+
+     /**
+      * Fetch the next entry in the directory.
+      *
+      * @return null If we have reached the end of the directory.
+      */
+     File.DirectoryIterator.prototype._next = function _next() {
+       // Bailout if the directory does not exist
+       if (!this._exists) {
+         throw File.Error.noSuchFile("DirectoryIterator.prototype.next", this._path);
+       }
+       // Bailout if the iterator is closed.
+       if (this._closed) {
+         return null;
+       }
+       // If this is the first entry, we have obtained it already
+       // during construction.
+       if (this._first) {
+         this._first = false;
+         return this._findData;
+       }
+
+       if (WinFile.FindNextFile(this._handle, this._findDataPtr)) {
+         return this._findData;
+       } else {
+         let error = ctypes.winLastError;
+         this.close();
+         if (error == Const.ERROR_NO_MORE_FILES) {
+            return null;
+         } else {
+            throw new File.Error("iter (FindNextFile)", error, this._path);
+         }
+       }
+     },
+
+     /**
+      * Return the next entry in the directory, if any such entry is
+      * available.
+      *
+      * Skip special directories "." and "..".
+      *
+      * @return {File.Entry} The next entry in the directory.
+      * @throws {StopIteration} Once all files in the directory have been
+      * encountered.
+      */
+     File.DirectoryIterator.prototype.next = function next() {
+         // FIXME: If we start supporting "\\?\"-prefixed paths, do not forget
+         // that "." and ".." are absolutely normal file names if _path starts
+         // with such prefix
+         for (let entry = this._next(); entry != null; entry = this._next()) {
+           let name = entry.cFileName.readString();
+           if (name == "." || name == "..") {
+             continue;
+           }
+           return new File.DirectoryIterator.Entry(entry, this._path);
+         }
+         throw StopIteration;
+     };
+
+     File.DirectoryIterator.prototype.close = function close() {
+       if (this._closed) {
+         return;
+       }
+       this._closed = true;
+       if (this._handle) {
+         // We might not have a handle if the iterator is closed
+         // before being used.
+         throw_on_zero("FindClose",
+           WinFile.FindClose(this._handle),
+           this._path);
+         this._handle = null;
+       }
+     };
+
+    /**
+     * Determine whether the directory exists.
+     *
+     * @return {boolean}
+     */
+     File.DirectoryIterator.prototype.exists = function exists() {
+       return this._exists;
+     };
+
+     File.DirectoryIterator.Entry = function Entry(win_entry, parent) {
+       if (!win_entry.dwFileAttributes || !win_entry.ftCreationTime ||
+           !win_entry.ftLastAccessTime || !win_entry.ftLastWriteTime)
+        throw new TypeError();
+
+       // Copy the relevant part of |win_entry| to ensure that
+       // our data is not overwritten prematurely.
+       let isDir = !!(win_entry.dwFileAttributes & Const.FILE_ATTRIBUTE_DIRECTORY);
+       let isSymLink = !!(win_entry.dwFileAttributes & Const.FILE_ATTRIBUTE_REPARSE_POINT);
+
+       let winCreationDate = FILETIME_to_Date(win_entry.ftCreationTime, this._path);
+       let winLastWriteDate = FILETIME_to_Date(win_entry.ftLastWriteTime, this._path);
+       let winLastAccessDate = FILETIME_to_Date(win_entry.ftLastAccessTime, this._path);
+
+       let name = win_entry.cFileName.readString();
+       if (!name) {
+         throw new TypeError("Empty name");
+       }
+
+       if (!parent) {
+         throw new TypeError("Empty parent");
+       }
+       this._parent = parent;
+
+       let path = Path.join(this._parent, name);
+
+       SysAll.AbstractEntry.call(this, isDir, isSymLink, name,
+         winCreationDate, winLastWriteDate,
+         winLastAccessDate, path);
+     };
+     File.DirectoryIterator.Entry.prototype = Object.create(SysAll.AbstractEntry.prototype);
+
+     /**
+      * Return a version of an instance of
+      * File.DirectoryIterator.Entry that can be sent from a worker
+      * thread to the main thread. Note that deserialization is
+      * asymmetric and returns an object with a different
+      * implementation.
+      */
+     File.DirectoryIterator.Entry.toMsg = function toMsg(value) {
+       if (!value instanceof File.DirectoryIterator.Entry) {
+         throw new TypeError("parameter of " +
+           "File.DirectoryIterator.Entry.toMsg must be a " +
+           "File.DirectoryIterator.Entry");
+       }
+       let serialized = {};
+       for (let key in File.DirectoryIterator.Entry.prototype) {
+         serialized[key] = value[key];
+       }
+       return serialized;
+     };
+
+
+     /**
+      * Information on a file.
+      *
+      * To obtain the latest information on a file, use |File.stat|
+      * (for an unopened file) or |File.prototype.stat| (for an
+      * already opened file).
+      *
+      * @constructor
+      */
+     File.Info = function Info(stat, path) {
+       let isDir = !!(stat.dwFileAttributes & Const.FILE_ATTRIBUTE_DIRECTORY);
+       let isSymLink = !!(stat.dwFileAttributes & Const.FILE_ATTRIBUTE_REPARSE_POINT);
+
+       let winBirthDate = FILETIME_to_Date(stat.ftCreationTime, this._path);
+       let lastAccessDate = FILETIME_to_Date(stat.ftLastAccessTime, this._path);
+       let lastWriteDate = FILETIME_to_Date(stat.ftLastWriteTime, this._path);
+
+       let value = ctypes.UInt64.join(stat.nFileSizeHigh, stat.nFileSizeLow);
+       let size = Type.uint64_t.importFromC(value);
+       let winAttributes = {
+         readOnly: !!(stat.dwFileAttributes & Const.FILE_ATTRIBUTE_READONLY),
+         system: !!(stat.dwFileAttributes & Const.FILE_ATTRIBUTE_SYSTEM),
+         hidden: !!(stat.dwFileAttributes & Const.FILE_ATTRIBUTE_HIDDEN),
+       };
+
+       SysAll.AbstractInfo.call(this, path, isDir, isSymLink, size,
+         winBirthDate, lastAccessDate, lastWriteDate, winAttributes);
+     };
+     File.Info.prototype = Object.create(SysAll.AbstractInfo.prototype);
+
+     /**
+      * Return a version of an instance of File.Info that can be sent
+      * from a worker thread to the main thread. Note that deserialization
+      * is asymmetric and returns an object with a different implementation.
+      */
+     File.Info.toMsg = function toMsg(stat) {
+       if (!stat instanceof File.Info) {
+         throw new TypeError("parameter of File.Info.toMsg must be a File.Info");
+       }
+       let serialized = {};
+       for (let key in File.Info.prototype) {
+         serialized[key] = stat[key];
+       }
+       return serialized;
+     };
+
+
+     /**
+      * Fetch the information on a file.
+      *
+      * Performance note: if you have opened the file already,
+      * method |File.prototype.stat| is generally much faster
+      * than method |File.stat|.
+      *
+      * Platform-specific note: under Windows, if the file is
+      * already opened without sharing of the read capability,
+      * this function will fail.
+      *
+      * @return {File.Information}
+      */
+     File.stat = function stat(path) {
+       let file = File.open(path, FILE_STAT_MODE, FILE_STAT_OPTIONS);
+       try {
+         return file.stat();
+       } finally {
+         file.close();
+       }
+     };
+     // All of the following is required to ensure that File.stat
+     // also works on directories.
+     const FILE_STAT_MODE = {
+       read: true
+     };
+     const FILE_STAT_OPTIONS = {
+       // Directories can be opened neither for reading(!) nor for writing
+       winAccess: 0,
+       // Directories can only be opened with backup semantics(!)
+       winFlags: Const.FILE_FLAG_BACKUP_SEMANTICS,
+       winDisposition: Const.OPEN_EXISTING
+     };
+
+     /**
+      * Set the file's access permission bits.
+      */
+     File.setPermissions = function setPermissions(path, options = {}) {
+       if (!("winAttributes" in options)) {
+         return;
+       }
+       let oldAttributes = WinFile.GetFileAttributes(path);
+       if (oldAttributes == Const.INVALID_FILE_ATTRIBUTES) {
+         throw new File.Error("setPermissions", ctypes.winLastError, path);
+       }
+       let newAttributes = toFileAttributes(options.winAttributes, oldAttributes);
+       throw_on_zero("setPermissions",
+                     WinFile.SetFileAttributes(path, newAttributes),
+                     path);
+     };
+
+     /**
+      * Set the last access and modification date of the file.
+      * The time stamp resolution is 1 second at best, but might be worse
+      * depending on the platform.
+      *
+      * Performance note: if you have opened the file already in write mode,
+      * method |File.prototype.stat| is generally much faster
+      * than method |File.stat|.
+      *
+      * Platform-specific note: under Windows, if the file is
+      * already opened without sharing of the write capability,
+      * this function will fail.
+      *
+      * @param {string} path The full name of the file to set the dates for.
+      * @param {Date,number=} accessDate The last access date. If numeric,
+      * milliseconds since epoch. If omitted or null, then the current date
+      * will be used.
+      * @param {Date,number=} modificationDate The last modification date. If
+      * numeric, milliseconds since epoch. If omitted or null, then the current
+      * date will be used.
+      *
+      * @throws {TypeError} In case of invalid paramters.
+      * @throws {OS.File.Error} In case of I/O error.
+      */
+     File.setDates = function setDates(path, accessDate, modificationDate) {
+       let file = File.open(path, FILE_SETDATES_MODE, FILE_SETDATES_OPTIONS);
+       try {
+         return file.setDates(accessDate, modificationDate);
+       } finally {
+         file.close();
+       }
+     };
+     // All of the following is required to ensure that File.setDates
+     // also works on directories.
+     const FILE_SETDATES_MODE = {
+       write: true
+     };
+     const FILE_SETDATES_OPTIONS = {
+       winAccess: Const.GENERIC_WRITE,
+       // Directories can only be opened with backup semantics(!)
+       winFlags: Const.FILE_FLAG_BACKUP_SEMANTICS,
+       winDisposition: Const.OPEN_EXISTING
+     };
+
+     File.read = exports.OS.Shared.AbstractFile.read;
+     File.writeAtomic = exports.OS.Shared.AbstractFile.writeAtomic;
+     File.openUnique = exports.OS.Shared.AbstractFile.openUnique;
+     File.makeDir = exports.OS.Shared.AbstractFile.makeDir;
+
+     /**
+      * Remove an existing directory and its contents.
+      *
+      * @param {string} path The name of the directory.
+      * @param {*=} options Additional options.
+      *   - {bool} ignoreAbsent If |false|, throw an error if the directory doesn't
+      *     exist. |true| by default.
+      *   - {boolean} ignorePermissions If |true|, remove the file even when lacking write
+      *     permission.
+      *
+      * @throws {OS.File.Error} In case of I/O error, in particular if |path| is
+      *         not a directory.
+      */
+     File.removeDir = function(path, options = {}) {
+       // We can't use File.stat here because it will follow the symlink.
+       let attributes = WinFile.GetFileAttributes(path);
+       if (attributes == Const.INVALID_FILE_ATTRIBUTES) {
+         if ((!("ignoreAbsent" in options) || options.ignoreAbsent) &&
+             ctypes.winLastError == Const.ERROR_FILE_NOT_FOUND) {
+           return;
+         }
+         throw new File.Error("removeEmptyDir", ctypes.winLastError, path);
+       }
+       if (attributes & Const.FILE_ATTRIBUTE_REPARSE_POINT) {
+         // Unlike Unix symlinks, NTFS junctions or NTFS symlinks to
+         // directories are directories themselves. OS.File.remove()
+         // will not work for them.
+         OS.File.removeEmptyDir(path, options);
+         return;
+       }
+       exports.OS.Shared.AbstractFile.removeRecursive(path, options);
+     };
+
+     /**
+      * Get the current directory by getCurrentDirectory.
+      */
+     File.getCurrentDirectory = function getCurrentDirectory() {
+       // This function is more complicated than one could hope.
+       //
+       // This is due to two facts:
+       // - the maximal length of a path under Windows is not completely
+       //  specified (there is a constant MAX_PATH, but it is quite possible
+       //  to create paths that are much larger, see bug 744413);
+       // - if we attempt to call |GetCurrentDirectory| with a buffer that
+       //  is too short, it returns the length of the current directory, but
+       //  this length might be insufficient by the time we can call again
+       //  the function with a larger buffer, in the (unlikely but possible)
+       //  case in which the process changes directory to a directory with
+       //  a longer name between both calls.
+       //
+       let buffer_size = 4096;
+       while (true) {
+         let array = new (ctypes.ArrayType(ctypes.char16_t, buffer_size))();
+         let expected_size = throw_on_zero("getCurrentDirectory",
+           WinFile.GetCurrentDirectory(buffer_size, array)
+         );
+         if (expected_size <= buffer_size) {
+           return array.readString();
+         }
+         // At this point, we are in a case in which our buffer was not
+         // large enough to hold the name of the current directory.
+         // Consequently, we need to increase the size of the buffer.
+         // Note that, even in crazy scenarios, the loop will eventually
+         // converge, as the length of the paths cannot increase infinitely.
+         buffer_size = expected_size + 1 /* to store \0 */;
+       }
+     };
+
+     /**
+      * Set the current directory by setCurrentDirectory.
+      */
+     File.setCurrentDirectory = function setCurrentDirectory(path) {
+       throw_on_zero("setCurrentDirectory",
+         WinFile.SetCurrentDirectory(path),
+         path);
+     };
+
+     /**
+      * Get/set the current directory by |curDir|.
+      */
+     Object.defineProperty(File, "curDir", {
+         set: function(path) {
+           this.setCurrentDirectory(path);
+         },
+         get: function() {
+           return this.getCurrentDirectory();
+         }
+       }
+     );
+
+     // Utility functions, used for error-handling
+
+     /**
+      * Turn the result of |open| into an Error or a File
+      * @param {number} maybe The result of the |open| operation that may
+      * represent either an error or a success. If -1, this function raises
+      * an error holding ctypes.winLastError, otherwise it returns the opened file.
+      * @param {string=} path The path of the file.
+      */
+     function error_or_file(maybe, path) {
+       if (maybe == Const.INVALID_HANDLE_VALUE) {
+         throw new File.Error("open", ctypes.winLastError, path);
+       }
+       return new File(maybe, path);
+     }
+
+     /**
+      * Utility function to sort errors represented as "0" from successes.
+      *
+      * @param {string=} operation The name of the operation. If unspecified,
+      * the name of the caller function.
+      * @param {number} result The result of the operation that may
+      * represent either an error or a success. If 0, this function raises
+      * an error holding ctypes.winLastError, otherwise it returns |result|.
+      * @param {string=} path The path of the file.
+      */
+     function throw_on_zero(operation, result, path) {
+       if (result == 0) {
+         throw new File.Error(operation, ctypes.winLastError, path);
+       }
+       return result;
+     }
+
+     /**
+      * Utility function to sort errors represented as "-1" from successes.
+      *
+      * @param {string=} operation The name of the operation. If unspecified,
+      * the name of the caller function.
+      * @param {number} result The result of the operation that may
+      * represent either an error or a success. If -1, this function raises
+      * an error holding ctypes.winLastError, otherwise it returns |result|.
+      * @param {string=} path The path of the file.
+      */
+     function throw_on_negative(operation, result, path) {
+       if (result < 0) {
+         throw new File.Error(operation, ctypes.winLastError, path);
+       }
+       return result;
+     }
+
+     /**
+      * Utility function to sort errors represented as |null| from successes.
+      *
+      * @param {string=} operation The name of the operation. If unspecified,
+      * the name of the caller function.
+      * @param {pointer} result The result of the operation that may
+      * represent either an error or a success. If |null|, this function raises
+      * an error holding ctypes.winLastError, otherwise it returns |result|.
+      * @param {string=} path The path of the file.
+      */
+     function throw_on_null(operation, result, path) {
+       if (result == null || (result.isNull && result.isNull())) {
+         throw new File.Error(operation, ctypes.winLastError, path);
+       }
+       return result;
+     }
+
+     /**
+      * Helper used by both versions of setPermissions
+      */
+     function toFileAttributes(winAttributes, oldDwAttrs) {
+       if ("readOnly" in winAttributes) {
+         if (winAttributes.readOnly) {
+           oldDwAttrs |= Const.FILE_ATTRIBUTE_READONLY;
+         } else {
+           oldDwAttrs &= ~Const.FILE_ATTRIBUTE_READONLY;
+         }
+       }
+       if ("system" in winAttributes) {
+         if (winAttributes.system) {
+           oldDwAttrs |= Const.FILE_ATTRIBUTE_SYSTEM;
+         } else {
+           oldDwAttrs &= ~Const.FILE_ATTRIBUTE_SYSTEM;
+         }
+       }
+       if ("hidden" in winAttributes) {
+         if (winAttributes.hidden) {
+           oldDwAttrs |= Const.FILE_ATTRIBUTE_HIDDEN;
+         } else {
+           oldDwAttrs &= ~Const.FILE_ATTRIBUTE_HIDDEN;
+         }
+       }
+       return oldDwAttrs;
+     }
+
+     File.Win = exports.OS.Win.File;
+     File.Error = SysAll.Error;
+     exports.OS.File = File;
+     exports.OS.Shared.Type = Type;
+
+     Object.defineProperty(File, "POS_START", { value: SysAll.POS_START });
+     Object.defineProperty(File, "POS_CURRENT", { value: SysAll.POS_CURRENT });
+     Object.defineProperty(File, "POS_END", { value: SysAll.POS_END });
+   })(this);
+}
diff --git a/toolkit/components/osfile/modules/ospath.jsm b/toolkit/components/osfile/modules/ospath.jsm
new file mode 100644
index 000000000..68bbe4345
--- /dev/null
+++ b/toolkit/components/osfile/modules/ospath.jsm
@@ -0,0 +1,45 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * Handling native paths.
+ *
+ * This module contains a number of functions destined to simplify
+ * working with native paths through a cross-platform API. Functions
+ * of this module will only work with the following assumptions:
+ *
+ * - paths are valid;
+ * - paths are defined with one of the grammars that this module can
+ *   parse (see later);
+ * - all path concatenations go through function |join|.
+ */
+
+"use strict";
+
+if (typeof Components == "undefined") {
+  let Path;
+  if (OS.Constants.Win) {
+    Path = require("resource://gre/modules/osfile/ospath_win.jsm");
+  } else {
+    Path = require("resource://gre/modules/osfile/ospath_unix.jsm");
+  }
+  module.exports = Path;
+} else {
+  let Cu = Components.utils;
+  let Scope = {};
+  Cu.import("resource://gre/modules/osfile/osfile_shared_allthreads.jsm", Scope);
+
+  let Path = {};
+  if (Scope.OS.Constants.Win) {
+    Cu.import("resource://gre/modules/osfile/ospath_win.jsm", Path);
+  } else {
+    Cu.import("resource://gre/modules/osfile/ospath_unix.jsm", Path);
+  }
+
+  this.EXPORTED_SYMBOLS = [];
+  for (let k in Path) {
+    this.EXPORTED_SYMBOLS.push(k);
+    this[k] = Path[k];
+  }
+}
diff --git a/toolkit/components/osfile/modules/ospath_unix.jsm b/toolkit/components/osfile/modules/ospath_unix.jsm
new file mode 100644
index 000000000..1d574baed
--- /dev/null
+++ b/toolkit/components/osfile/modules/ospath_unix.jsm
@@ -0,0 +1,202 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * Handling native paths.
+ *
+ * This module contains a number of functions destined to simplify
+ * working with native paths through a cross-platform API. Functions
+ * of this module will only work with the following assumptions:
+ *
+ * - paths are valid;
+ * - paths are defined with one of the grammars that this module can
+ *   parse (see later);
+ * - all path concatenations go through function |join|.
+ */
+
+"use strict";
+
+// Boilerplate used to be able to import this module both from the main
+// thread and from worker threads.
+if (typeof Components != "undefined") {
+  Components.utils.importGlobalProperties(["URL"]);
+  // Global definition of |exports|, to keep everybody happy.
+  // In non-main thread, |exports| is provided by the module
+  // loader.
+  this.exports = {};
+} else if (typeof module == "undefined" || typeof exports == "undefined") {
+  throw new Error("Please load this module using require()");
+}
+
+var EXPORTED_SYMBOLS = [
+  "basename",
+  "dirname",
+  "join",
+  "normalize",
+  "split",
+  "toFileURI",
+  "fromFileURI",
+];
+
+/**
+ * Return the final part of the path.
+ * The final part of the path is everything after the last "/".
+ */
+var basename = function(path) {
+  return path.slice(path.lastIndexOf("/") + 1);
+};
+exports.basename = basename;
+
+/**
+ * Return the directory part of the path.
+ * The directory part of the path is everything before the last
+ * "/". If the last few characters of this part are also "/",
+ * they are ignored.
+ *
+ * If the path contains no directory, return ".".
+ */
+var dirname = function(path) {
+  let index = path.lastIndexOf("/");
+  if (index == -1) {
+    return ".";
+  }
+  while (index >= 0 && path[index] == "/") {
+    --index;
+  }
+  return path.slice(0, index + 1);
+};
+exports.dirname = dirname;
+
+/**
+ * Join path components.
+ * This is the recommended manner of getting the path of a file/subdirectory
+ * in a directory.
+ *
+ * Example: Obtaining $TMP/foo/bar in an OS-independent manner
+ *  var tmpDir = OS.Constants.Path.tmpDir;
+ *  var path = OS.Path.join(tmpDir, "foo", "bar");
+ *
+ * Under Unix, this will return "/tmp/foo/bar".
+ *
+ * Empty components are ignored, i.e. `OS.Path.join("foo", "", "bar)` is the
+ * same as `OS.Path.join("foo", "bar")`.
+ */
+var join = function(...path) {
+  // If there is a path that starts with a "/", eliminate everything before
+  let paths = [];
+  for (let subpath of path) {
+    if (subpath == null) {
+      throw new TypeError("invalid path component");
+    }
+    if (subpath.length == 0) {
+      continue;
+    } else if (subpath[0] == "/") {
+      paths = [subpath];
+    } else {
+      paths.push(subpath);
+    }
+  }
+  return paths.join("/");
+};
+exports.join = join;
+
+/**
+ * Normalize a path by removing any unneeded ".", "..", "//".
+ */
+var normalize = function(path) {
+  let stack = [];
+  let absolute;
+  if (path.length >= 0 && path[0] == "/") {
+    absolute = true;
+  } else {
+    absolute = false;
+  }
+  path.split("/").forEach(function(v) {
+    switch (v) {
+    case "":  case ".":// fallthrough
+      break;
+    case "..":
+      if (stack.length == 0) {
+        if (absolute) {
+          throw new Error("Path is ill-formed: attempting to go past root");
+        } else {
+          stack.push("..");
+        }
+      } else {
+        if (stack[stack.length - 1] == "..") {
+          stack.push("..");
+        } else {
+          stack.pop();
+        }
+      }
+      break;
+    default:
+      stack.push(v);
+    }
+  });
+  let string = stack.join("/");
+  return absolute ? "/" + string : string;
+};
+exports.normalize = normalize;
+
+/**
+ * Return the components of a path.
+ * You should generally apply this function to a normalized path.
+ *
+ * @return {{
+ *   {bool} absolute |true| if the path is absolute, |false| otherwise
+ *   {array} components the string components of the path
+ * }}
+ *
+ * Other implementations may add additional OS-specific informations.
+ */
+var split = function(path) {
+  return {
+    absolute: path.length && path[0] == "/",
+    components: path.split("/")
+  };
+};
+exports.split = split;
+
+/**
+ * Returns the file:// URI file path of the given local file path.
+ */
+// The case of %3b is designed to match Services.io, but fundamentally doesn't matter.
+var toFileURIExtraEncodings = {';': '%3b', '?': '%3F', '#': '%23'};
+var toFileURI = function toFileURI(path) {
+  // Per https://url.spec.whatwg.org we should not encode [] in the path
+  let dontNeedEscaping = {'%5B': '[', '%5D': ']'};
+  let uri = encodeURI(this.normalize(path)).replace(/%(5B|5D)/gi,
+    match => dontNeedEscaping[match]);
+
+  // add a prefix, and encodeURI doesn't escape a few characters that we do
+  // want to escape, so fix that up
+  let prefix = "file://";
+  uri = prefix + uri.replace(/[;?#]/g, match => toFileURIExtraEncodings[match]);
+
+  return uri;
+};
+exports.toFileURI = toFileURI;
+
+/**
+ * Returns the local file path from a given file URI.
+ */
+var fromFileURI = function fromFileURI(uri) {
+  let url = new URL(uri);
+  if (url.protocol != 'file:') {
+    throw new Error("fromFileURI expects a file URI");
+  }
+  let path = this.normalize(decodeURIComponent(url.pathname));
+  return path;
+};
+exports.fromFileURI = fromFileURI;
+
+
+//////////// Boilerplate
+if (typeof Components != "undefined") {
+  this.EXPORTED_SYMBOLS = EXPORTED_SYMBOLS;
+  for (let symbol of EXPORTED_SYMBOLS) {
+    this[symbol] = exports[symbol];
+  }
+}
diff --git a/toolkit/components/osfile/modules/ospath_win.jsm b/toolkit/components/osfile/modules/ospath_win.jsm
new file mode 100644
index 000000000..31a87b115
--- /dev/null
+++ b/toolkit/components/osfile/modules/ospath_win.jsm
@@ -0,0 +1,373 @@
+/**
+ * Handling native paths.
+ *
+ * This module contains a number of functions destined to simplify
+ * working with native paths through a cross-platform API. Functions
+ * of this module will only work with the following assumptions:
+ *
+ * - paths are valid;
+ * - paths are defined with one of the grammars that this module can
+ *   parse (see later);
+ * - all path concatenations go through function |join|.
+ *
+ * Limitations of this implementation.
+ *
+ * Windows supports 6 distinct grammars for paths. For the moment, this
+ * implementation supports the following subset:
+ *
+ * - drivename:backslash-separated components
+ * - backslash-separated components
+ * - \\drivename\ followed by backslash-separated components
+ *
+ * Additionally, |normalize| can convert a path containing slash-
+ * separated components to a path containing backslash-separated
+ * components.
+ */
+
+"use strict";
+
+// Boilerplate used to be able to import this module both from the main
+// thread and from worker threads.
+if (typeof Components != "undefined") {
+  Components.utils.importGlobalProperties(["URL"]);
+  // Global definition of |exports|, to keep everybody happy.
+  // In non-main thread, |exports| is provided by the module
+  // loader.
+  this.exports = {};
+} else if (typeof module == "undefined" || typeof exports == "undefined") {
+  throw new Error("Please load this module using require()");
+}
+
+var EXPORTED_SYMBOLS = [
+  "basename",
+  "dirname",
+  "join",
+  "normalize",
+  "split",
+  "winGetDrive",
+  "winIsAbsolute",
+  "toFileURI",
+  "fromFileURI",
+];
+
+/**
+ * Return the final part of the path.
+ * The final part of the path is everything after the last "\\".
+ */
+var basename = function(path) {
+  if (path.startsWith("\\\\")) {
+    // UNC-style path
+    let index = path.lastIndexOf("\\");
+    if (index != 1) {
+      return path.slice(index + 1);
+    }
+    return ""; // Degenerate case
+  }
+  return path.slice(Math.max(path.lastIndexOf("\\"),
+                             path.lastIndexOf(":")) + 1);
+};
+exports.basename = basename;
+
+/**
+ * Return the directory part of the path.
+ *
+ * If the path contains no directory, return the drive letter,
+ * or "." if the path contains no drive letter or if option
+ * |winNoDrive| is set.
+ *
+ * Otherwise, return everything before the last backslash,
+ * including the drive/server name.
+ *
+ *
+ * @param {string} path The path.
+ * @param {*=} options Platform-specific options controlling the behavior
+ * of this function. This implementation supports the following options:
+ *  - |winNoDrive| If |true|, also remove the letter from the path name.
+ */
+var dirname = function(path, options) {
+  let noDrive = (options && options.winNoDrive);
+
+  // Find the last occurrence of "\\"
+  let index = path.lastIndexOf("\\");
+  if (index == -1) {
+    // If there is no directory component...
+    if (!noDrive) {
+      // Return the drive path if possible, falling back to "."
+      return this.winGetDrive(path) || ".";
+    } else {
+      // Or just "."
+      return ".";
+    }
+  }
+
+  if (index == 1 && path.charAt(0) == "\\") {
+    // The path is reduced to a UNC drive
+    if (noDrive) {
+      return ".";
+    } else {
+      return path;
+    }
+  }
+
+  // Ignore any occurrence of "\\: immediately before that one
+  while (index >= 0 && path[index] == "\\") {
+    --index;
+  }
+
+  // Compute what is left, removing the drive name if necessary
+  let start;
+  if (noDrive) {
+    start = (this.winGetDrive(path) || "").length;
+  } else {
+    start = 0;
+  }
+  return path.slice(start, index + 1);
+};
+exports.dirname = dirname;
+
+/**
+ * Join path components.
+ * This is the recommended manner of getting the path of a file/subdirectory
+ * in a directory.
+ *
+ * Example: Obtaining $TMP/foo/bar in an OS-independent manner
+ *  var tmpDir = OS.Constants.Path.tmpDir;
+ *  var path = OS.Path.join(tmpDir, "foo", "bar");
+ *
+ * Under Windows, this will return "$TMP\foo\bar".
+ *
+ * Empty components are ignored, i.e. `OS.Path.join("foo", "", "bar)` is the
+ * same as `OS.Path.join("foo", "bar")`.
+ */
+var join = function(...path) {
+  let paths = [];
+  let root;
+  let absolute = false;
+  for (let subpath of path) {
+    if (subpath == null) {
+      throw new TypeError("invalid path component");
+    }
+    if (subpath == "") {
+      continue;
+    }
+    let drive = this.winGetDrive(subpath);
+    if (drive) {
+      root = drive;
+      let component = trimBackslashes(subpath.slice(drive.length));
+      if (component) {
+        paths = [component];
+      } else {
+        paths = [];
+      }
+      absolute = true;
+    } else if (this.winIsAbsolute(subpath)) {
+      paths = [trimBackslashes(subpath)];
+      absolute = true;
+    } else {
+      paths.push(trimBackslashes(subpath));
+    }
+  }
+  let result = "";
+  if (root) {
+    result += root;
+  }
+  if (absolute) {
+    result += "\\";
+  }
+  result += paths.join("\\");
+  return result;
+};
+exports.join = join;
+
+/**
+ * Return the drive name of a path, or |null| if the path does
+ * not contain a drive name.
+ *
+ * Drive name appear either as "DriveName:..." (the return drive
+ * name includes the ":") or "\\\\DriveName..." (the returned drive name
+ * includes "\\\\").
+ */
+var winGetDrive = function(path) {
+  if (path == null) {
+    throw new TypeError("path is invalid");
+  }
+
+  if (path.startsWith("\\\\")) {
+    // UNC path
+    if (path.length == 2) {
+      return null;
+    }
+    let index = path.indexOf("\\", 2);
+    if (index == -1) {
+      return path;
+    }
+    return path.slice(0, index);
+  }
+  // Non-UNC path
+  let index = path.indexOf(":");
+  if (index <= 0) return null;
+  return path.slice(0, index + 1);
+};
+exports.winGetDrive = winGetDrive;
+
+/**
+ * Return |true| if the path is absolute, |false| otherwise.
+ *
+ * We consider that a path is absolute if it starts with "\\"
+ * or "driveletter:\\".
+ */
+var winIsAbsolute = function(path) {
+  let index = path.indexOf(":");
+  return path.length > index + 1 && path[index + 1] == "\\";
+};
+exports.winIsAbsolute = winIsAbsolute;
+
+/**
+ * Normalize a path by removing any unneeded ".", "..", "\\".
+ * Also convert any "/" to a "\\".
+ */
+var normalize = function(path) {
+  let stack = [];
+
+  if (!path.startsWith("\\\\")) {
+    // Normalize "/" to "\\"
+    path = path.replace(/\//g, "\\");
+  }
+
+  // Remove the drive (we will put it back at the end)
+  let root = this.winGetDrive(path);
+  if (root) {
+    path = path.slice(root.length);
+  }
+
+  // Remember whether we need to restore a leading "\\" or drive name.
+  let absolute = this.winIsAbsolute(path);
+
+  // And now, fill |stack| from the components,
+  // popping whenever there is a ".."
+  path.split("\\").forEach(function loop(v) {
+    switch (v) {
+    case "":  case ".": // Ignore
+      break;
+    case "..":
+      if (stack.length == 0) {
+        if (absolute) {
+          throw new Error("Path is ill-formed: attempting to go past root");
+        } else {
+         stack.push("..");
+        }
+      } else {
+        if (stack[stack.length - 1] == "..") {
+          stack.push("..");
+        } else {
+          stack.pop();
+        }
+      }
+      break;
+    default:
+      stack.push(v);
+    }
+  });
+
+  // Put everything back together
+  let result = stack.join("\\");
+  if (absolute || root) {
+    result = "\\" + result;
+  }
+  if (root) {
+    result = root + result;
+  }
+  return result;
+};
+exports.normalize = normalize;
+
+/**
+ * Return the components of a path.
+ * You should generally apply this function to a normalized path.
+ *
+ * @return {{
+ *   {bool} absolute |true| if the path is absolute, |false| otherwise
+ *   {array} components the string components of the path
+ *   {string?} winDrive the drive or server for this path
+ * }}
+ *
+ * Other implementations may add additional OS-specific informations.
+ */
+var split = function(path) {
+  return {
+    absolute: this.winIsAbsolute(path),
+    winDrive: this.winGetDrive(path),
+    components: path.split("\\")
+  };
+};
+exports.split = split;
+
+/**
+ * Return the file:// URI file path of the given local file path.
+ */
+// The case of %3b is designed to match Services.io, but fundamentally doesn't matter.
+var toFileURIExtraEncodings = {';': '%3b', '?': '%3F', '#': '%23'};
+var toFileURI = function toFileURI(path) {
+  // URI-escape forward slashes and convert backward slashes to forward
+  path = this.normalize(path).replace(/[\\\/]/g, m => (m=='\\')? '/' : '%2F');
+  // Per https://url.spec.whatwg.org we should not encode [] in the path
+  let dontNeedEscaping = {'%5B': '[', '%5D': ']'};
+  let uri = encodeURI(path).replace(/%(5B|5D)/gi,
+    match => dontNeedEscaping[match]);
+
+  // add a prefix, and encodeURI doesn't escape a few characters that we do
+  // want to escape, so fix that up
+  let prefix = "file:///";
+  uri = prefix + uri.replace(/[;?#]/g, match => toFileURIExtraEncodings[match]);
+
+  // turn e.g., file:///C: into file:///C:/
+  if (uri.charAt(uri.length - 1) === ':') {
+    uri += "/"
+  }
+
+  return uri;
+};
+exports.toFileURI = toFileURI;
+
+/**
+ * Returns the local file path from a given file URI.
+ */
+var fromFileURI = function fromFileURI(uri) {
+  let url = new URL(uri);
+  if (url.protocol != 'file:') {
+    throw new Error("fromFileURI expects a file URI");
+  }
+
+  // strip leading slash, since Windows paths don't start with one
+  uri = url.pathname.substr(1);
+
+  let path = decodeURI(uri);
+  // decode a few characters where URL's parsing is overzealous
+  path = path.replace(/%(3b|3f|23)/gi,
+        match => decodeURIComponent(match));
+  path = this.normalize(path);
+
+  // this.normalize() does not remove the trailing slash if the path
+  // component is a drive letter. eg. 'C:\'' will not get normalized.
+  if (path.endsWith(":\\")) {
+    path = path.substr(0, path.length - 1);
+  }
+  return this.normalize(path);
+};
+exports.fromFileURI = fromFileURI;
+
+/**
+* Utility function: Remove any leading/trailing backslashes
+* from a string.
+*/
+var trimBackslashes = function trimBackslashes(string) {
+  return string.replace(/^\\+|\\+$/g,'');
+};
+
+//////////// Boilerplate
+if (typeof Components != "undefined") {
+  this.EXPORTED_SYMBOLS = EXPORTED_SYMBOLS;
+  for (let symbol of EXPORTED_SYMBOLS) {
+    this[symbol] = exports[symbol];
+  }
+}
diff --git a/toolkit/components/osfile/moz.build b/toolkit/components/osfile/moz.build
new file mode 100644
index 000000000..d17da2d60
--- /dev/null
+++ b/toolkit/components/osfile/moz.build
@@ -0,0 +1,35 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+DIRS += [
+    'modules',
+]
+
+MOCHITEST_CHROME_MANIFESTS += ['tests/mochi/chrome.ini']
+XPCSHELL_TESTS_MANIFESTS += ['tests/xpcshell/xpcshell.ini']
+
+SOURCES += [
+    'NativeOSFileInternals.cpp',
+]
+
+XPIDL_MODULE = 'toolkit_osfile'
+
+XPIDL_SOURCES += [
+    'nsINativeOSFileInternals.idl',
+]
+
+EXPORTS.mozilla += [
+    'NativeOSFileInternals.h',
+]
+
+EXTRA_PP_JS_MODULES += [
+    'osfile.jsm',
+]
+
+FINAL_LIBRARY = 'xul'
+
+with Files('**'):
+    BUG_COMPONENT = ('Toolkit', 'OS.File')
diff --git a/toolkit/components/osfile/nsINativeOSFileInternals.idl b/toolkit/components/osfile/nsINativeOSFileInternals.idl
new file mode 100644
index 000000000..c1bf8be14
--- /dev/null
+++ b/toolkit/components/osfile/nsINativeOSFileInternals.idl
@@ -0,0 +1,93 @@
+/* -*- Mode: c++; c-basic-offset: 2; indent-tabs-mode: nil; tab-width: 40 -*- */
+/* vim: set ts=2 et sw=2 tw=40: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsISupports.idl"
+
+/**
+ * The result of a successful asynchronous operation.
+ */
+[scriptable, builtinclass, uuid(08B4CF29-3D65-4E79-B522-A694C322ED07)]
+interface nsINativeOSFileResult: nsISupports
+{
+  /**
+   * The actual value produced by the operation.
+   *
+   * Actual type of this value depends on the options passed to the
+   * operation.
+   */
+  [implicit_jscontext]
+  readonly attribute jsval result;
+
+  /**
+   * Delay between when the operation was requested on the main thread and
+   * when the operation was started off main thread.
+   */
+  readonly attribute double dispatchDurationMS;
+
+  /**
+   * Duration of the off main thread execution.
+   */
+  readonly attribute double executionDurationMS;
+};
+
+/**
+ * A callback invoked in case of success.
+ */
+[scriptable, function, uuid(2C1922CA-CA1B-4099-8B61-EC23CFF49412)]
+interface nsINativeOSFileSuccessCallback: nsISupports
+{
+  void complete(in nsINativeOSFileResult result);
+};
+
+/**
+ * A callback invoked in case of error.
+ */
+[scriptable, function, uuid(F612E0FC-6736-4D24-AA50-FD661B3B40B6)]
+interface nsINativeOSFileErrorCallback: nsISupports
+{
+  /**
+   * @param operation The name of the failed operation. Provided to aid
+   * debugging only, may change without notice.
+   * @param OSstatus The OS status of the operation (errno under Unix,
+   * GetLastError under Windows).
+   */
+  void complete(in ACString operation, in long OSstatus);
+};
+
+/**
+ * A service providing native implementations of some of the features
+ * of OS.File.
+ */
+[scriptable, builtinclass, uuid(913362AD-1526-4623-9E6B-A2EB08AFBBB9)]
+interface nsINativeOSFileInternalsService: nsISupports
+{
+  /**
+   * Implementation of OS.File.read
+   *
+   * @param path The absolute path to the file to read.
+   * @param options An object that may contain some of the following fields
+   * - {number} bytes The maximal number of bytes to read.
+   * - {string} encoding If provided, return the result as a string, decoded
+   *   using this encoding. Otherwise, pass the result as an ArrayBuffer.
+   *   Invalid encodings cause onError to be called with the platform-specific
+   *   "invalid argument" constant.
+   * - {string} compression Unimplemented at the moment.
+   * @param onSuccess The success callback.
+   * @param onError The error callback.
+   */
+  [implicit_jscontext]
+  void read(in AString path, in jsval options,
+            in nsINativeOSFileSuccessCallback onSuccess,
+            in nsINativeOSFileErrorCallback onError);
+};
+
+
+%{ C++
+
+#define NATIVE_OSFILE_INTERNALS_SERVICE_CID {0x63A69303,0x8A64,0x45A9,{0x84, 0x8C, 0xD4, 0xE2, 0x79, 0x27, 0x94, 0xE6}}
+#define NATIVE_OSFILE_INTERNALS_SERVICE_CONTRACTID "@mozilla.org/toolkit/osfile/native-internals;1"
+
+%}
diff --git a/toolkit/components/osfile/osfile.jsm b/toolkit/components/osfile/osfile.jsm
new file mode 100644
index 000000000..20f21591a
--- /dev/null
+++ b/toolkit/components/osfile/osfile.jsm
@@ -0,0 +1,32 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * Common front for various implementations of OS.File
+ */
+
+if (typeof Components != "undefined") {
+  this.EXPORTED_SYMBOLS = ["OS"];
+  Components.utils.import("resource://gre/modules/osfile/osfile_async_front.jsm", this);
+} else {
+  // At this stage, we need to import all sources at once to avoid
+  // a unique failure on tbpl + talos that seems caused by a
+  // what looks like a nested event loop bug (see bug 794091).
+#ifdef XP_WIN
+  importScripts(
+    "resource://gre/modules/workers/require.js",
+    "resource://gre/modules/osfile/osfile_win_back.jsm",
+    "resource://gre/modules/osfile/osfile_shared_front.jsm",
+    "resource://gre/modules/osfile/osfile_win_front.jsm"
+  );
+#else
+  importScripts(
+    "resource://gre/modules/workers/require.js",
+    "resource://gre/modules/osfile/osfile_unix_back.jsm",
+    "resource://gre/modules/osfile/osfile_shared_front.jsm",
+    "resource://gre/modules/osfile/osfile_unix_front.jsm"
+  );
+#endif
+  OS.Path = require("resource://gre/modules/osfile/ospath.jsm");
+}
diff --git a/toolkit/components/osfile/tests/mochi/.eslintrc.js b/toolkit/components/osfile/tests/mochi/.eslintrc.js
new file mode 100644
index 000000000..8c0f4f574
--- /dev/null
+++ b/toolkit/components/osfile/tests/mochi/.eslintrc.js
@@ -0,0 +1,7 @@
+"use strict";
+
+module.exports = {
+  "extends": [
+    "../../../../../testing/mochitest/chrome.eslintrc.js"
+  ]
+};
diff --git a/toolkit/components/osfile/tests/mochi/chrome.ini b/toolkit/components/osfile/tests/mochi/chrome.ini
new file mode 100644
index 000000000..1da463316
--- /dev/null
+++ b/toolkit/components/osfile/tests/mochi/chrome.ini
@@ -0,0 +1,15 @@
+[DEFAULT]
+skip-if = os == 'android'
+support-files =
+  main_test_osfile_async.js
+  worker_handler.js
+  worker_test_osfile_comms.js
+  worker_test_osfile_front.js
+  worker_test_osfile_shared.js
+  worker_test_osfile_unix.js
+  worker_test_osfile_win.js
+
+[test_osfile_async.xul]
+[test_osfile_back.xul]
+[test_osfile_comms.xul]
+[test_osfile_front.xul]
diff --git a/toolkit/components/osfile/tests/mochi/main_test_osfile_async.js b/toolkit/components/osfile/tests/mochi/main_test_osfile_async.js
new file mode 100644
index 000000000..b940a032a
--- /dev/null
+++ b/toolkit/components/osfile/tests/mochi/main_test_osfile_async.js
@@ -0,0 +1,443 @@
+"use strict";
+
+Components.utils.import("resource://gre/modules/osfile.jsm");
+Components.utils.import("resource://gre/modules/Promise.jsm");
+Components.utils.import("resource://gre/modules/Task.jsm");
+Components.utils.import("resource://gre/modules/AsyncShutdown.jsm");
+
+// The following are used to compare against a well-tested reference
+// implementation of file I/O.
+Components.utils.import("resource://gre/modules/NetUtil.jsm");
+Components.utils.import("resource://gre/modules/FileUtils.jsm");
+Components.utils.import("resource://gre/modules/Services.jsm");
+
+var myok = ok;
+var myis = is;
+var myinfo = info;
+var myisnot = isnot;
+
+var isPromise = function ispromise(value) {
+  return value != null && typeof value == "object" && "then" in value;
+};
+
+var maketest = function(prefix, test) {
+  let utils = {
+    ok: function ok(t, m) {
+      myok(t, prefix + ": " + m);
+    },
+    is: function is(l, r, m) {
+      myis(l, r, prefix + ": " + m);
+    },
+    isnot: function isnot(l, r, m) {
+      myisnot(l, r, prefix + ": " + m);
+    },
+    info: function info(m) {
+      myinfo(prefix + ": " + m);
+    },
+    fail: function fail(m) {
+      utils.ok(false, m);
+    },
+    okpromise: function okpromise(t, m) {
+      return t.then(
+        function onSuccess() {
+          util.ok(true, m);
+        },
+        function onFailure() {
+          util.ok(false, m);
+        }
+      );
+    }
+  };
+  return function runtest() {
+    utils.info("Entering");
+    try {
+      let result = test.call(this, utils);
+      if (!isPromise(result)) {
+        throw new TypeError("The test did not return a promise");
+      }
+      utils.info("This was a promise");
+      // The test returns a promise
+      result = result.then(function test_complete() {
+        utils.info("Complete");
+      }, function catch_uncaught_errors(err) {
+        utils.fail("Uncaught error " + err);
+        if (err && typeof err == "object" && "message" in err) {
+          utils.fail("(" + err.message + ")");
+        }
+        if (err && typeof err == "object" && "stack" in err) {
+          utils.fail("at " + err.stack);
+        }
+      });
+      return result;
+    } catch (x) {
+      utils.fail("Error " + x + " at " + x.stack);
+      return null;
+    }
+  };
+};
+
+/**
+ * Fetch asynchronously the contents of a file using xpcom.
+ *
+ * Used for comparing xpcom-based results to os.file-based results.
+ *
+ * @param {string} path The _absolute_ path to the file.
+ * @return {promise}
+ * @resolves {string} The contents of the file.
+ */
+var reference_fetch_file = function reference_fetch_file(path, test) {
+  test.info("Fetching file " + path);
+  let promise = Promise.defer();
+  let file = new FileUtils.File(path);
+  NetUtil.asyncFetch({
+    uri: NetUtil.newURI(file),
+    loadUsingSystemPrincipal: true
+  }, function(stream, status) {
+      if (!Components.isSuccessCode(status)) {
+        promise.reject(status);
+        return;
+      }
+      let result, reject;
+      try {
+        result = NetUtil.readInputStreamToString(stream, stream.available());
+      } catch (x) {
+        reject = x;
+      }
+      stream.close();
+      if (reject) {
+        promise.reject(reject);
+      } else {
+        promise.resolve(result);
+      }
+    });
+
+  return promise.promise;
+};
+
+/**
+ * Compare asynchronously the contents two files using xpcom.
+ *
+ * Used for comparing xpcom-based results to os.file-based results.
+ *
+ * @param {string} a The _absolute_ path to the first file.
+ * @param {string} b The _absolute_ path to the second file.
+ *
+ * @resolves {null}
+ */
+var reference_compare_files = function reference_compare_files(a, b, test) {
+  test.info("Comparing files " + a + " and " + b);
+  let a_contents = yield reference_fetch_file(a, test);
+  let b_contents = yield reference_fetch_file(b, test);
+  is(a_contents, b_contents, "Contents of files " + a + " and " + b + " match");
+};
+
+var reference_dir_contents = function reference_dir_contents(path) {
+  let result = [];
+  let entries = new FileUtils.File(path).directoryEntries;
+  while (entries.hasMoreElements()) {
+    let entry = entries.getNext().QueryInterface(Components.interfaces.nsILocalFile);
+    result.push(entry.path);
+  }
+  return result;
+};
+
+// Set/Unset OS.Shared.DEBUG, OS.Shared.TEST and a console listener.
+function toggleDebugTest (pref, consoleListener) {
+  Services.prefs.setBoolPref("toolkit.osfile.log", pref);
+  Services.prefs.setBoolPref("toolkit.osfile.log.redirect", pref);
+  Services.console[pref ? "registerListener" : "unregisterListener"](
+    consoleListener);
+}
+
+var test = maketest("Main", function main(test) {
+  return Task.spawn(function() {
+    SimpleTest.waitForExplicitFinish();
+    yield test_stat();
+    yield test_debug();
+    yield test_info_features_detect();
+    yield test_position();
+    yield test_iter();
+    yield test_exists();
+    yield test_debug_test();
+    info("Test is over");
+    SimpleTest.finish();
+  });
+});
+
+/**
+ * A file that we know exists and that can be used for reading.
+ */
+var EXISTING_FILE = OS.Path.join("chrome", "toolkit", "components",
+  "osfile", "tests", "mochi", "main_test_osfile_async.js");
+
+/**
+ * Test OS.File.stat and OS.File.prototype.stat
+ */
+var test_stat = maketest("stat", function stat(test) {
+  return Task.spawn(function() {
+    // Open a file and stat it
+    let file = yield OS.File.open(EXISTING_FILE);
+    let stat1;
+
+    try {
+      test.info("Stating file");
+      stat1 = yield file.stat();
+      test.ok(true, "stat has worked " + stat1);
+      test.ok(stat1, "stat is not empty");
+    } finally {
+      yield file.close();
+    }
+
+    // Stat the same file without opening it
+    test.info("Stating a file without opening it");
+    let stat2 = yield OS.File.stat(EXISTING_FILE);
+    test.ok(true, "stat 2 has worked " + stat2);
+    test.ok(stat2, "stat 2 is not empty");
+    for (let key in stat2) {
+      test.is("" + stat1[key], "" + stat2[key], "Stat field " + key + "is the same");
+    }
+  });
+});
+
+/**
+ * Test feature detection using OS.File.Info.prototype on main thread
+ */
+var test_info_features_detect = maketest("features_detect", function features_detect(test) {
+  return Task.spawn(function() {
+    if (OS.Constants.Win) {
+      // see if winBirthDate is defined
+      if ("winBirthDate" in OS.File.Info.prototype) {
+        test.ok(true, "winBirthDate is defined");
+      } else {
+        test.fail("winBirthDate not defined though we are under Windows");
+      }
+    } else if (OS.Constants.libc) {
+      // see if unixGroup is defined
+      if ("unixGroup" in OS.File.Info.prototype) {
+        test.ok(true, "unixGroup is defined");
+      } else {
+        test.fail("unixGroup is not defined though we are under Unix");
+      }
+    }
+  });
+});
+
+/**
+ * Test file.{getPosition, setPosition}
+ */
+var test_position = maketest("position", function position(test) {
+  return Task.spawn(function() {
+    let file = yield OS.File.open(EXISTING_FILE);
+
+    try {
+      let view = yield file.read();
+      test.info("First batch of content read");
+      let CHUNK_SIZE = 178;// An arbitrary number of bytes to read from the file
+      let pos = yield file.getPosition();
+      test.info("Obtained position");
+      test.is(pos, view.byteLength, "getPosition returned the end of the file");
+      pos = yield file.setPosition(-CHUNK_SIZE, OS.File.POS_END);
+      test.info("Changed position");
+      test.is(pos, view.byteLength - CHUNK_SIZE, "setPosition returned the correct position");
+
+      let view2 = yield file.read();
+      test.info("Read the end of the file");
+      for (let i = 0; i < CHUNK_SIZE; ++i) {
+        if (view2[i] != view[i + view.byteLength - CHUNK_SIZE]) {
+          test.is(view2[i], view[i], "setPosition put us in the right position");
+        }
+      }
+    } finally {
+      yield file.close();
+    }
+  });
+});
+
+/**
+ * Test OS.File.prototype.{DirectoryIterator}
+ */
+var test_iter = maketest("iter", function iter(test) {
+  return Task.spawn(function() {
+    let currentDir = yield OS.File.getCurrentDirectory();
+
+    // Trivial walks through the directory
+    test.info("Preparing iteration");
+    let iterator = new OS.File.DirectoryIterator(currentDir);
+    let temporary_file_name = OS.Path.join(currentDir, "empty-temporary-file.tmp");
+    try {
+      yield OS.File.remove(temporary_file_name);
+    } catch (err) {
+      // Ignore errors removing file
+    }
+    let allFiles1 = yield iterator.nextBatch();
+    test.info("Obtained all files through nextBatch");
+    test.isnot(allFiles1.length, 0, "There is at least one file");
+    test.isnot(allFiles1[0].path, null, "Files have a path");
+
+    // Ensure that we have the same entries with |reference_dir_contents|
+    let referenceEntries = new Set();
+    for (let entry of reference_dir_contents(currentDir)) {
+      referenceEntries.add(entry);
+    }
+    test.is(referenceEntries.size, allFiles1.length, "All the entries in the directory have been listed");
+    for (let entry of allFiles1) {
+      test.ok(referenceEntries.has(entry.path), "File " + entry.path + " effectively exists");
+      // Ensure that we have correct isDir and isSymLink
+      // Current directory is {objdir}/_tests/testing/mochitest/, assume it has some dirs and symlinks.
+      var f = new FileUtils.File(entry.path);
+      test.is(entry.isDir, f.isDirectory(), "Get file " + entry.path + " isDir correctly");
+      test.is(entry.isSymLink, f.isSymlink(), "Get file " + entry.path + " isSymLink correctly");
+    }
+
+    yield iterator.close();
+    test.info("Closed iterator");
+
+    test.info("Double closing DirectoryIterator");
+    iterator = new OS.File.DirectoryIterator(currentDir);
+    yield iterator.close();
+    yield iterator.close(); //double closing |DirectoryIterator|
+    test.ok(true, "|DirectoryIterator| was closed twice successfully");
+
+    let allFiles2 = [];
+    let i = 0;
+    iterator = new OS.File.DirectoryIterator(currentDir);
+    yield iterator.forEach(function(entry, index) {
+      test.is(i++, index, "Getting the correct index");
+      allFiles2.push(entry);
+    });
+    test.info("Obtained all files through forEach");
+    is(allFiles1.length, allFiles2.length, "Both runs returned the same number of files");
+    for (let i = 0; i < allFiles1.length; ++i) {
+      if (allFiles1[i].path != allFiles2[i].path) {
+        test.is(allFiles1[i].path, allFiles2[i].path, "Both runs return the same files");
+        break;
+      }
+    }
+
+    // Testing batch iteration + whether an iteration can be stopped early
+    let BATCH_LENGTH = 10;
+    test.info("Getting some files through nextBatch");
+    yield iterator.close();
+
+    iterator = new OS.File.DirectoryIterator(currentDir);
+    let someFiles1 = yield iterator.nextBatch(BATCH_LENGTH);
+    let someFiles2 = yield iterator.nextBatch(BATCH_LENGTH);
+    yield iterator.close();
+
+    iterator = new OS.File.DirectoryIterator(currentDir);
+    yield iterator.forEach(function cb(entry, index, iterator) {
+      if (index < BATCH_LENGTH) {
+        test.is(entry.path, someFiles1[index].path, "Both runs return the same files (part 1)");
+      } else if (index < 2*BATCH_LENGTH) {
+        test.is(entry.path, someFiles2[index - BATCH_LENGTH].path, "Both runs return the same files (part 2)");
+      } else if (index == 2 * BATCH_LENGTH) {
+        test.info("Attempting to stop asynchronous forEach");
+        return iterator.close();
+      } else {
+        test.fail("Can we stop an asynchronous forEach? " + index);
+      }
+      return null;
+    });
+    yield iterator.close();
+
+    // Ensuring that we find new files if they appear
+    let file = yield OS.File.open(temporary_file_name, { write: true } );
+    file.close();
+    iterator = new OS.File.DirectoryIterator(currentDir);
+    try {
+      let files = yield iterator.nextBatch();
+      is(files.length, allFiles1.length + 1, "The directory iterator has noticed the new file");
+      let exists = yield iterator.exists();
+      test.ok(exists, "After nextBatch, iterator detects that the directory exists");
+    } finally {
+      yield iterator.close();
+    }
+
+    // Ensuring that opening a non-existing directory fails consistently
+    // once iteration starts.
+    try {
+      iterator = null;
+      iterator = new OS.File.DirectoryIterator("/I do not exist");
+      let exists = yield iterator.exists();
+      test.ok(!exists, "Before any iteration, iterator detects that the directory doesn't exist");
+      let exn = null;
+      try {
+        yield iterator.next();
+      } catch (ex if ex instanceof OS.File.Error && ex.becauseNoSuchFile) {
+        exn = ex;
+        let exists = yield iterator.exists();
+        test.ok(!exists, "After one iteration, iterator detects that the directory doesn't exist");
+      }
+      test.ok(exn, "Iterating through a directory that does not exist has failed with becauseNoSuchFile");
+    } finally {
+      if (iterator) {
+        iterator.close();
+      }
+    }
+    test.ok(!!iterator, "The directory iterator for a non-existing directory was correctly created");
+  });
+});
+
+/**
+ * Test OS.File.prototype.{exists}
+ */
+var test_exists = maketest("exists", function exists(test) {
+  return Task.spawn(function() {
+    let fileExists = yield OS.File.exists(EXISTING_FILE);
+    test.ok(fileExists, "file exists");
+    fileExists = yield OS.File.exists(EXISTING_FILE + ".tmp");
+    test.ok(!fileExists, "file does not exists");
+  });
+});
+
+/**
+ * Test changes to OS.Shared.DEBUG flag.
+ */
+var test_debug = maketest("debug", function debug(test) {
+  return Task.spawn(function() {
+    function testSetDebugPref (pref) {
+      try {
+        Services.prefs.setBoolPref("toolkit.osfile.log", pref);
+      } catch (x) {
+        test.fail("Setting OS.Shared.DEBUG to " + pref +
+          " should not cause error.");
+      } finally {
+        test.is(OS.Shared.DEBUG, pref, "OS.Shared.DEBUG is set correctly.");
+      }
+    }
+    testSetDebugPref(true);
+    let workerDEBUG = yield OS.File.GET_DEBUG();
+    test.is(workerDEBUG, true, "Worker's DEBUG is set.");
+    testSetDebugPref(false);
+    workerDEBUG = yield OS.File.GET_DEBUG();
+    test.is(workerDEBUG, false, "Worker's DEBUG is unset.");
+  });
+});
+
+/**
+ * Test logging in the main thread with set OS.Shared.DEBUG and
+ * OS.Shared.TEST flags.
+ */
+var test_debug_test = maketest("debug_test", function debug_test(test) {
+  return Task.spawn(function () {
+    // Create a console listener.
+    let consoleListener = {
+      observe: function (aMessage) {
+        // Ignore unexpected messages.
+        if (!(aMessage instanceof Components.interfaces.nsIConsoleMessage)) {
+          return;
+        }
+        if (aMessage.message.indexOf("TEST OS") < 0) {
+          return;
+        }
+        test.ok(true, "DEBUG TEST messages are logged correctly.");
+      }
+    };
+    toggleDebugTest(true, consoleListener);
+    // Execution of OS.File.exist method will trigger OS.File.LOG several times.
+    let fileExists = yield OS.File.exists(EXISTING_FILE);
+    toggleDebugTest(false, consoleListener);
+  });
+});
+
+
diff --git a/toolkit/components/osfile/tests/mochi/test_osfile_async.xul b/toolkit/components/osfile/tests/mochi/test_osfile_async.xul
new file mode 100644
index 000000000..1ef103f02
--- /dev/null
+++ b/toolkit/components/osfile/tests/mochi/test_osfile_async.xul
@@ -0,0 +1,23 @@
+<?xml version="1.0"?>
+<!--
+  Any copyright is dedicated to the Public Domain.
+  http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<window title="Testing async I/O with OS.File"
+        xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+        onload="test();">
+
+  <script type="application/javascript"
+          src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"/>
+  <script type="application/javascript"
+          src="chrome://mochikit/content/tests/SimpleTest/EventUtils.js"/>
+  <script type="application/javascript"
+          src="main_test_osfile_async.js"/>
+
+  <body xmlns="http://www.w3.org/1999/xhtml">
+    <p id="display"></p>
+    <div id="content" style="display:none;"></div>
+    <pre id="test"></pre>
+  </body>
+  <label id="test-result"/>
+</window>
diff --git a/toolkit/components/osfile/tests/mochi/test_osfile_back.xul b/toolkit/components/osfile/tests/mochi/test_osfile_back.xul
new file mode 100644
index 000000000..b6af6303f
--- /dev/null
+++ b/toolkit/components/osfile/tests/mochi/test_osfile_back.xul
@@ -0,0 +1,46 @@
+<?xml version="1.0"?>
+<!--
+  Any copyright is dedicated to the Public Domain.
+  http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<window title="Testing OS.File on a chrome worker thread"
+        xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+        onload="test();">
+
+  <script type="application/javascript"
+          src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"/>
+  <script type="application/javascript"
+          src="chrome://mochikit/content/tests/SimpleTest/EventUtils.js"/>
+  <script type="application/javascript"
+          src="worker_handler.js"/>
+  <script type="application/javascript">
+  <![CDATA[
+
+let worker;
+
+function test() {
+  ok(true, "test_osfile.xul: Starting test");
+  if (navigator.platform.indexOf("Win") != -1) {
+    ok(true, "test_osfile.xul: Using Windows test suite");
+    worker = new ChromeWorker("worker_test_osfile_win.js");
+  } else {
+    ok(true, "test_osfile.xul: Using Unix test suite");
+    worker = new ChromeWorker("worker_test_osfile_unix.js");
+  }
+  SimpleTest.waitForExplicitFinish();
+  ok(true, "test_osfile.xul: Chrome worker created");
+  dump("MAIN: go\n");
+  worker_handler(worker);
+  worker.postMessage(0);
+  ok(true, "test_osfile.xul: Test in progress");
+};
+]]>
+  </script>
+
+  <body xmlns="http://www.w3.org/1999/xhtml">
+    <p id="display"></p>
+    <div id="content" style="display:none;"></div>
+    <pre id="test"></pre>
+  </body>
+  <label id="test-result"/>
+</window>
diff --git a/toolkit/components/osfile/tests/mochi/test_osfile_comms.xul b/toolkit/components/osfile/tests/mochi/test_osfile_comms.xul
new file mode 100644
index 000000000..88e474ce2
--- /dev/null
+++ b/toolkit/components/osfile/tests/mochi/test_osfile_comms.xul
@@ -0,0 +1,84 @@
+<?xml version="1.0"?>
+<!--
+  Any copyright is dedicated to the Public Domain.
+  http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<window title="Testing OS.File on a chrome worker thread"
+        xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+        onload="test();">
+
+  <script type="application/javascript"
+          src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"/>
+  <script type="application/javascript"
+          src="chrome://mochikit/content/tests/SimpleTest/EventUtils.js"/>
+  <script type="application/javascript">
+  <![CDATA[
+
+"use strict";
+
+let worker;
+
+let test = function test() {
+  SimpleTest.info("test_osfile_comms.xul: Starting test");
+  Components.utils.import("resource://gre/modules/ctypes.jsm");
+  Components.utils.import("resource://gre/modules/osfile.jsm");
+  worker = new ChromeWorker("worker_test_osfile_comms.js");
+  SimpleTest.waitForExplicitFinish();
+  try {
+    worker.onerror = function onerror(error) {
+      SimpleTest.ok(false, "received error "+error);
+    }
+   worker.onmessage = function onmessage(msg) {
+    Components.utils.forceShrinkingGC();
+    switch (msg.data.kind) {
+      case "is":
+        SimpleTest.ok(msg.data.outcome, msg.data.description +
+        " ("+ msg.data.a + " ==? " + msg.data.b + ")" );
+        return;
+      case "isnot":
+        SimpleTest.ok(msg.data.outcome, msg.data.description +
+        " ("+ msg.data.a + " !=? " + msg.data.b + ")" );
+        return;
+      case "ok":
+        SimpleTest.ok(msg.data.condition, msg.data.description);
+        return;
+      case "info":
+        SimpleTest.info(msg.data.description);
+        return;
+      case "finish":
+        SimpleTest.finish();
+        return;
+      case "value":
+        SimpleTest.ok(true, "test_osfile_comms.xul: Received value " + JSON.stringify(msg.data.value));
+        let type = eval(msg.data.typename);
+        let check = eval(msg.data.check);
+        let value = msg.data.value;
+        let deserialized = type.fromMsg(value);
+        check(deserialized, "Main thread test: ");
+       return;
+      default:
+        SimpleTest.ok(false, "test_osfile_comms.xul: wrong message "+JSON.stringify(msg.data));
+        return;
+      }
+    };
+    worker.postMessage(0)
+    ok(true, "Worker launched");
+  } catch(x) {
+    // As we have set |waitForExplicitFinish|, we add this fallback
+    // in case of uncaught error, to ensure that the test does not
+    // just freeze.
+    ok(false, "Caught exception " + x + " at " + x.stack);
+    SimpleTest.finish();
+  }
+};
+
+]]>
+  </script>
+
+  <body xmlns="http://www.w3.org/1999/xhtml">
+    <p id="display"></p>
+    <div id="content" style="display:none;"></div>
+    <pre id="test"></pre>
+  </body>
+  <label id="test-result"/>
+</window>
diff --git a/toolkit/components/osfile/tests/mochi/test_osfile_front.xul b/toolkit/components/osfile/tests/mochi/test_osfile_front.xul
new file mode 100644
index 000000000..10d3fbd01
--- /dev/null
+++ b/toolkit/components/osfile/tests/mochi/test_osfile_front.xul
@@ -0,0 +1,44 @@
+<?xml version="1.0"?>
+<!--
+  Any copyright is dedicated to the Public Domain.
+  http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<window title="Testing OS.File on a chrome worker thread"
+        xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+        onload="test();">
+
+  <script type="application/javascript"
+          src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"/>
+  <script type="application/javascript"
+          src="chrome://mochikit/content/tests/SimpleTest/EventUtils.js"/>
+  <script type="application/javascript"
+          src="worker_handler.js"/>
+  <script type="application/javascript">
+  <![CDATA[
+
+let worker;
+let main = this;
+
+function test() {
+  info("test_osfile_front.xul: Starting test");
+
+  // Test the OS.File worker
+
+  worker = new ChromeWorker("worker_test_osfile_front.js");
+  SimpleTest.waitForExplicitFinish();
+  info("test_osfile_front.xul: Chrome worker created");
+  dump("MAIN: go\n");
+  worker_handler(worker);
+  worker.postMessage(0);
+  ok(true, "test_osfile_front.xul: Test in progress");
+};
+]]>
+  </script>
+
+  <body xmlns="http://www.w3.org/1999/xhtml">
+    <p id="display"></p>
+    <div id="content" style="display:none;"></div>
+    <pre id="test"></pre>
+  </body>
+  <label id="test-result"/>
+</window>
diff --git a/toolkit/components/osfile/tests/mochi/worker_handler.js b/toolkit/components/osfile/tests/mochi/worker_handler.js
new file mode 100644
index 000000000..6c513e6ba
--- /dev/null
+++ b/toolkit/components/osfile/tests/mochi/worker_handler.js
@@ -0,0 +1,34 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+function worker_handler(worker) {
+  worker.onerror = function(error) {
+    error.preventDefault();
+    ok(false, "Worker error " + error.message);
+  }
+  worker.onmessage = function(msg) {
+    ok(true, "MAIN: onmessage " + JSON.stringify(msg.data));
+    switch (msg.data.kind) {
+    case "is":
+      SimpleTest.ok(msg.data.outcome, msg.data.description +
+         "( "+ msg.data.a + " ==? " + msg.data.b + ")" );
+      return;
+    case "isnot":
+      SimpleTest.ok(msg.data.outcome, msg.data.description +
+      "( "+ msg.data.a + " !=? " + msg.data.b + ")" );
+         return;
+    case "ok":
+      SimpleTest.ok(msg.data.condition, msg.data.description);
+      return;
+    case "info":
+      SimpleTest.info(msg.data.description);
+      return;
+    case "finish":
+      SimpleTest.finish();
+      return;
+    default:
+      SimpleTest.ok(false, "test_osfile.xul: wrong message " + JSON.stringify(msg.data));
+      return;
+    }
+  };
+}
diff --git a/toolkit/components/osfile/tests/mochi/worker_test_osfile_comms.js b/toolkit/components/osfile/tests/mochi/worker_test_osfile_comms.js
new file mode 100644
index 000000000..5e8bdd9ca
--- /dev/null
+++ b/toolkit/components/osfile/tests/mochi/worker_test_osfile_comms.js
@@ -0,0 +1,145 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+importScripts('worker_test_osfile_shared.js');
+
+// The set of samples for communications test. Declare as a global
+// variable to prevent this from being garbage-collected too early.
+var samples;
+
+self.onmessage = function(msg) {
+  info("Initializing");
+  self.onmessage = function on_unexpected_message(msg) {
+    throw new Error("Unexpected message " + JSON.stringify(msg.data));
+  };
+  importScripts("resource://gre/modules/osfile.jsm");
+  info("Initialization complete");
+
+  samples = [
+    { typename: "OS.Shared.Type.char.in_ptr",
+      valuedescr: "String",
+      value: "This is a test",
+      type: OS.Shared.Type.char.in_ptr,
+      check: function check_string(candidate, prefix) {
+        is(candidate, "This is a test", prefix);
+      }},
+    { typename: "OS.Shared.Type.char.in_ptr",
+      valuedescr: "Typed array",
+      value: (function() {
+                let view = new Uint8Array(15);
+                for (let i = 0; i < 15; ++i) {
+                  view[i] = i;
+                }
+                return view;
+              })(),
+      type: OS.Shared.Type.char.in_ptr,
+      check: function check_ArrayBuffer(candidate, prefix) {
+        for (let i = 0; i < 15; ++i) {
+          is(candidate[i], i % 256, prefix + "Checking that the contents of the ArrayBuffer were preserved");
+        }
+      }},
+    { typename: "OS.Shared.Type.char.in_ptr",
+      valuedescr: "Pointer",
+      value: new OS.Shared.Type.char.in_ptr.implementation(1),
+      type: OS.Shared.Type.char.in_ptr,
+      check: function check_ptr(candidate, prefix) {
+        let address = ctypes.cast(candidate, ctypes.uintptr_t).value.toString();
+        is(address, "1", prefix + "Checking that the pointer address was preserved");
+      }},
+    { typename: "OS.Shared.Type.char.in_ptr",
+      valuedescr: "C array",
+      value: (function() {
+                let buf = new (ctypes.ArrayType(ctypes.uint8_t, 15))();
+                for (let i = 0; i < 15; ++i) {
+                  buf[i] = i % 256;
+                }
+                return buf;
+              })(),
+      type: OS.Shared.Type.char.in_ptr,
+      check: function check_array(candidate, prefix) {
+        let cast = ctypes.cast(candidate, ctypes.uint8_t.ptr);
+        for (let i = 0; i < 15; ++i) {
+          is(cast.contents, i % 256, prefix + "Checking that the contents of the C array were preserved, index " + i);
+          cast = cast.increment();
+        }
+      }
+    },
+    { typename: "OS.File.Error",
+      valuedescr: "OS Error",
+      type: OS.File.Error,
+      value: new OS.File.Error("foo", 1),
+      check: function check_error(candidate, prefix) {
+        ok(candidate instanceof OS.File.Error,
+          prefix + "Error is an OS.File.Error");
+        ok(candidate.unixErrno == 1 || candidate.winLastError == 1,
+           prefix + "Error code is correct");
+        try {
+          let string = candidate.toString();
+          info(prefix + ".toString() works " + string);
+        } catch (x) {
+          ok(false, prefix + ".toString() fails " + x);
+        }
+      }
+    }
+  ];
+  samples.forEach(function test(sample) {
+    let type = sample.type;
+    let value = sample.value;
+    let check = sample.check;
+    info("Testing handling of type " + sample.typename + " communicating " + sample.valuedescr);
+
+    // 1. Test serialization
+    let serialized;
+    let exn;
+    try {
+      serialized = type.toMsg(value);
+    } catch (ex) {
+      exn = ex;
+    }
+    is(exn, null, "Can I serialize the following value? " + value +
+      " aka " + JSON.stringify(value));
+    if (exn) {
+      return;
+    }
+
+    if ("data" in serialized) {
+      // Unwrap from `Meta`
+      serialized = serialized.data;
+    }
+
+    // 2. Test deserialization
+    let deserialized;
+    try {
+      deserialized = type.fromMsg(serialized);
+    } catch (ex) {
+      exn = ex;
+    }
+    is(exn, null, "Can I deserialize the following message? " + serialized
+     + " aka " + JSON.stringify(serialized));
+    if (exn) {
+      return;
+    }
+
+    // 3. Local test deserialized value
+    info("Running test on deserialized value " + deserialized +
+      " aka " + JSON.stringify(deserialized));
+    check(deserialized, "Local test: ");
+
+    // 4. Test sending serialized
+    info("Attempting to send message");
+    try {
+      self.postMessage({kind:"value",
+        typename: sample.typename,
+        value: serialized,
+        check: check.toSource()});
+    } catch (ex) {
+      exn = ex;
+    }
+    is(exn, null, "Can I send the following message? " + serialized
+     + " aka " + JSON.stringify(serialized));
+  });
+
+  finish();
+ };
diff --git a/toolkit/components/osfile/tests/mochi/worker_test_osfile_front.js b/toolkit/components/osfile/tests/mochi/worker_test_osfile_front.js
new file mode 100644
index 000000000..29e613510
--- /dev/null
+++ b/toolkit/components/osfile/tests/mochi/worker_test_osfile_front.js
@@ -0,0 +1,566 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+importScripts('worker_test_osfile_shared.js');
+importScripts("resource://gre/modules/workers/require.js");
+
+var SharedAll = require("resource://gre/modules/osfile/osfile_shared_allthreads.jsm");
+SharedAll.Config.DEBUG = true;
+
+function should_throw(f) {
+  try {
+    f();
+  } catch (x) {
+    return x;
+  }
+  return null;
+}
+
+self.onmessage = function onmessage_start(msg) {
+  self.onmessage = function onmessage_ignored(msg) {
+    log("ignored message " + JSON.stringify(msg.data));
+  };
+  try {
+    test_init();
+    test_open_existing_file();
+    test_open_non_existing_file();
+    test_flush_open_file();
+    test_copy_existing_file();
+    test_position();
+    test_move_file();
+    test_iter_dir();
+    test_info();
+    test_path();
+    test_exists_file();
+    test_remove_file();
+  } catch (x) {
+    log("Catching error: " + x);
+    log("Stack: " + x.stack);
+    log("Source: " + x.toSource());
+    ok(false, x.toString() + "\n" + x.stack);
+  }
+  finish();
+};
+
+function test_init() {
+  info("Starting test_init");
+  importScripts("resource://gre/modules/osfile.jsm");
+}
+
+/**
+ * Test that we can open an existing file.
+ */
+function test_open_existing_file()
+{
+  info("Starting test_open_existing");
+  let file = OS.File.open("chrome/toolkit/components/osfile/tests/mochi/worker_test_osfile_unix.js");
+  file.close();
+}
+
+/**
+ * Test that opening a file that does not exist fails with the right error.
+ */
+function test_open_non_existing_file()
+{
+  info("Starting test_open_non_existing");
+  let exn;
+  try {
+    let file = OS.File.open("/I do not exist");
+  } catch (x) {
+    exn = x;
+    info("test_open_non_existing_file: Exception detail " + exn);
+  }
+  ok(!!exn, "test_open_non_existing_file: Exception was raised ");
+  ok(exn instanceof OS.File.Error, "test_open_non_existing_file: Exception was a OS.File.Error");
+  ok(exn.becauseNoSuchFile, "test_open_non_existing_file: Exception confirms that the file does not exist");
+}
+
+/**
+ * Test that to ensure that |foo.flush()| does not
+ * cause an error, where |foo| is an open file.
+ */
+function test_flush_open_file()
+{
+  info("Starting test_flush_open_file");
+  let tmp = "test_flush.tmp";
+  let file = OS.File.open(tmp, {create: true, write: true});
+  file.flush();
+  file.close();
+  OS.File.remove(tmp);
+}
+
+/**
+ * Utility function for comparing two files (or a prefix of two files).
+ *
+ * This function returns nothing but fails of both files (or prefixes)
+ * are not identical.
+ *
+ * @param {string} test The name of the test (used for logging).
+ * @param {string} sourcePath The name of the first file.
+ * @param {string} destPath The name of the second file.
+ * @param {number=} prefix If specified, only compare the |prefix|
+ * first bytes of |sourcePath| and |destPath|.
+ */
+function compare_files(test, sourcePath, destPath, prefix)
+{
+  info(test + ": Comparing " + sourcePath + " and " + destPath);
+  let source = OS.File.open(sourcePath);
+  let dest = OS.File.open(destPath);
+  info("Files are open");
+  let sourceResult, destResult;
+  try {
+    if (prefix != undefined) {
+      sourceResult = source.read(prefix);
+      destResult = dest.read(prefix);
+    } else {
+      sourceResult = source.read();
+      destResult = dest.read();
+    }
+    is(sourceResult.length, destResult.length, test + ": Both files have the same size");
+    for (let i = 0; i < sourceResult.length; ++i) {
+      if (sourceResult[i] != destResult[i]) {
+        is(sourceResult[i] != destResult[i], test + ": Comparing char " + i);
+        break;
+      }
+    }
+  } finally {
+    source.close();
+    dest.close();
+  }
+  info(test + ": Comparison complete");
+}
+
+/**
+ * Test that copying a file using |copy| works.
+ */
+function test_copy_existing_file()
+{
+  let src_file_name =
+    OS.Path.join("chrome", "toolkit", "components", "osfile", "tests", "mochi",
+                 "worker_test_osfile_front.js");
+  let tmp_file_name = "test_osfile_front.tmp";
+  info("Starting test_copy_existing");
+  OS.File.copy(src_file_name, tmp_file_name);
+
+  info("test_copy_existing: Copy complete");
+  compare_files("test_copy_existing", src_file_name, tmp_file_name);
+
+  // Create a bogus file with arbitrary content, then attempt to overwrite
+  // it with |copy|.
+  let dest = OS.File.open(tmp_file_name, {trunc: true});
+  let buf = new Uint8Array(50);
+  dest.write(buf);
+  dest.close();
+
+  OS.File.copy(src_file_name, tmp_file_name);
+
+  compare_files("test_copy_existing 2", src_file_name, tmp_file_name);
+
+  // Attempt to overwrite with noOverwrite
+  let exn;
+  try {
+    OS.File.copy(src_file_name, tmp_file_name, {noOverwrite: true});
+  } catch(x) {
+    exn = x;
+  }
+  ok(!!exn, "test_copy_existing: noOverwrite prevents overwriting existing files");
+
+  info("test_copy_existing: Cleaning up");
+  OS.File.remove(tmp_file_name);
+}
+
+/**
+ * Test that moving a file works.
+ */
+function test_move_file()
+{
+  info("test_move_file: Starting");
+  // 1. Copy file into a temporary file
+  let src_file_name =
+    OS.Path.join("chrome", "toolkit", "components", "osfile", "tests", "mochi",
+                 "worker_test_osfile_front.js");
+  let tmp_file_name = "test_osfile_front.tmp";
+  let tmp2_file_name = "test_osfile_front.tmp2";
+  OS.File.copy(src_file_name, tmp_file_name);
+
+  info("test_move_file: Copy complete");
+
+  // 2. Move
+  OS.File.move(tmp_file_name, tmp2_file_name);
+
+  info("test_move_file: Move complete");
+
+  // 3. Check that destination exists
+  compare_files("test_move_file", src_file_name, tmp2_file_name);
+
+  // 4. Check that original file does not exist anymore
+  let exn;
+  try {
+    OS.File.open(tmp_file_name);
+  } catch (x) {
+    exn = x;
+  }
+  ok(!!exn, "test_move_file: Original file has been removed");
+
+  info("test_move_file: Cleaning up");
+  OS.File.remove(tmp2_file_name);
+}
+
+function test_iter_dir()
+{
+  info("test_iter_dir: Starting");
+
+  // Create a file, to be sure that it exists
+  let tmp_file_name = "test_osfile_front.tmp";
+  let tmp_file = OS.File.open(tmp_file_name, {write: true, trunc:true});
+  tmp_file.close();
+
+  let parent = OS.File.getCurrentDirectory();
+  info("test_iter_dir: directory " + parent);
+  let iterator = new OS.File.DirectoryIterator(parent);
+  info("test_iter_dir: iterator created");
+  let encountered_tmp_file = false;
+  for (let entry in iterator) {
+    // Checking that |name| can be decoded properly
+    info("test_iter_dir: encountering entry " + entry.name);
+
+    if (entry.name == tmp_file_name) {
+      encountered_tmp_file = true;
+      isnot(entry.isDir, "test_iter_dir: The temporary file is not a directory");
+      isnot(entry.isSymLink, "test_iter_dir: The temporary file is not a link");
+    }
+
+    let file;
+    let success = true;
+    try {
+      file = OS.File.open(entry.path);
+    } catch (x) {
+      if (x.becauseNoSuchFile) {
+        success = false;
+      }
+    }
+    if (file) {
+      file.close();
+    }
+    ok(success, "test_iter_dir: Entry " + entry.path + " exists");
+
+    if (OS.Win) {
+      // We assume that the files are at least as recent as 2009.
+      // Since this test was written in 2011 and some of our packaging
+      // sets dates arbitrarily to 2010, this should be safe.
+      let year = new Date().getFullYear();
+      let creation = entry.winCreationDate;
+      ok(creation, "test_iter_dir: Windows creation date exists: " + creation);
+      ok(creation.getFullYear() >= 2009 && creation.getFullYear() <= year, "test_iter_dir: consistent creation date");
+
+      let lastWrite = entry.winLastWriteDate;
+      ok(lastWrite, "test_iter_dir: Windows lastWrite date exists: " + lastWrite);
+      ok(lastWrite.getFullYear() >= 2009 && lastWrite.getFullYear() <= year, "test_iter_dir: consistent lastWrite date");
+
+      let lastAccess = entry.winLastAccessDate;
+      ok(lastAccess, "test_iter_dir: Windows lastAccess date exists: " + lastAccess);
+      ok(lastAccess.getFullYear() >= 2009 && lastAccess.getFullYear() <= year, "test_iter_dir: consistent lastAccess date");
+    }
+
+  }
+  ok(encountered_tmp_file, "test_iter_dir: We have found the temporary file");
+
+  info("test_iter_dir: Cleaning up");
+  iterator.close();
+
+  // Testing nextBatch()
+  iterator = new OS.File.DirectoryIterator(parent);
+  let allentries = [];
+  for (let x in iterator) {
+    allentries.push(x);
+  }
+  iterator.close();
+
+  ok(allentries.length >= 14, "test_iter_dir: Meta-check: the test directory should contain at least 14 items");
+
+  iterator = new OS.File.DirectoryIterator(parent);
+  let firstten = iterator.nextBatch(10);
+  is(firstten.length, 10, "test_iter_dir: nextBatch(10) returns 10 items");
+  for (let i = 0; i < firstten.length; ++i) {
+    is(allentries[i].path, firstten[i].path, "test_iter_dir: Checking that batch returns the correct entries");
+  }
+  let nextthree = iterator.nextBatch(3);
+  is(nextthree.length, 3, "test_iter_dir: nextBatch(3) returns 3 items");
+  for (let i = 0; i < nextthree.length; ++i) {
+    is(allentries[i + firstten.length].path, nextthree[i].path, "test_iter_dir: Checking that batch 2 returns the correct entries");
+  }
+  let everythingelse = iterator.nextBatch();
+  ok(everythingelse.length >= 1, "test_iter_dir: nextBatch() returns at least one item");
+  for (let i = 0; i < everythingelse.length; ++i) {
+    is(allentries[i + firstten.length + nextthree.length].path, everythingelse[i].path, "test_iter_dir: Checking that batch 3 returns the correct entries");
+  }
+  is(iterator.nextBatch().length, 0, "test_iter_dir: Once there is nothing left, nextBatch returns an empty array");
+  iterator.close();
+
+  iterator = new OS.File.DirectoryIterator(parent);
+  iterator.close();
+  is(iterator.nextBatch().length, 0, "test_iter_dir: nextBatch on closed iterator returns an empty array");
+
+  iterator = new OS.File.DirectoryIterator(parent);
+  let allentries2 = iterator.nextBatch();
+  is(allentries.length, allentries2.length, "test_iter_dir: Checking that getBatch(null) returns the right number of entries");
+  for (let i = 0; i < allentries.length; ++i) {
+    is(allentries[i].path, allentries2[i].path, "test_iter_dir: Checking that getBatch(null) returns everything in the right order");
+  }
+  iterator.close();
+
+  // Test forEach
+  iterator = new OS.File.DirectoryIterator(parent);
+  let index = 0;
+  iterator.forEach(
+    function cb(entry, aIndex, aIterator) {
+      is(index, aIndex, "test_iter_dir: Checking that forEach index is correct");
+      ok(iterator == aIterator, "test_iter_dir: Checking that right iterator is passed");
+      if (index < 10) {
+        is(allentries[index].path, entry.path, "test_iter_dir: Checking that forEach entry is correct");
+      } else if (index == 10) {
+        iterator.close();
+      } else {
+        ok(false, "test_iter_dir: Checking that forEach can be stopped early");
+      }
+      ++index;
+  });
+  iterator.close();
+
+  //test for prototype |OS.File.DirectoryIterator.unixAsFile|
+  if ("unixAsFile" in OS.File.DirectoryIterator.prototype) {
+    info("testing property unixAsFile");
+    let path = OS.Path.join("chrome", "toolkit", "components", "osfile", "tests", "mochi");
+    iterator = new OS.File.DirectoryIterator(path);
+
+    let dir_file = iterator.unixAsFile();// return |File|
+    let stat0 = dir_file.stat();
+    let stat1 = OS.File.stat(path);
+
+    let unix_info_to_string = function unix_info_to_string(info) {
+      return "| " + info.unixMode + " | " + info.unixOwner + " | " + info.unixGroup + " | " + info.creationDate + " | " + info.lastModificationDate + " | " + info.lastAccessDate + " | " + info.size + " |";
+    };
+
+    let s0_string = unix_info_to_string(stat0);
+    let s1_string = unix_info_to_string(stat1);
+
+    ok(stat0.isDir, "unixAsFile returned a directory");
+    is(s0_string, s1_string, "unixAsFile returned the correct file");
+    dir_file.close();
+    iterator.close();
+  }
+  info("test_iter_dir: Complete");
+}
+
+function test_position() {
+  info("test_position: Starting");
+
+  ok("POS_START" in OS.File, "test_position: POS_START exists");
+  ok("POS_CURRENT" in OS.File, "test_position: POS_CURRENT exists");
+  ok("POS_END" in OS.File, "test_position: POS_END exists");
+
+  let ARBITRARY_POSITION = 321;
+  let src_file_name =
+    OS.Path.join("chrome", "toolkit", "components", "osfile", "tests", "mochi",
+                 "worker_test_osfile_front.js");
+
+  let file = OS.File.open(src_file_name);
+  is(file.getPosition(), 0, "test_position: Initial position is 0");
+
+  let size = 0 + file.stat().size; // Hack: We can remove this 0 + once 776259 has landed
+
+  file.setPosition(ARBITRARY_POSITION, OS.File.POS_START);
+  is(file.getPosition(), ARBITRARY_POSITION, "test_position: Setting position from start");
+
+  file.setPosition(0, OS.File.POS_START);
+  is(file.getPosition(), 0, "test_position: Setting position from start back to 0");
+
+  file.setPosition(ARBITRARY_POSITION);
+  is(file.getPosition(), ARBITRARY_POSITION, "test_position: Setting position without argument");
+
+  file.setPosition(-ARBITRARY_POSITION, OS.File.POS_END);
+  is(file.getPosition(), size - ARBITRARY_POSITION, "test_position: Setting position from end");
+
+  file.setPosition(ARBITRARY_POSITION, OS.File.POS_CURRENT);
+  is(file.getPosition(), size, "test_position: Setting position from current");
+
+  file.close();
+  info("test_position: Complete");
+}
+
+function test_info() {
+  info("test_info: Starting");
+
+  let filename = "test_info.tmp";
+  let size = 261;// An arbitrary file length
+  let start = new Date();
+
+ // Cleanup any leftover from previous tests
+  try {
+    OS.File.remove(filename);
+    info("test_info: Cleaned up previous garbage");
+  } catch (x) {
+    if (!x.becauseNoSuchFile) {
+      throw x;
+    }
+    info("test_info: No previous garbage");
+  }
+
+  let file = OS.File.open(filename, {trunc: true});
+  let buf = new ArrayBuffer(size);
+  file._write(buf, size);
+  file.close();
+
+  // Test OS.File.stat on new file
+  let stat = OS.File.stat(filename);
+  ok(!!stat, "test_info: info acquired");
+  ok(!stat.isDir, "test_info: file is not a directory");
+  is(stat.isSymLink, false, "test_info: file is not a link");
+  is(stat.size.toString(), size, "test_info: correct size");
+
+  let stop = new Date();
+
+  // We round down/up by 1s as file system precision is lower than
+  // Date precision (no clear specifications about that, but it seems
+  // that this can be a little over 1 second under ext3 and 2 seconds
+  // under FAT).
+  let SLOPPY_FILE_SYSTEM_ADJUSTMENT = 3000;
+  let startMs = start.getTime() - SLOPPY_FILE_SYSTEM_ADJUSTMENT;
+  let stopMs  = stop.getTime() + SLOPPY_FILE_SYSTEM_ADJUSTMENT;
+  info("Testing stat with bounds [ " + startMs + ", " + stopMs +" ]");
+
+  (function() {
+    let birth;
+    if ("winBirthDate" in stat) {
+      birth = stat.winBirthDate;
+    } else if ("macBirthDate" in stat) {
+      birth = stat.macBirthDate;
+    } else {
+      ok(true, "Skipping birthdate test");
+      return;
+    }
+    ok(birth.getTime() <= stopMs,
+    "test_info: platformBirthDate is consistent");
+    // Note: Previous versions of this test checked whether the file
+    // has been created after the start of the test. Unfortunately,
+    // this sometimes failed under Windows, in specific circumstances:
+    // if the file has been removed at the start of the test and
+    // recreated immediately, the Windows file system detects this and
+    // decides that the file was actually truncated rather than
+    // recreated, hence that it should keep its previous creation
+    // date.  Debugging hilarity ensues.
+  });
+
+  let change = stat.lastModificationDate;
+  info("Testing lastModificationDate: " + change);
+  ok(change.getTime() >= startMs && change.getTime() <= stopMs,
+     "test_info: lastModificationDate is consistent");
+
+  // Test OS.File.prototype.stat on new file
+  file = OS.File.open(filename);
+  try {
+    stat = file.stat();
+  } finally {
+    file.close();
+  }
+
+  ok(!!stat, "test_info: info acquired 2");
+  ok(!stat.isDir, "test_info: file is not a directory 2");
+  ok(!stat.isSymLink, "test_info: file is not a link 2");
+  is(stat.size.toString(), size, "test_info: correct size 2");
+
+  stop = new Date();
+
+  // Round up/down as above
+  startMs = start.getTime() - SLOPPY_FILE_SYSTEM_ADJUSTMENT;
+  stopMs  = stop.getTime() + SLOPPY_FILE_SYSTEM_ADJUSTMENT;
+  info("Testing stat 2 with bounds [ " + startMs + ", " + stopMs +" ]");
+
+  let access = stat.lastAccessDate;
+  info("Testing lastAccessDate: " + access);
+  ok(access.getTime() >= startMs && access.getTime() <= stopMs,
+     "test_info: lastAccessDate is consistent");
+
+  change = stat.lastModificationDate;
+  info("Testing lastModificationDate 2: " + change);
+  ok(change.getTime() >= startMs && change.getTime() <= stopMs,
+     "test_info: lastModificationDate 2 is consistent");
+
+  // Test OS.File.stat on directory
+  stat = OS.File.stat(OS.File.getCurrentDirectory());
+  ok(!!stat, "test_info: info on directory acquired");
+  ok(stat.isDir, "test_info: directory is a directory");
+
+  info("test_info: Complete");
+}
+
+// Note that most of the features of path are tested in
+// worker_test_osfile_{unix, win}.js
+function test_path()
+{
+  info("test_path: starting");
+  let abcd = OS.Path.join("a", "b", "c", "d");
+  is(OS.Path.basename(abcd), "d", "basename of a/b/c/d");
+
+  let abc = OS.Path.join("a", "b", "c");
+  is(OS.Path.dirname(abcd), abc, "dirname of a/b/c/d");
+
+  let abdotsc = OS.Path.join("a", "b", "..", "c");
+  is(OS.Path.normalize(abdotsc), OS.Path.join("a", "c"), "normalize a/b/../c");
+
+  let adotsdotsdots = OS.Path.join("a", "..", "..", "..");
+  is(OS.Path.normalize(adotsdotsdots), OS.Path.join("..", ".."), "normalize a/../../..");
+
+  info("test_path: Complete");
+}
+
+/**
+ * Test the file |exists| method.
+ */
+function test_exists_file()
+{
+  let file_name = OS.Path.join("chrome", "toolkit", "components" ,"osfile",
+                               "tests", "mochi", "test_osfile_front.xul");
+  info("test_exists_file: starting");
+  ok(OS.File.exists(file_name), "test_exists_file: file exists (OS.File.exists)");
+  ok(!OS.File.exists(file_name + ".tmp"), "test_exists_file: file does not exists (OS.File.exists)");
+
+  let dir_name = OS.Path.join("chrome", "toolkit", "components" ,"osfile",
+                               "tests", "mochi");
+  ok(OS.File.exists(dir_name), "test_exists_file: directory exists");
+  ok(!OS.File.exists(dir_name) + ".tmp", "test_exists_file: directory does not exist");
+
+  info("test_exists_file: complete");
+}
+
+/**
+ * Test the file |remove| method.
+ */
+function test_remove_file()
+{
+  let absent_file_name = "test_osfile_front_absent.tmp";
+
+  // Check that removing absent files is handled correctly
+  let exn = should_throw(function() {
+    OS.File.remove(absent_file_name, {ignoreAbsent: false});
+  });
+  ok(!!exn, "test_remove_file: throws if there is no such file");
+
+  exn = should_throw(function() {
+    OS.File.remove(absent_file_name, {ignoreAbsent: true});
+    OS.File.remove(absent_file_name);
+  });
+  ok(!exn, "test_remove_file: ignoreAbsent works");
+
+  if (OS.Win) {
+    let file_name = "test_osfile_front_file_to_remove.tmp";
+    let file = OS.File.open(file_name, {write: true});
+    file.close();
+    ok(OS.File.exists(file_name), "test_remove_file: test file exists");
+    OS.Win.File.SetFileAttributes(file_name,
+                                  OS.Constants.Win.FILE_ATTRIBUTE_READONLY);
+    OS.File.remove(file_name);
+    ok(!OS.File.exists(file_name),
+       "test_remove_file: test file has been removed");
+  }
+}
diff --git a/toolkit/components/osfile/tests/mochi/worker_test_osfile_shared.js b/toolkit/components/osfile/tests/mochi/worker_test_osfile_shared.js
new file mode 100644
index 000000000..da82d4b0a
--- /dev/null
+++ b/toolkit/components/osfile/tests/mochi/worker_test_osfile_shared.js
@@ -0,0 +1,32 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+function log(text) {
+  dump("WORKER " + text + "\n");
+}
+
+function send(message) {
+  self.postMessage(message);
+}
+
+function finish() {
+  send({kind: "finish"});
+}
+
+function ok(condition, description) {
+  send({kind: "ok", condition: !!condition, description: "" + description});
+}
+
+function is(a, b, description) {
+  let outcome = a == b; // Need to decide outcome here, as not everything can be serialized
+  send({kind: "is", outcome: outcome, description: "" + description, a: "" + a, b: "" + b});
+}
+
+function isnot(a, b, description) {
+  let outcome = a != b; // Need to decide outcome here, as not everything can be serialized
+  send({kind: "isnot", outcome: outcome, description: "" + description, a: "" + a, b: "" + b});
+}
+
+function info(description) {
+  send({kind: "info", description: "" + description});
+}
diff --git a/toolkit/components/osfile/tests/mochi/worker_test_osfile_unix.js b/toolkit/components/osfile/tests/mochi/worker_test_osfile_unix.js
new file mode 100644
index 000000000..9fe2d0b4e
--- /dev/null
+++ b/toolkit/components/osfile/tests/mochi/worker_test_osfile_unix.js
@@ -0,0 +1,201 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+importScripts('worker_test_osfile_shared.js');
+
+self.onmessage = function(msg) {
+  log("received message "+JSON.stringify(msg.data));
+  self.onmessage = function(msg) {
+    log("ignored message "+JSON.stringify(msg.data));
+  };
+  test_init();
+  test_getcwd();
+  test_open_close();
+  test_create_file();
+  test_access();
+  test_read_write();
+  test_passing_undefined();
+  finish();
+};
+
+function test_init() {
+  info("Starting test_init");
+  importScripts("resource://gre/modules/osfile.jsm");
+}
+
+function test_open_close() {
+  info("Starting test_open_close");
+  is(typeof OS.Unix.File.open, "function", "OS.Unix.File.open is a function");
+  let file = OS.Unix.File.open("chrome/toolkit/components/osfile/tests/mochi/worker_test_osfile_unix.js", OS.Constants.libc.O_RDONLY, 0);
+  isnot(file, -1, "test_open_close: opening succeeded");
+  info("Close: "+OS.Unix.File.close.toSource());
+  let result = OS.Unix.File.close(file);
+  is(result, 0, "test_open_close: close succeeded");
+
+  file = OS.Unix.File.open("/i do not exist", OS.Constants.libc.O_RDONLY, 0);
+  is(file, -1, "test_open_close: opening of non-existing file failed");
+  is(ctypes.errno, OS.Constants.libc.ENOENT, "test_open_close: error is ENOENT");
+}
+
+function test_create_file()
+{
+  info("Starting test_create_file");
+  let file = OS.Unix.File.open("test.tmp", OS.Constants.libc.O_RDWR
+                               | OS.Constants.libc.O_CREAT
+                               | OS.Constants.libc.O_TRUNC,
+                               OS.Constants.libc.S_IRWXU);
+  isnot(file, -1, "test_create_file: file created");
+  OS.Unix.File.close(file);
+}
+
+function test_access()
+{
+  info("Starting test_access");
+  let file = OS.Unix.File.open("test1.tmp", OS.Constants.libc.O_RDWR
+                               | OS.Constants.libc.O_CREAT
+                               | OS.Constants.libc.O_TRUNC,
+                               OS.Constants.libc.S_IRWXU);
+  let result = OS.Unix.File.access("test1.tmp", OS.Constants.libc.R_OK | OS.Constants.libc.W_OK | OS.Constants.libc.X_OK | OS.Constants.libc.F_OK);
+  is(result, 0, "first call to access() succeeded");
+  OS.Unix.File.close(file);
+
+  file = OS.Unix.File.open("test1.tmp", OS.Constants.libc.O_WRONLY
+                               | OS.Constants.libc.O_CREAT
+                               | OS.Constants.libc.O_TRUNC,
+                               OS.Constants.libc.S_IWUSR);
+
+  info("test_access: preparing second call to access()");
+  result = OS.Unix.File.access("test2.tmp", OS.Constants.libc.R_OK
+                        | OS.Constants.libc.W_OK
+                        | OS.Constants.libc.X_OK
+                        | OS.Constants.libc.F_OK);
+  is(result, -1, "test_access: second call to access() failed as expected");
+  is(ctypes.errno, OS.Constants.libc.ENOENT, "This is the correct error");
+  OS.Unix.File.close(file);
+}
+
+function test_getcwd()
+{
+  let array = new (ctypes.ArrayType(ctypes.char, 32768))();
+  let path = OS.Unix.File.getcwd(array, array.length);
+  if (ctypes.char.ptr(path).isNull()) {
+    ok(false, "test_get_cwd: getcwd returned null, errno: " + ctypes.errno);
+  }
+  let path2;
+  if (OS.Unix.File.get_current_dir_name) {
+    path2 = OS.Unix.File.get_current_dir_name();
+  } else {
+    path2 = OS.Unix.File.getwd_auto(null);
+  }
+  if (ctypes.char.ptr(path2).isNull()) {
+    ok(false, "test_get_cwd: getwd_auto/get_current_dir_name returned null, errno: " + ctypes.errno);
+  }
+  is(path.readString(), path2.readString(), "test_get_cwd: getcwd and getwd return the same path");
+}
+
+function test_read_write()
+{
+  let output_name = "osfile_copy.tmp";
+  // Copy file
+  let input = OS.Unix.File.open(
+    "chrome/toolkit/components/osfile/tests/mochi/worker_test_osfile_unix.js",
+    OS.Constants.libc.O_RDONLY, 0);
+  isnot(input, -1, "test_read_write: input file opened");
+  let output = OS.Unix.File.open("osfile_copy.tmp", OS.Constants.libc.O_RDWR
+    | OS.Constants.libc.O_CREAT
+    | OS.Constants.libc.O_TRUNC,
+    OS.Constants.libc.S_IRWXU);
+  isnot(output, -1, "test_read_write: output file opened");
+
+  let array = new (ctypes.ArrayType(ctypes.char, 4096))();
+  let bytes = -1;
+  let total = 0;
+  while (true) {
+    bytes = OS.Unix.File.read(input, array, 4096);
+    ok(bytes != undefined, "test_read_write: bytes is defined");
+    isnot(bytes, -1, "test_read_write: no read error");
+    let write_from = 0;
+    if (bytes == 0) {
+      break;
+    }
+    while (bytes > 0) {
+      let ptr = array.addressOfElement(write_from);
+      // Note: |write| launches an exception in case of error
+      let written = OS.Unix.File.write(output, array, bytes);
+      isnot(written, -1, "test_read_write: no write error");
+      write_from += written;
+      bytes -= written;
+    }
+    total += write_from;
+  }
+  info("test_read_write: copy complete " + total);
+
+  // Compare files
+  let result;
+  info("SEEK_SET: " + OS.Constants.libc.SEEK_SET);
+  info("Input: " + input + "(" + input.toSource() + ")");
+  info("Output: " + output + "(" + output.toSource() + ")");
+  result = OS.Unix.File.lseek(input, 0, OS.Constants.libc.SEEK_SET);
+  info("Result of lseek: " + result);
+  isnot(result, -1, "test_read_write: input seek succeeded " + ctypes.errno);
+  result = OS.Unix.File.lseek(output, 0, OS.Constants.libc.SEEK_SET);
+  isnot(result, -1, "test_read_write: output seek succeeded " + ctypes.errno);
+
+  let array2 = new (ctypes.ArrayType(ctypes.char, 4096))();
+  let bytes2 = -1;
+  let pos = 0;
+  while (true) {
+    bytes = OS.Unix.File.read(input, array, 4096);
+    isnot(bytes, -1, "test_read_write: input read succeeded");
+    bytes2 = OS.Unix.File.read(output, array2, 4096);
+    isnot(bytes, -1, "test_read_write: output read succeeded");
+    is(bytes > 0, bytes2 > 0, "Both files contain data or neither does "+bytes+", "+bytes2);
+    if (bytes == 0) {
+      break;
+    }
+    if (bytes != bytes2) {
+      // This would be surprising, but theoretically possible with a
+      // remote file system, I believe.
+      bytes = Math.min(bytes, bytes2);
+      pos += bytes;
+      result = OS.Unix.File.lseek(input, pos, OS.Constants.libc.SEEK_SET);
+      isnot(result, -1, "test_read_write: input seek succeeded");
+      result = OS.Unix.File.lseek(output, pos, OS.Constants.libc.SEEK_SET);
+      isnot(result, -1, "test_read_write: output seek succeeded");
+    } else {
+      pos += bytes;
+    }
+    for (let i = 0; i < bytes; ++i) {
+      if (array[i] != array2[i]) {
+        ok(false, "Files do not match at position " + i
+           + " ("+array[i] + "/"+array2[i] + ")");
+      }
+    }
+  }
+  info("test_read_write test complete");
+  result = OS.Unix.File.close(input);
+  isnot(result, -1, "test_read_write: input close succeeded");
+  result = OS.Unix.File.close(output);
+  isnot(result, -1, "test_read_write: output close succeeded");
+  result = OS.Unix.File.unlink(output_name);
+  isnot(result, -1, "test_read_write: input remove succeeded");
+  info("test_read_write cleanup complete");
+}
+
+function test_passing_undefined()
+{
+  info("Testing that an exception gets thrown when an FFI function is passed undefined");
+  let exceptionRaised = false;
+
+  try {
+    let file = OS.Unix.File.open(undefined, OS.Constants.libc.O_RDWR
+                                            | OS.Constants.libc.O_CREAT
+                                            | OS.Constants.libc.O_TRUNC,
+                                            OS.Constants.libc.S_IRWXU);
+  } catch(e if e instanceof TypeError && e.message.indexOf("open") > -1) {
+    exceptionRaised = true;
+  }
+
+  ok(exceptionRaised, "test_passing_undefined: exception gets thrown")
+}
+
diff --git a/toolkit/components/osfile/tests/mochi/worker_test_osfile_win.js b/toolkit/components/osfile/tests/mochi/worker_test_osfile_win.js
new file mode 100644
index 000000000..f41fdecfe
--- /dev/null
+++ b/toolkit/components/osfile/tests/mochi/worker_test_osfile_win.js
@@ -0,0 +1,211 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+importScripts('worker_test_osfile_shared.js');
+
+self.onmessage = function(msg) {
+  self.onmessage = function(msg) {
+    log("ignored message "+JSON.stringify(msg.data));
+  };
+
+  test_init();
+  test_GetCurrentDirectory();
+  test_OpenClose();
+  test_CreateFile();
+  test_ReadWrite();
+  test_passing_undefined();
+  finish();
+};
+
+function test_init() {
+  info("Starting test_init");
+  importScripts("resource://gre/modules/osfile.jsm");
+}
+
+function test_OpenClose() {
+  info("Starting test_OpenClose");
+  is(typeof OS.Win.File.CreateFile, "function", "OS.Win.File.CreateFile is a function");
+  is(OS.Win.File.CloseHandle(OS.Constants.Win.INVALID_HANDLE_VALUE), true, "CloseHandle returns true given the invalid handle");
+  is(OS.Win.File.FindClose(OS.Constants.Win.INVALID_HANDLE_VALUE), true, "FindClose returns true given the invalid handle");
+  isnot(OS.Constants.Win.GENERIC_READ, undefined, "GENERIC_READ exists");
+  isnot(OS.Constants.Win.FILE_SHARE_READ, undefined, "FILE_SHARE_READ exists");
+  isnot(OS.Constants.Win.FILE_ATTRIBUTE_NORMAL, undefined, "FILE_ATTRIBUTE_NORMAL exists");
+  let file = OS.Win.File.CreateFile(
+    "chrome\\toolkit\\components\\osfile\\tests\\mochi\\worker_test_osfile_win.js",
+    OS.Constants.Win.GENERIC_READ,
+    0,
+    null,
+    OS.Constants.Win.OPEN_EXISTING,
+    0,
+    null);
+  info("test_OpenClose: Passed open");
+  isnot(file, OS.Constants.Win.INVALID_HANDLE_VALUE, "test_OpenClose: file opened");
+  let result = OS.Win.File.CloseHandle(file);
+  isnot(result, 0, "test_OpenClose: close succeeded");
+
+  file = OS.Win.File.CreateFile(
+    "\\I do not exist",
+    OS.Constants.Win.GENERIC_READ,
+    OS.Constants.Win.FILE_SHARE_READ,
+    null,
+    OS.Constants.Win.OPEN_EXISTING,
+    OS.Constants.Win.FILE_ATTRIBUTE_NORMAL,
+    null);
+  is(file, OS.Constants.Win.INVALID_HANDLE_VALUE, "test_OpenClose: cannot open non-existing file");
+  is(ctypes.winLastError, OS.Constants.Win.ERROR_FILE_NOT_FOUND, "test_OpenClose: error is ERROR_FILE_NOT_FOUND");
+}
+
+function test_CreateFile()
+{
+  info("Starting test_CreateFile");
+  let file = OS.Win.File.CreateFile(
+    "test.tmp",
+    OS.Constants.Win.GENERIC_READ | OS.Constants.Win.GENERIC_WRITE,
+    OS.Constants.Win.FILE_SHARE_READ | OS.Constants.FILE_SHARE_WRITE,
+    null,
+    OS.Constants.Win.CREATE_ALWAYS,
+    OS.Constants.Win.FILE_ATTRIBUTE_NORMAL,
+    null);
+  isnot(file, OS.Constants.Win.INVALID_HANDLE_VALUE, "test_CreateFile: opening succeeded");
+  let result = OS.Win.File.CloseHandle(file);
+  isnot(result, 0, "test_CreateFile: close succeeded");
+}
+
+function test_GetCurrentDirectory()
+{
+  let array = new (ctypes.ArrayType(ctypes.char16_t, 4096))();
+  let result = OS.Win.File.GetCurrentDirectory(4096, array);
+  ok(result < array.length, "test_GetCurrentDirectory: length sufficient");
+  ok(result > 0, "test_GetCurrentDirectory: length != 0");
+}
+
+function test_ReadWrite()
+{
+  info("Starting test_ReadWrite");
+  let output_name = "osfile_copy.tmp";
+  // Copy file
+  let input = OS.Win.File.CreateFile(
+    "chrome\\toolkit\\components\\osfile\\tests\\mochi\\worker_test_osfile_win.js",
+     OS.Constants.Win.GENERIC_READ,
+     0,
+     null,
+     OS.Constants.Win.OPEN_EXISTING,
+     0,
+     null);
+  isnot(input, OS.Constants.Win.INVALID_HANDLE_VALUE, "test_ReadWrite: input file opened");
+  let output = OS.Win.File.CreateFile(
+     "osfile_copy.tmp",
+     OS.Constants.Win.GENERIC_READ | OS.Constants.Win.GENERIC_WRITE,
+     0,
+     null,
+     OS.Constants.Win.CREATE_ALWAYS,
+     OS.Constants.Win.FILE_ATTRIBUTE_NORMAL,
+     null);
+  isnot(output, OS.Constants.Win.INVALID_HANDLE_VALUE, "test_ReadWrite: output file opened");
+  let array = new (ctypes.ArrayType(ctypes.char, 4096))();
+  let bytes_read = new ctypes.uint32_t(0);
+  let bytes_read_ptr = bytes_read.address();
+  log("We have a pointer for bytes read: "+bytes_read_ptr);
+  let bytes_written = new ctypes.uint32_t(0);
+  let bytes_written_ptr = bytes_written.address();
+  log("We have a pointer for bytes written: "+bytes_written_ptr);
+  log("test_ReadWrite: buffer and pointers ready");
+  let result;
+  while (true) {
+    log("test_ReadWrite: reading");
+    result = OS.Win.File.ReadFile(input, array, 4096, bytes_read_ptr, null);
+    isnot (result, 0, "test_ReadWrite: read success");
+    let write_from = 0;
+    let bytes_left = bytes_read;
+    log("test_ReadWrite: read chunk complete " + bytes_left.value);
+    if (bytes_left.value == 0) {
+      break;
+    }
+    while (bytes_left.value > 0) {
+      log("test_ReadWrite: writing "+bytes_left.value);
+      let ptr = array.addressOfElement(write_from);
+      // Note: |WriteFile| launches an exception in case of error
+      result = OS.Win.File.WriteFile(output, array, bytes_left, bytes_written_ptr, null);
+      isnot (result, 0, "test_ReadWrite: write success");
+      write_from += bytes_written;
+      bytes_left -= bytes_written;
+    }
+  }
+  info("test_ReadWrite: copy complete");
+
+  // Compare files
+  result = OS.Win.File.SetFilePointer(input, 0, null, OS.Constants.Win.FILE_BEGIN);
+  isnot (result, OS.Constants.Win.INVALID_SET_FILE_POINTER, "test_ReadWrite: input reset");
+
+  result = OS.Win.File.SetFilePointer(output, 0, null, OS.Constants.Win.FILE_BEGIN);
+  isnot (result, OS.Constants.Win.INVALID_SET_FILE_POINTER, "test_ReadWrite: output reset");
+
+  let array2 = new (ctypes.ArrayType(ctypes.char, 4096))();
+  let bytes_read2 = new ctypes.uint32_t(0);
+  let bytes_read2_ptr = bytes_read2.address();
+  let pos = 0;
+  while (true) {
+    result = OS.Win.File.ReadFile(input, array, 4096, bytes_read_ptr, null);
+    isnot(result, 0, "test_ReadWrite: input read succeeded");
+
+    result = OS.Win.File.ReadFile(output, array2, 4096, bytes_read2_ptr, null);
+    isnot(result, 0, "test_ReadWrite: output read succeeded");
+
+    is(bytes_read.value > 0, bytes_read2.value > 0,
+       "Both files contain data or neither does " + bytes_read.value + ", " + bytes_read2.value);
+    if (bytes_read.value == 0) {
+      break;
+    }
+    let bytes;
+    if (bytes_read.value != bytes_read2.value) {
+      // This would be surprising, but theoretically possible with a
+      // remote file system, I believe.
+      bytes = Math.min(bytes_read.value, bytes_read2.value);
+      pos += bytes;
+      result = OS.Win.File.SetFilePointer(input,  pos, null, OS.Constants.Win.FILE_BEGIN);
+      isnot(result, 0, "test_ReadWrite: input seek succeeded");
+
+      result = OS.Win.File.SetFilePointer(output, pos, null, OS.Constants.Win.FILE_BEGIN);
+      isnot(result, 0, "test_ReadWrite: output seek succeeded");
+
+    } else {
+      bytes = bytes_read.value;
+      pos += bytes;
+    }
+    for (let i = 0; i < bytes; ++i) {
+      if (array[i] != array2[i]) {
+        ok(false, "Files do not match at position " + i
+           + " ("+array[i] + "/"+array2[i] + ")");
+      }
+    }
+  }
+  info("test_ReadWrite test complete");
+  result = OS.Win.File.CloseHandle(input);
+  isnot(result, 0, "test_ReadWrite: inpout close succeeded");
+  result = OS.Win.File.CloseHandle(output);
+  isnot(result, 0, "test_ReadWrite: outpout close succeeded");
+  result = OS.Win.File.DeleteFile(output_name);
+  isnot(result, 0, "test_ReadWrite: output remove succeeded");
+  info("test_ReadWrite cleanup complete");
+}
+
+function test_passing_undefined()
+{
+  info("Testing that an exception gets thrown when an FFI function is passed undefined");
+  let exceptionRaised = false;
+
+  try {
+    let file = OS.Win.File.CreateFile(
+      undefined,
+      OS.Constants.Win.GENERIC_READ,
+      0,
+      null,
+      OS.Constants.Win.OPEN_EXISTING,
+      0,
+      null);
+  } catch(e if e instanceof TypeError && e.message.indexOf("CreateFile") > -1) {
+    exceptionRaised = true;
+  }
+
+  ok(exceptionRaised, "test_passing_undefined: exception gets thrown")
+}
diff --git a/toolkit/components/osfile/tests/xpcshell/.eslintrc.js b/toolkit/components/osfile/tests/xpcshell/.eslintrc.js
new file mode 100644
index 000000000..d35787cd2
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/.eslintrc.js
@@ -0,0 +1,7 @@
+"use strict";
+
+module.exports = {
+  "extends": [
+    "../../../../../testing/xpcshell/xpcshell.eslintrc.js"
+  ]
+};
diff --git a/toolkit/components/osfile/tests/xpcshell/head.js b/toolkit/components/osfile/tests/xpcshell/head.js
new file mode 100644
index 000000000..eef29962a
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/head.js
@@ -0,0 +1,99 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+var {utils: Cu, interfaces: Ci} = Components;
+
+var {XPCOMUtils} = Cu.import("resource://gre/modules/XPCOMUtils.jsm", {});
+
+// Bug 1014484 can only be reproduced by loading OS.File first from the
+// CommonJS loader, so we do not want OS.File to be loaded eagerly for
+// all the tests in this directory.
+XPCOMUtils.defineLazyModuleGetter(this, "OS",
+  "resource://gre/modules/osfile.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "FileUtils",
+  "resource://gre/modules/FileUtils.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "NetUtil",
+  "resource://gre/modules/NetUtil.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "Services",
+  "resource://gre/modules/Services.jsm");
+
+var {Promise} = Cu.import("resource://gre/modules/Promise.jsm", {});
+var {Task} = Cu.import("resource://gre/modules/Task.jsm", {});
+
+Services.prefs.setBoolPref("toolkit.osfile.log", true);
+
+/**
+ * As add_task, but execute the test both with native operations and
+ * without.
+ */
+function add_test_pair(generator) {
+  add_task(function*() {
+    do_print("Executing test " + generator.name + " with native operations");
+    Services.prefs.setBoolPref("toolkit.osfile.native", true);
+    return Task.spawn(generator);
+  });
+  add_task(function*() {
+    do_print("Executing test " + generator.name + " without native operations");
+    Services.prefs.setBoolPref("toolkit.osfile.native", false);
+    return Task.spawn(generator);
+  });
+}
+
+/**
+ * Fetch asynchronously the contents of a file using xpcom.
+ *
+ * Used for comparing xpcom-based results to os.file-based results.
+ *
+ * @param {string} path The _absolute_ path to the file.
+ * @return {promise}
+ * @resolves {string} The contents of the file.
+ */
+function reference_fetch_file(path, test) {
+  do_print("Fetching file " + path);
+  let deferred = Promise.defer();
+  let file = new FileUtils.File(path);
+  NetUtil.asyncFetch({
+    uri: NetUtil.newURI(file),
+    loadUsingSystemPrincipal: true
+  }, function(stream, status) {
+      if (!Components.isSuccessCode(status)) {
+        deferred.reject(status);
+        return;
+      }
+      let result, reject;
+      try {
+        result = NetUtil.readInputStreamToString(stream, stream.available());
+      } catch (x) {
+        reject = x;
+      }
+      stream.close();
+      if (reject) {
+        deferred.reject(reject);
+      } else {
+        deferred.resolve(result);
+      }
+    });
+
+  return deferred.promise;
+};
+
+/**
+ * Compare asynchronously the contents two files using xpcom.
+ *
+ * Used for comparing xpcom-based results to os.file-based results.
+ *
+ * @param {string} a The _absolute_ path to the first file.
+ * @param {string} b The _absolute_ path to the second file.
+ *
+ * @resolves {null}
+ */
+function reference_compare_files(a, b, test) {
+  return Task.spawn(function*() {
+    do_print("Comparing files " + a + " and " + b);
+    let a_contents = yield reference_fetch_file(a, test);
+    let b_contents = yield reference_fetch_file(b, test);
+    do_check_eq(a_contents, b_contents);
+  });
+};
diff --git a/toolkit/components/osfile/tests/xpcshell/test_available_free_space.js b/toolkit/components/osfile/tests/xpcshell/test_available_free_space.js
new file mode 100644
index 000000000..08e67763b
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_available_free_space.js
@@ -0,0 +1,38 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+Components.utils.import("resource://gre/modules/osfile.jsm");
+Components.utils.import("resource://gre/modules/Services.jsm");
+
+do_register_cleanup(function() {
+  Services.prefs.setBoolPref("toolkit.osfile.log", false);
+});
+
+function run_test() {
+  Services.prefs.setBoolPref("toolkit.osfile.log", true);
+
+  run_next_test();
+}
+
+/**
+ * Test OS.File.getAvailableFreeSpace
+ */
+add_task(function() {
+  // Set up profile. We will use profile path to query for available free
+  // space.
+  do_get_profile();
+
+  let dir = OS.Constants.Path.profileDir;
+
+  // Sanity checking for the test
+  do_check_true((yield OS.File.exists(dir)));
+
+  // Query for available bytes for user
+  let availableBytes = yield OS.File.getAvailableFreeSpace(dir);
+
+  do_check_true(!!availableBytes);
+  do_check_true(availableBytes > 0);
+});
diff --git a/toolkit/components/osfile/tests/xpcshell/test_compression.js b/toolkit/components/osfile/tests/xpcshell/test_compression.js
new file mode 100644
index 000000000..b40235615
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_compression.js
@@ -0,0 +1,98 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+Components.utils.import("resource://gre/modules/osfile.jsm");
+
+function run_test() {
+  do_test_pending();
+  run_next_test();
+}
+
+add_task(function test_compress_lz4() {
+  let path = OS.Path.join(OS.Constants.Path.tmpDir, "compression.lz");
+  let length = 1024;
+  let array = new Uint8Array(length);
+  for (let i = 0; i < array.byteLength; ++i) {
+    array[i] = i;
+  }
+  let arrayAsString = Array.prototype.join.call(array);
+
+  do_print("Writing data with lz4 compression");
+  let bytes = yield OS.File.writeAtomic(path, array, { compression: "lz4" });
+  do_print("Compressed " + length + " bytes into " + bytes);
+
+  do_print("Reading back with lz4 decompression");
+  let decompressed = yield OS.File.read(path, { compression: "lz4" });
+  do_print("Decompressed into " + decompressed.byteLength + " bytes");
+  do_check_eq(arrayAsString, Array.prototype.join.call(decompressed));
+});
+
+add_task(function test_uncompressed() {
+  do_print("Writing data without compression");
+  let path = OS.Path.join(OS.Constants.Path.tmpDir, "no_compression.tmp");
+  let array = new Uint8Array(1024);
+  for (let i = 0; i < array.byteLength; ++i) {
+    array[i] = i;
+  }
+  let bytes = yield OS.File.writeAtomic(path, array); // No compression
+
+  let exn;
+  // Force decompression, reading should fail
+  try {
+    yield OS.File.read(path, { compression: "lz4" });
+  } catch (ex) {
+    exn = ex;
+  }
+  do_check_true(!!exn);
+  // Check the exception message (and that it contains the file name)
+  do_check_true(exn.message.indexOf(`Invalid header (no magic number) - Data: ${ path }`) != -1);
+});
+
+add_task(function test_no_header() {
+  let path = OS.Path.join(OS.Constants.Path.tmpDir, "no_header.tmp");
+  let array = new Uint8Array(8).fill(0,0);  // Small array with no header
+
+  do_print("Writing data with no header");
+
+  let bytes = yield OS.File.writeAtomic(path, array); // No compression
+  let exn;
+  // Force decompression, reading should fail
+  try {
+    yield OS.File.read(path, { compression: "lz4" });
+  } catch (ex) {
+    exn = ex;
+  }
+  do_check_true(!!exn);
+  // Check the exception message (and that it contains the file name)
+  do_check_true(exn.message.indexOf(`Buffer is too short (no header) - Data: ${ path }`) != -1);
+});
+
+add_task(function test_invalid_content() {
+  let path = OS.Path.join(OS.Constants.Path.tmpDir, "invalid_content.tmp");
+  let arr1 = new Uint8Array([109, 111, 122, 76, 122, 52, 48, 0]);
+  let arr2 = new Uint8Array(248).fill(1,0);
+
+  let array = new Uint8Array(arr1.length + arr2.length);
+  array.set(arr1);
+  array.set(arr2, arr1.length);
+
+  do_print("Writing invalid data (with a valid header and only ones after that)");
+
+  let bytes = yield OS.File.writeAtomic(path, array); // No compression
+  let exn;
+  // Force decompression, reading should fail
+  try {
+    yield OS.File.read(path, { compression: "lz4" });
+  } catch (ex) {
+    exn = ex;
+  }
+  do_check_true(!!exn);
+  // Check the exception message (and that it contains the file name)
+  do_check_true(exn.message.indexOf(`Invalid content: Decompression stopped at 0 - Data: ${ path }`) != -1);
+});
+
+add_task(function() {
+  do_test_finished();
+});
\ No newline at end of file
diff --git a/toolkit/components/osfile/tests/xpcshell/test_constants.js b/toolkit/components/osfile/tests/xpcshell/test_constants.js
new file mode 100644
index 000000000..e92f33ab8
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_constants.js
@@ -0,0 +1,31 @@
+"use strict";
+
+Components.utils.import("resource://gre/modules/osfile.jsm");
+Components.utils.import("resource://gre/modules/Services.jsm");
+Components.utils.import("resource://gre/modules/Services.jsm", this);
+
+function run_test() {
+  run_next_test();
+}
+
+// Test that OS.Constants is defined correctly.
+add_task(function* check_definition() {
+  do_check_true(OS.Constants!=null);
+  do_check_true(!!OS.Constants.Win || !!OS.Constants.libc);
+  do_check_true(OS.Constants.Path!=null);
+  do_check_true(OS.Constants.Sys!=null);
+  //check system name
+  if (OS.Constants.Sys.Name == "Gonk") {
+  // Services.appinfo.OS doesn't know the difference between Gonk and Android
+    do_check_eq(Services.appinfo.OS, "Android");
+  } else {
+    do_check_eq(Services.appinfo.OS, OS.Constants.Sys.Name);
+  }
+
+  //check if using DEBUG build
+  if (Components.classes["@mozilla.org/xpcom/debug;1"].getService(Components.interfaces.nsIDebug2).isDebugBuild == true) {
+    do_check_true(OS.Constants.Sys.DEBUG);
+  } else {
+    do_check_true(typeof(OS.Constants.Sys.DEBUG) == 'undefined');
+  }
+});
diff --git a/toolkit/components/osfile/tests/xpcshell/test_creationDate.js b/toolkit/components/osfile/tests/xpcshell/test_creationDate.js
new file mode 100644
index 000000000..9c4fa1dfc
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_creationDate.js
@@ -0,0 +1,31 @@
+"use strict";
+
+function run_test() {
+  do_test_pending();
+  run_next_test();
+}
+
+/**
+ * Test to ensure that deprecation warning is issued on use
+ * of creationDate.
+ */
+add_task(function test_deprecatedCreationDate () {
+  let deferred = Promise.defer();
+  let consoleListener = {
+    observe: function (aMessage) {
+      if(aMessage.message.indexOf("Field 'creationDate' is deprecated.") > -1) {
+        do_print("Deprecation message printed");
+        do_check_true(true);
+        Services.console.unregisterListener(consoleListener);
+        deferred.resolve();
+      }
+    }
+  };
+  let currentDir = yield OS.File.getCurrentDirectory();
+  let path = OS.Path.join(currentDir, "test_creationDate.js");
+
+  Services.console.registerListener(consoleListener);
+  (yield OS.File.stat(path)).creationDate;
+});
+
+add_task(do_test_finished);
diff --git a/toolkit/components/osfile/tests/xpcshell/test_duration.js b/toolkit/components/osfile/tests/xpcshell/test_duration.js
new file mode 100644
index 000000000..305c03da8
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_duration.js
@@ -0,0 +1,91 @@
+var {OS} = Components.utils.import("resource://gre/modules/osfile.jsm", {});
+var {Services} = Components.utils.import("resource://gre/modules/Services.jsm", {});
+
+/**
+ * Test optional duration reporting that can be used for telemetry.
+ */
+add_task(function* duration() {
+  Services.prefs.setBoolPref("toolkit.osfile.log", true);
+  // Options structure passed to a OS.File copy method.
+  let copyOptions = {
+    // This field should be overridden with the actual duration
+    // measurement.
+    outExecutionDuration: null
+  };
+  let currentDir = yield OS.File.getCurrentDirectory();
+  let pathSource = OS.Path.join(currentDir, "test_duration.js");
+  let copyFile = pathSource + ".bak";
+  function testOptions(options, name) {
+    do_print("Checking outExecutionDuration for operation: " + name);
+    do_print(name + ": Gathered method duration time: " +
+      options.outExecutionDuration + "ms");
+    // Making sure that duration was updated.
+    do_check_eq(typeof options.outExecutionDuration, "number");
+    do_check_true(options.outExecutionDuration >= 0);
+  };
+  // Testing duration of OS.File.copy.
+  yield OS.File.copy(pathSource, copyFile, copyOptions);
+  testOptions(copyOptions, "OS.File.copy");
+  yield OS.File.remove(copyFile);
+
+  // Trying an operation where options are cloned.
+  let pathDest = OS.Path.join(OS.Constants.Path.tmpDir,
+    "osfile async test read writeAtomic.tmp");
+  let tmpPath = pathDest + ".tmp";
+  let readOptions = {
+    outExecutionDuration: null
+  };
+  let contents = yield OS.File.read(pathSource, undefined, readOptions);
+  testOptions(readOptions, "OS.File.read");
+  // Options structure passed to a OS.File writeAtomic method.
+  let writeAtomicOptions = {
+    // This field should be first initialized with the actual
+    // duration measurement then progressively incremented.
+    outExecutionDuration: null,
+    tmpPath: tmpPath
+  };
+  yield OS.File.writeAtomic(pathDest, contents, writeAtomicOptions);
+  testOptions(writeAtomicOptions, "OS.File.writeAtomic");
+  yield OS.File.remove(pathDest);
+
+  do_print("Ensuring that we can use outExecutionDuration to accumulate durations");
+
+  let ARBITRARY_BASE_DURATION = 5;
+  copyOptions = {
+    // This field should now be incremented with the actual duration
+    // measurement.
+    outExecutionDuration: ARBITRARY_BASE_DURATION
+  };
+  let backupDuration = ARBITRARY_BASE_DURATION;
+  // Testing duration of OS.File.copy.
+  yield OS.File.copy(pathSource, copyFile, copyOptions);
+
+  do_check_true(copyOptions.outExecutionDuration >= backupDuration);
+
+  backupDuration = copyOptions.outExecutionDuration;
+  yield OS.File.remove(copyFile, copyOptions);
+  do_check_true(copyOptions.outExecutionDuration >= backupDuration);
+
+  // Trying an operation where options are cloned.
+  // Options structure passed to a OS.File writeAtomic method.
+  writeAtomicOptions = {
+    // This field should be overridden with the actual duration
+    // measurement.
+    outExecutionDuration: copyOptions.outExecutionDuration,
+    tmpPath: tmpPath
+  };
+  backupDuration = writeAtomicOptions.outExecutionDuration;
+
+  yield OS.File.writeAtomic(pathDest, contents, writeAtomicOptions);
+  do_check_true(copyOptions.outExecutionDuration >= backupDuration);
+  OS.File.remove(pathDest);
+
+  // Testing an operation that doesn't take arguments at all
+  let file = yield OS.File.open(pathSource);
+  yield file.stat();
+  yield file.close();
+});
+
+function run_test() {
+  run_next_test();
+}
diff --git a/toolkit/components/osfile/tests/xpcshell/test_exception.js b/toolkit/components/osfile/tests/xpcshell/test_exception.js
new file mode 100644
index 000000000..1282adb3e
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_exception.js
@@ -0,0 +1,89 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/**
+ * Test that functions throw the appropriate exceptions.
+ */
+
+"use strict";
+
+var EXISTING_FILE = do_get_file("xpcshell.ini").path;
+
+
+// Tests on |open|
+
+add_test_pair(function test_typeerror() {
+  let exn;
+  try {
+    let fd = yield OS.File.open("/tmp", {no_such_key: 1});
+    do_print("Fd: " + fd);
+  } catch (ex) {
+    exn = ex;
+  }
+  do_print("Exception: " + exn);
+  do_check_true(exn.constructor.name == "TypeError");
+});
+
+// Tests on |read|
+
+add_test_pair(function* test_bad_encoding() {
+  do_print("Testing with a wrong encoding");
+  try {
+    yield OS.File.read(EXISTING_FILE, { encoding: "baby-speak-encoded" });
+    do_throw("Should have thrown with an ex.becauseInvalidArgument");
+  } catch (ex if ex.becauseInvalidArgument) {
+    do_print("Wrong encoding caused the correct exception");
+  }
+
+  try {
+    yield OS.File.read(EXISTING_FILE, { encoding: 4 });
+    do_throw("Should have thrown a TypeError");
+  } catch (ex if ex.constructor.name == "TypeError") {
+    // Note that TypeError doesn't carry across compartments
+    do_print("Non-string encoding caused the correct exception");
+  }
+ });
+
+add_test_pair(function* test_bad_compression() {
+  do_print("Testing with a non-existing compression");
+  try {
+    yield OS.File.read(EXISTING_FILE, { compression: "mmmh-crunchy" });
+    do_throw("Should have thrown with an ex.becauseInvalidArgument");
+  } catch (ex if ex.becauseInvalidArgument) {
+    do_print("Wrong encoding caused the correct exception");
+  }
+
+  do_print("Testing with a bad type for option compression");
+  try {
+    yield OS.File.read(EXISTING_FILE, { compression: 5 });
+    do_throw("Should have thrown a TypeError");
+  } catch (ex if ex.constructor.name == "TypeError") {
+    // Note that TypeError doesn't carry across compartments
+    do_print("Non-string encoding caused the correct exception");
+  }
+});
+
+add_test_pair(function* test_bad_bytes() {
+  do_print("Testing with a bad type for option bytes");
+  try {
+    yield OS.File.read(EXISTING_FILE, { bytes: "five" });
+    do_throw("Should have thrown a TypeError");
+  } catch (ex if ex.constructor.name == "TypeError") {
+    // Note that TypeError doesn't carry across compartments
+    do_print("Non-number bytes caused the correct exception");
+  }
+});
+
+add_test_pair(function* read_non_existent() {
+  do_print("Testing with a non-existent file");
+  try {
+    yield OS.File.read("I/do/not/exist");
+    do_throw("Should have thrown with an ex.becauseNoSuchFile");
+  } catch (ex if ex.becauseNoSuchFile) {
+    do_print("Correct exceptions");
+  }
+});
+
+function run_test() {
+  run_next_test();
+}
diff --git a/toolkit/components/osfile/tests/xpcshell/test_file_URL_conversion.js b/toolkit/components/osfile/tests/xpcshell/test_file_URL_conversion.js
new file mode 100644
index 000000000..3ec42065b
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_file_URL_conversion.js
@@ -0,0 +1,114 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+function run_test() {
+  Components.utils.import("resource://gre/modules/Services.jsm");
+  Components.utils.import("resource://gre/modules/osfile.jsm");
+  Components.utils.import("resource://gre/modules/FileUtils.jsm");
+
+  let isWindows = ("@mozilla.org/windows-registry-key;1" in Components.classes);
+
+  // Test cases for filePathToURI
+  let paths = isWindows ? [
+    'C:\\',
+    'C:\\test',
+    'C:\\test\\',
+    'C:\\test%2f',
+    'C:\\test\\test\\test',
+    'C:\\test;+%',
+    'C:\\test?action=index\\',
+    'C:\\test\ test',
+    '\\\\C:\\a\\b\\c',
+    '\\\\Server\\a\\b\\c',
+
+    // note that per http://support.microsoft.com/kb/177506 (under more info),
+    // the following characters are allowed on Windows:
+    'C:\\char^',
+    'C:\\char&',
+    'C:\\char\'',
+    'C:\\char@',
+    'C:\\char{',
+    'C:\\char}',
+    'C:\\char[',
+    'C:\\char]',
+    'C:\\char,',
+    'C:\\char$',
+    'C:\\char=',
+    'C:\\char!',
+    'C:\\char-',
+    'C:\\char#',
+    'C:\\char(',
+    'C:\\char)',
+    'C:\\char%',
+    'C:\\char.',
+    'C:\\char+',
+    'C:\\char~',
+    'C:\\char_'
+  ] : [
+    '/',
+    '/test',
+    '/test/',
+    '/test%2f',
+    '/test/test/test',
+    '/test;+%',
+    '/test?action=index/',
+    '/test\ test',
+    '/punctuation/;,/?:@&=+$-_.!~*\'()[]"#',
+    '/CasePreserving'
+  ];
+
+  // some additional URIs to test, beyond those generated from paths
+  let uris = isWindows ? [
+    'file:///C:/test/',
+    'file://localhost/C:/test',
+    'file:///c:/test/test.txt',
+    //'file:///C:/foo%2f', // trailing, encoded slash
+    'file:///C:/%3f%3F',
+    'file:///C:/%3b%3B',
+    'file:///C:/%3c%3C', // not one of the special-cased ? or ;
+    'file:///C:/%78', // 'x', not usually uri encoded
+    'file:///C:/test#frag', // a fragment identifier
+    'file:///C:/test?action=index' // an actual query component
+  ] : [
+    'file:///test/',
+    'file://localhost/test',
+    'file:///test/test.txt',
+    'file:///foo%2f', // trailing, encoded slash
+    'file:///%3f%3F',
+    'file:///%3b%3B',
+    'file:///%3c%3C', // not one of the special-cased ? or ;
+    'file:///%78', // 'x', not usually uri encoded
+    'file:///test#frag', // a fragment identifier
+    'file:///test?action=index' // an actual query component
+  ];
+
+  for (let path of paths) {
+    // convert that to a uri using FileUtils and Services, which toFileURI is trying to model
+    let file = FileUtils.File(path);
+    let uri = Services.io.newFileURI(file).spec;
+    do_check_eq(uri, OS.Path.toFileURI(path));
+
+    // keep the resulting URI to try the reverse, except for "C:\" for which the
+    // behavior of nsIFileURL and OS.File is inconsistent
+    if (path != "C:\\") {
+      uris.push(uri);
+    }
+  }
+
+  for (let uri of uris) {
+    // convert URIs to paths with nsIFileURI, which fromFileURI is trying to model
+    let path = Services.io.newURI(uri, null, null).QueryInterface(Components.interfaces.nsIFileURL).file.path;
+    do_check_eq(path, OS.Path.fromFileURI(uri));
+  }
+
+  // check that non-file URLs aren't allowed
+  let thrown = false;
+  try {
+    OS.Path.fromFileURI('http://test.com')
+  } catch (e) {
+    do_check_eq(e.message, "fromFileURI expects a file URI");
+    thrown = true;
+  }
+  do_check_true(thrown);
+}
diff --git a/toolkit/components/osfile/tests/xpcshell/test_loader.js b/toolkit/components/osfile/tests/xpcshell/test_loader.js
new file mode 100644
index 000000000..dcfa819be
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_loader.js
@@ -0,0 +1,37 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * Test that OS.File can be loaded using the CommonJS loader.
+ */
+
+var { Loader } = Components.utils.import('resource://gre/modules/commonjs/toolkit/loader.js', {});
+
+function run_test() {
+  run_next_test();
+}
+
+
+add_task(function*() {
+  let dataDir = Services.io.newFileURI(do_get_file("test_loader/", true)).spec + "/";
+  let loader = Loader.Loader({
+    paths: {'': dataDir }
+  });
+
+  let require = Loader.Require(loader, Loader.Module('module_test_loader', 'foo'));
+  do_print("Require is ready");
+  try {
+    require('module_test_loader');
+  } catch (error) {
+    dump('Bootstrap error: ' +
+         (error.message ? error.message : String(error)) + '\n' +
+         (error.stack || error.fileName + ': ' + error.lineNumber) + '\n');
+
+    throw error;
+  }
+
+  do_print("Require has worked");
+});
+
diff --git a/toolkit/components/osfile/tests/xpcshell/test_loader/module_test_loader.js b/toolkit/components/osfile/tests/xpcshell/test_loader/module_test_loader.js
new file mode 100644
index 000000000..18356d6ad
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_loader/module_test_loader.js
@@ -0,0 +1,9 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Load OS.File from a module loaded with the CommonJS/addon-sdk loader
+
+var {Cu} = require("chrome");
+Cu.import('resource://gre/modules/osfile.jsm');
diff --git a/toolkit/components/osfile/tests/xpcshell/test_logging.js b/toolkit/components/osfile/tests/xpcshell/test_logging.js
new file mode 100644
index 000000000..133909e0b
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_logging.js
@@ -0,0 +1,74 @@
+"use strict";
+
+Components.utils.import("resource://gre/modules/osfile.jsm");
+Components.utils.import("resource://gre/modules/Services.jsm");
+
+/**
+ * Tests logging by passing OS.Shared.LOG both an object with its own
+ * toString method, and one with the default.
+ */
+function run_test() {
+  do_test_pending();
+  let messageCount = 0;
+
+  do_print("Test starting");
+
+  // Create a console listener.
+  let consoleListener = {
+    observe: function (aMessage) {
+      //Ignore unexpected messages.
+      if (!(aMessage instanceof Components.interfaces.nsIConsoleMessage)) {
+        return;
+      }
+      // This is required, as printing to the |Services.console|
+      // while in the observe function causes an exception.
+      do_execute_soon(function() {
+        do_print("Observing message " + aMessage.message);
+        if (aMessage.message.indexOf("TEST OS") < 0) {
+          return;
+        }
+
+        ++messageCount;
+        if(messageCount == 1) {
+          do_check_eq(aMessage.message, "TEST OS {\"name\":\"test\"}\n");
+        }
+        if(messageCount == 2) {
+          do_check_eq(aMessage.message, "TEST OS name is test\n");
+          toggleConsoleListener(false);
+          do_test_finished();
+        }
+      });
+    }
+  };
+
+  // Set/Unset the console listener.
+  function toggleConsoleListener (pref) {
+    do_print("Setting console listener: " + pref);
+    Services.prefs.setBoolPref("toolkit.osfile.log", pref);
+    Services.prefs.setBoolPref("toolkit.osfile.log.redirect", pref);
+    Services.console[pref ? "registerListener" : "unregisterListener"](
+      consoleListener);
+  }
+
+  toggleConsoleListener(true);
+
+  let objectDefault = {name: "test"};
+  let CustomToString = function() {
+    this.name = "test";
+  };
+  CustomToString.prototype.toString = function() {
+    return "name is " + this.name;
+  };
+  let objectCustom = new CustomToString();
+
+  do_print(OS.Shared.LOG.toSource());
+
+  do_print("Logging 1");
+  OS.Shared.LOG(objectDefault);
+
+  do_print("Logging 2");
+  OS.Shared.LOG(objectCustom);
+  // Once both messages are observed OS.Shared.DEBUG, and OS.Shared.TEST
+  // are reset to false.
+}
+
diff --git a/toolkit/components/osfile/tests/xpcshell/test_makeDir.js b/toolkit/components/osfile/tests/xpcshell/test_makeDir.js
new file mode 100644
index 000000000..5b9740a7f
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_makeDir.js
@@ -0,0 +1,142 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+Components.utils.import("resource://gre/modules/osfile.jsm");
+Components.utils.import("resource://gre/modules/Services.jsm");
+
+var Path = OS.Path;
+var profileDir;
+
+do_register_cleanup(function() {
+  Services.prefs.setBoolPref("toolkit.osfile.log", false);
+});
+
+function run_test() {
+  run_next_test();
+}
+
+/**
+ * Test OS.File.makeDir
+ */
+
+add_task(function init() {
+  // Set up profile. We create the directory in the profile, because the profile
+  // is removed after every test run.
+  do_get_profile();
+  profileDir = OS.Constants.Path.profileDir;
+  Services.prefs.setBoolPref("toolkit.osfile.log", true);
+});
+
+/**
+ * Basic use
+ */
+
+add_task(function* test_basic() {
+  let dir = Path.join(profileDir, "directory");
+
+  // Sanity checking for the test
+  do_check_false((yield OS.File.exists(dir)));
+
+  // Make a directory
+  yield OS.File.makeDir(dir);
+
+  //check if the directory exists
+  yield OS.File.stat(dir);
+
+  // Make a directory that already exists, this should succeed
+  yield OS.File.makeDir(dir);
+
+  // Make a directory with ignoreExisting
+  yield OS.File.makeDir(dir, {ignoreExisting: true});
+
+  // Make a directory with ignoreExisting false
+  let exception = null;
+  try {
+    yield OS.File.makeDir(dir, {ignoreExisting: false});
+  } catch (ex) {
+    exception = ex;
+  }
+
+  do_check_true(!!exception);
+  do_check_true(exception instanceof OS.File.Error);
+  do_check_true(exception.becauseExists);
+});
+
+// Make a root directory that already exists
+add_task(function* test_root() {
+  if (OS.Constants.Win) {
+    yield OS.File.makeDir("C:");
+    yield OS.File.makeDir("C:\\");
+  } else {
+    yield OS.File.makeDir("/");
+  }
+});
+
+/**
+ * Creating subdirectories
+ */
+add_task(function test_option_from() {
+  let dir = Path.join(profileDir, "a", "b", "c");
+
+  // Sanity checking for the test
+  do_check_false((yield OS.File.exists(dir)));
+
+  // Make a directory
+  yield OS.File.makeDir(dir, {from: profileDir});
+
+  //check if the directory exists
+  yield OS.File.stat(dir);
+
+  // Make a directory that already exists, this should succeed
+  yield OS.File.makeDir(dir);
+
+  // Make a directory with ignoreExisting
+  yield OS.File.makeDir(dir, {ignoreExisting: true});
+
+  // Make a directory with ignoreExisting false
+  let exception = null;
+  try {
+    yield OS.File.makeDir(dir, {ignoreExisting: false});
+  } catch (ex) {
+    exception = ex;
+  }
+
+  do_check_true(!!exception);
+  do_check_true(exception instanceof OS.File.Error);
+  do_check_true(exception.becauseExists);
+
+  // Make a directory without |from| and fail
+  let dir2 = Path.join(profileDir, "g", "h", "i");
+  exception = null;
+  try {
+    yield OS.File.makeDir(dir2);
+  } catch (ex) {
+    exception = ex;
+  }
+
+  do_check_true(!!exception);
+  do_check_true(exception instanceof OS.File.Error);
+  do_check_true(exception.becauseNoSuchFile);
+
+  // Test edge cases on paths
+
+  let dir3 = Path.join(profileDir, "d", "", "e", "f");
+  do_check_false((yield OS.File.exists(dir3)));
+  yield OS.File.makeDir(dir3, {from: profileDir});
+  do_check_true((yield OS.File.exists(dir3)));
+
+  let dir4;
+  if (OS.Constants.Win) {
+    // Test that we can create a directory recursively even
+    // if we have too many "\\".
+    dir4 = profileDir + "\\\\g";
+  } else {
+    dir4 = profileDir + "////g";
+  }
+  do_check_false((yield OS.File.exists(dir4)));
+  yield OS.File.makeDir(dir4, {from: profileDir});
+  do_check_true((yield OS.File.exists(dir4)));
+});
diff --git a/toolkit/components/osfile/tests/xpcshell/test_open.js b/toolkit/components/osfile/tests/xpcshell/test_open.js
new file mode 100644
index 000000000..78772ad09
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_open.js
@@ -0,0 +1,70 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+Components.utils.import("resource://gre/modules/osfile.jsm");
+
+function run_test() {
+  run_next_test();
+}
+
+/**
+ * Test OS.File.open for reading:
+ * - with an existing file (should succeed);
+ * - with a non-existing file (should fail);
+ * - with inconsistent arguments (should fail).
+ */
+add_task(function() {
+  // Attempt to open a file that does not exist, ensure that it yields the
+  // appropriate error.
+  try {
+    let fd = yield OS.File.open(OS.Path.join(".", "This file does not exist"));
+    do_check_true(false, "File opening 1 succeeded (it should fail)");
+  } catch (err if err instanceof OS.File.Error && err.becauseNoSuchFile) {
+    do_print("File opening 1 failed " + err);
+  }
+
+  // Attempt to open a file with the wrong args, so that it fails before
+  // serialization, ensure that it yields the appropriate error.
+  do_print("Attempting to open a file with wrong arguments");
+  try {
+    let fd = yield OS.File.open(1, 2, 3);
+    do_check_true(false, "File opening 2 succeeded (it should fail)" + fd);
+  } catch (err) {
+    do_print("File opening 2 failed " + err);
+    do_check_false(err instanceof OS.File.Error,
+                   "File opening 2 returned something that is not a file error");
+    do_check_true(err.constructor.name == "TypeError",
+                  "File opening 2 returned a TypeError");
+  }
+
+  // Attempt to open a file correctly
+  do_print("Attempting to open a file correctly");
+  let openedFile = yield OS.File.open(OS.Path.join(do_get_cwd().path, "test_open.js"));
+  do_print("File opened correctly");
+
+  do_print("Attempting to close a file correctly");
+  yield openedFile.close();
+
+  do_print("Attempting to close a file again");
+  yield openedFile.close();
+});
+
+/**
+ * Test the error thrown by OS.File.open when attempting to open a directory
+ * that does not exist.
+ */
+add_task(function test_error_attributes () {
+
+  let dir = OS.Path.join(do_get_profile().path, "test_osfileErrorAttrs");
+  let fpath = OS.Path.join(dir, "test_error_attributes.txt");
+
+  try {
+    yield OS.File.open(fpath, {truncate: true}, {});
+    do_check_true(false, "Opening path suceeded (it should fail) " + fpath);
+  } catch (err) {
+    do_check_true(err instanceof OS.File.Error);
+    do_check_true(err.becauseNoSuchFile);
+  }
+});
diff --git a/toolkit/components/osfile/tests/xpcshell/test_osfile_async.js b/toolkit/components/osfile/tests/xpcshell/test_osfile_async.js
new file mode 100644
index 000000000..0f86b2ea8
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_osfile_async.js
@@ -0,0 +1,16 @@
+"use strict";
+
+Components.utils.import("resource://gre/modules/osfile.jsm");
+
+/**
+ * A trivial test ensuring that we can call osfile from xpcshell.
+ * (see bug 808161)
+ */
+
+function run_test() {
+  do_test_pending();
+  OS.File.getCurrentDirectory().then(
+    do_test_finished,
+    do_test_finished
+  );
+}
diff --git a/toolkit/components/osfile/tests/xpcshell/test_osfile_async_append.js b/toolkit/components/osfile/tests/xpcshell/test_osfile_async_append.js
new file mode 100644
index 000000000..0aef2c58a
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_osfile_async_append.js
@@ -0,0 +1,122 @@
+"use strict";
+
+do_print("starting tests");
+
+Components.utils.import("resource://gre/modules/osfile.jsm");
+Components.utils.import("resource://gre/modules/Task.jsm");
+
+/**
+ * A test to check that the |append| mode flag is correctly implemented.
+ * (see bug 925865)
+ */
+
+function setup_mode(mode) {
+  // Complete mode.
+  let realMode = {
+    read: true,
+    write: true
+  };
+  for (let k in mode) {
+    realMode[k] = mode[k];
+  }
+  return realMode;
+}
+
+// Test append mode.
+function test_append(mode) {
+  let path = OS.Path.join(OS.Constants.Path.tmpDir,
+                          "test_osfile_async_append.tmp");
+
+  // Clear any left-over files from previous runs.
+  try {
+    yield OS.File.remove(path);
+  } catch (ex if ex.becauseNoSuchFile) {
+    // ignore
+  }
+
+  try {
+    mode = setup_mode(mode);
+    mode.append = true;
+    if (mode.trunc) {
+      // Pre-fill file with some data to see if |trunc| actually works.
+      yield OS.File.writeAtomic(path, new Uint8Array(500));
+    }
+    let file = yield OS.File.open(path, mode);
+    try {
+      yield file.write(new Uint8Array(1000));
+      yield file.setPosition(0, OS.File.POS_START);
+      yield file.read(100);
+      // Should be at offset 100, length 1000 now.
+      yield file.write(new Uint8Array(100));
+      // Should be at offset 1100, length 1100 now.
+      let stat = yield file.stat();
+      do_check_eq(1100, stat.size);
+    } finally {
+      yield file.close();
+    }
+  } catch(ex) {
+    try {
+      yield OS.File.remove(path);
+    } catch (ex if ex.becauseNoSuchFile) {
+      // ignore.
+    }
+  }
+}
+
+// Test no-append mode.
+function test_no_append(mode) {
+  let path = OS.Path.join(OS.Constants.Path.tmpDir,
+                          "test_osfile_async_noappend.tmp");
+
+  // Clear any left-over files from previous runs.
+  try {
+    yield OS.File.remove(path);
+  } catch (ex if ex.becauseNoSuchFile) {
+    // ignore
+  }
+
+  try {
+    mode = setup_mode(mode);
+    mode.append = false;
+    if (mode.trunc) {
+      // Pre-fill file with some data to see if |trunc| actually works.
+      yield OS.File.writeAtomic(path, new Uint8Array(500));
+    }
+    let file = yield OS.File.open(path, mode);
+    try {
+      yield file.write(new Uint8Array(1000));
+      yield file.setPosition(0, OS.File.POS_START);
+      yield file.read(100);
+      // Should be at offset 100, length 1000 now.
+      yield file.write(new Uint8Array(100));
+      // Should be at offset 200, length 1000 now.
+      let stat = yield file.stat();
+      do_check_eq(1000, stat.size);
+    } finally {
+      yield file.close();
+    }
+  } finally {
+    try {
+      yield OS.File.remove(path);
+    } catch (ex if ex.becauseNoSuchFile) {
+      // ignore.
+    }
+  }
+}
+
+var test_flags = [
+  {},
+  {create:true},
+  {trunc:true}
+];
+function run_test() {
+  do_test_pending();
+
+  for (let t of test_flags) {
+    add_task(test_append.bind(null, t));
+    add_task(test_no_append.bind(null, t));
+  }
+  add_task(do_test_finished);
+
+  run_next_test();
+}
diff --git a/toolkit/components/osfile/tests/xpcshell/test_osfile_async_bytes.js b/toolkit/components/osfile/tests/xpcshell/test_osfile_async_bytes.js
new file mode 100644
index 000000000..68fa9152c
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_osfile_async_bytes.js
@@ -0,0 +1,39 @@
+"use strict";
+
+Components.utils.import("resource://gre/modules/osfile.jsm");
+Components.utils.import("resource://gre/modules/Task.jsm");
+
+function run_test() {
+  do_test_pending();
+  run_next_test();
+}
+
+/**
+ * Test to ensure that {bytes:} in options to |write| is correctly
+ * preserved.
+ */
+add_task(function* test_bytes() {
+  let path = OS.Path.join(OS.Constants.Path.tmpDir,
+                          "test_osfile_async_bytes.tmp");
+  let file = yield OS.File.open(path, {trunc: true, read: true, write: true});
+  try {
+    try {
+      // 1. Test write, by supplying {bytes:} options smaller than the actual
+      // buffer.
+      yield file.write(new Uint8Array(2048), {bytes: 1024});
+      do_check_eq((yield file.stat()).size, 1024);
+
+      // 2. Test that passing nullish values for |options| still works.
+      yield file.setPosition(0, OS.File.POS_END);
+      yield file.write(new Uint8Array(1024), null);
+      yield file.write(new Uint8Array(1024), undefined);
+      do_check_eq((yield file.stat()).size, 3072);
+    } finally {
+      yield file.close();
+    }
+  } finally {
+    yield OS.File.remove(path);
+  }
+});
+
+add_task(do_test_finished);
diff --git a/toolkit/components/osfile/tests/xpcshell/test_osfile_async_copy.js b/toolkit/components/osfile/tests/xpcshell/test_osfile_async_copy.js
new file mode 100644
index 000000000..9c52c8a80
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_osfile_async_copy.js
@@ -0,0 +1,113 @@
+"use strict";
+
+Components.utils.import("resource://gre/modules/osfile.jsm");
+Components.utils.import("resource://gre/modules/FileUtils.jsm");
+Components.utils.import("resource://gre/modules/NetUtil.jsm");
+Components.utils.import("resource://gre/modules/Services.jsm");
+Components.utils.import("resource://gre/modules/Promise.jsm");
+Components.utils.import("resource://gre/modules/Task.jsm");
+
+function run_test() {
+  do_test_pending();
+  run_next_test();
+}
+
+/**
+ * A file that we know exists and that can be used for reading.
+ */
+var EXISTING_FILE = "test_osfile_async_copy.js";
+
+/**
+ * Fetch asynchronously the contents of a file using xpcom.
+ *
+ * Used for comparing xpcom-based results to os.file-based results.
+ *
+ * @param {string} path The _absolute_ path to the file.
+ * @return {promise}
+ * @resolves {string} The contents of the file.
+ */
+var reference_fetch_file = function reference_fetch_file(path) {
+  let promise = Promise.defer();
+  let file = new FileUtils.File(path);
+  NetUtil.asyncFetch({
+    uri: NetUtil.newURI(file),
+    loadUsingSystemPrincipal: true
+  }, function(stream, status) {
+      if (!Components.isSuccessCode(status)) {
+        promise.reject(status);
+        return;
+      }
+      let result, reject;
+      try {
+        result = NetUtil.readInputStreamToString(stream, stream.available());
+      } catch (x) {
+        reject = x;
+      }
+      stream.close();
+      if (reject) {
+        promise.reject(reject);
+      } else {
+        promise.resolve(result);
+      }
+    });
+
+  return promise.promise;
+};
+
+/**
+ * Compare asynchronously the contents two files using xpcom.
+ *
+ * Used for comparing xpcom-based results to os.file-based results.
+ *
+ * @param {string} a The _absolute_ path to the first file.
+ * @param {string} b The _absolute_ path to the second file.
+ *
+ * @resolves {null}
+ */
+var reference_compare_files = function reference_compare_files(a, b) {
+  let a_contents = yield reference_fetch_file(a);
+  let b_contents = yield reference_fetch_file(b);
+  // Not using do_check_eq to avoid dumping the whole file to the log.
+  // It is OK to === compare here, as both variables contain a string.
+  do_check_true(a_contents === b_contents);
+};
+
+/**
+ * Test to ensure that OS.File.copy works.
+ */
+function test_copymove(options = {}) {
+  let source = OS.Path.join((yield OS.File.getCurrentDirectory()),
+                            EXISTING_FILE);
+  let dest = OS.Path.join(OS.Constants.Path.tmpDir,
+                          "test_osfile_async_copy_dest.tmp");
+  let dest2 = OS.Path.join(OS.Constants.Path.tmpDir,
+                           "test_osfile_async_copy_dest2.tmp");
+  try {
+    // 1. Test copy.
+    yield OS.File.copy(source, dest, options);
+    yield reference_compare_files(source, dest);
+    // 2. Test subsequent move.
+    yield OS.File.move(dest, dest2);
+    yield reference_compare_files(source, dest2);
+    // 3. Check that the moved file was really moved.
+    do_check_eq((yield OS.File.exists(dest)), false);
+  } finally {
+    try {
+      yield OS.File.remove(dest);
+    } catch (ex if ex.becauseNoSuchFile) {
+      // ignore
+    }
+    try {
+      yield OS.File.remove(dest2);
+    } catch (ex if ex.becauseNoSuchFile) {
+      // ignore
+    }
+  }
+}
+
+// Regular copy test.
+add_task(test_copymove);
+// Userland copy test.
+add_task(test_copymove.bind(null, {unixUserland: true}));
+
+add_task(do_test_finished);
diff --git a/toolkit/components/osfile/tests/xpcshell/test_osfile_async_flush.js b/toolkit/components/osfile/tests/xpcshell/test_osfile_async_flush.js
new file mode 100644
index 000000000..9ed087f4e
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_osfile_async_flush.js
@@ -0,0 +1,30 @@
+"use strict";
+
+Components.utils.import("resource://gre/modules/osfile.jsm");
+Components.utils.import("resource://gre/modules/Task.jsm");
+
+function run_test() {
+  do_test_pending();
+  run_next_test();
+}
+
+/**
+ * Test to ensure that |File.prototype.flush| is available in the async API.
+ */
+
+add_task(function test_flush() {
+  let path = OS.Path.join(OS.Constants.Path.tmpDir,
+                          "test_osfile_async_flush.tmp");
+  let file = yield OS.File.open(path, {trunc: true, write: true});
+  try {
+    try {
+      yield file.flush();
+    } finally {
+      yield file.close();
+    }
+  } finally {
+    yield OS.File.remove(path);
+  }
+});
+
+add_task(do_test_finished);
diff --git a/toolkit/components/osfile/tests/xpcshell/test_osfile_async_largefiles.js b/toolkit/components/osfile/tests/xpcshell/test_osfile_async_largefiles.js
new file mode 100644
index 000000000..a9ac776b0
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_osfile_async_largefiles.js
@@ -0,0 +1,153 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+Components.utils.import("resource://gre/modules/ctypes.jsm");
+Components.utils.import("resource://gre/modules/osfile.jsm");
+Components.utils.import("resource://gre/modules/Task.jsm");
+
+/**
+ * A test to check that .getPosition/.setPosition work with large files.
+ * (see bug 952997)
+ */
+
+// Test setPosition/getPosition.
+function test_setPosition(forward, current, backward) {
+  let path = OS.Path.join(OS.Constants.Path.tmpDir,
+                          "test_osfile_async_largefiles.tmp");
+
+  // Clear any left-over files from previous runs.
+  try {
+    yield OS.File.remove(path);
+  } catch (ex if ex.becauseNoSuchFile) {
+    // ignore
+  }
+
+  try {
+    let file = yield OS.File.open(path, {write:true, append:false});
+    try {
+      let pos = 0;
+
+      // 1. seek forward from start
+      do_print("Moving forward: " + forward);
+      yield file.setPosition(forward, OS.File.POS_START);
+      pos += forward;
+      do_check_eq((yield file.getPosition()), pos);
+
+      // 2. seek forward from current position
+      do_print("Moving current: " + current);
+      yield file.setPosition(current, OS.File.POS_CURRENT);
+      pos += current;
+      do_check_eq((yield file.getPosition()), pos);
+
+      // 3. seek backward from current position
+      do_print("Moving current backward: " + backward);
+      yield file.setPosition(-backward, OS.File.POS_CURRENT);
+      pos -= backward;
+      do_check_eq((yield file.getPosition()), pos);
+
+    } finally {
+      yield file.setPosition(0, OS.File.POS_START);
+      yield file.close();
+    }
+  } catch(ex) {
+    try {
+      yield OS.File.remove(path);
+    } catch (ex if ex.becauseNoSuchFile) {
+      // ignore.
+    }
+    do_throw(ex);
+  }
+}
+
+// Test setPosition/getPosition expected failures.
+function test_setPosition_failures() {
+  let path = OS.Path.join(OS.Constants.Path.tmpDir,
+                          "test_osfile_async_largefiles.tmp");
+
+  // Clear any left-over files from previous runs.
+  try {
+    yield OS.File.remove(path);
+  } catch (ex if ex.becauseNoSuchFile) {
+    // ignore
+  }
+
+  try {
+    let file = yield OS.File.open(path, {write:true, append:false});
+    try {
+      let pos = 0;
+
+      // 1. Use an invalid position value
+      try {
+        yield file.setPosition(0.5, OS.File.POS_START);
+        do_throw("Shouldn't have succeeded");
+      } catch (ex) {
+        do_check_true(ex.toString().includes("can't pass"));
+      }
+      // Since setPosition should have bailed, it shouldn't have moved the
+      // file pointer at all.
+      do_check_eq((yield file.getPosition()), 0);
+
+      // 2. Use an invalid position value
+      try {
+        yield file.setPosition(0xffffffff + 0.5, OS.File.POS_START);
+        do_throw("Shouldn't have succeeded");
+      } catch (ex) {
+        do_check_true(ex.toString().includes("can't pass"));
+      }
+      // Since setPosition should have bailed, it shouldn't have moved the
+      // file pointer at all.
+      do_check_eq((yield file.getPosition()), 0);
+
+      // 3. Use a position that cannot be represented as a double
+      try {
+        // Not all numbers after 9007199254740992 can be represented as a
+        // double. E.g. in js 9007199254740992 + 1 == 9007199254740992
+        yield file.setPosition(9007199254740992, OS.File.POS_START);
+        yield file.setPosition(1, OS.File.POS_CURRENT);
+        do_throw("Shouldn't have succeeded");
+      } catch (ex) {
+        do_print(ex.toString());
+        do_check_true(!!ex);
+      }
+
+    } finally {
+      yield file.setPosition(0, OS.File.POS_START);
+      yield file.close();
+      try {
+        yield OS.File.remove(path);
+      } catch (ex if ex.becauseNoSuchFile) {
+        // ignore.
+      }
+    }
+  } catch(ex) {
+    do_throw(ex);
+  }
+}
+
+function run_test() {
+  // First verify stuff works for small values.
+  add_task(test_setPosition.bind(null, 0, 100, 50));
+  add_task(test_setPosition.bind(null, 1000, 100, 50));
+  add_task(test_setPosition.bind(null, 1000, -100, -50));
+
+  if (OS.Constants.Win || ctypes.off_t.size >= 8) {
+    // Now verify stuff still works for large values.
+    // 1. Multiple small seeks, which add up to > MAXINT32
+    add_task(test_setPosition.bind(null, 0x7fffffff, 0x7fffffff, 0));
+    // 2. Plain large seek, that should end up at 0 again.
+    // 0xffffffff also happens to be the INVALID_SET_FILE_POINTER value on
+    // Windows, so this also tests the error handling
+    add_task(test_setPosition.bind(null, 0, 0xffffffff, 0xffffffff));
+    // 3. Multiple large seeks that should end up > MAXINT32.
+    add_task(test_setPosition.bind(null, 0xffffffff, 0xffffffff, 0xffffffff));
+    // 5. Multiple large seeks with negative offsets.
+    add_task(test_setPosition.bind(null, 0xffffffff, -0x7fffffff, 0x7fffffff));
+
+    // 6. Check failures
+    add_task(test_setPosition_failures);
+  }
+
+  run_next_test();
+}
diff --git a/toolkit/components/osfile/tests/xpcshell/test_osfile_async_setDates.js b/toolkit/components/osfile/tests/xpcshell/test_osfile_async_setDates.js
new file mode 100644
index 000000000..17d3afa7c
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_osfile_async_setDates.js
@@ -0,0 +1,211 @@
+"use strict";
+
+Components.utils.import("resource://gre/modules/osfile.jsm");
+Components.utils.import("resource://gre/modules/Task.jsm");
+
+/**
+ * A test to ensure that OS.File.setDates and OS.File.prototype.setDates are
+ * working correctly.
+ * (see bug 924916)
+ */
+
+function run_test() {
+  run_next_test();
+}
+
+// Non-prototypical tests, operating on path names.
+add_task(function* test_nonproto() {
+  // First, create a file we can mess with.
+  let path = OS.Path.join(OS.Constants.Path.tmpDir,
+                              "test_osfile_async_setDates_nonproto.tmp");
+  yield OS.File.writeAtomic(path, new Uint8Array(1));
+
+  try {
+    // 1. Try to set some well known dates.
+    // We choose multiples of 2000ms, because the time stamp resolution of
+    // the underlying OS might not support something more precise.
+    const accDate = 2000;
+    const modDate = 4000;
+    {
+      yield OS.File.setDates(path, accDate, modDate);
+      let stat = yield OS.File.stat(path);
+      do_check_eq(accDate, stat.lastAccessDate.getTime());
+      do_check_eq(modDate, stat.lastModificationDate.getTime());
+    }
+
+    // 2.1 Try to omit modificationDate (which should then default to
+    // |Date.now()|, expect for resolution differences).
+    {
+      yield OS.File.setDates(path, accDate);
+      let stat = yield OS.File.stat(path);
+      do_check_eq(accDate, stat.lastAccessDate.getTime());
+      do_check_neq(modDate, stat.lastModificationDate.getTime());
+    }
+
+    // 2.2 Try to omit accessDate as well (which should then default to
+    // |Date.now()|, expect for resolution differences).
+    {
+      yield OS.File.setDates(path);
+      let stat = yield OS.File.stat(path);
+      do_check_neq(accDate, stat.lastAccessDate.getTime());
+      do_check_neq(modDate, stat.lastModificationDate.getTime());
+    }
+
+    // 3. Repeat 1., but with Date objects this time
+    {
+      yield OS.File.setDates(path, new Date(accDate), new Date(modDate));
+      let stat = yield OS.File.stat(path);
+      do_check_eq(accDate, stat.lastAccessDate.getTime());
+      do_check_eq(modDate, stat.lastModificationDate.getTime());
+    }
+
+    // 4. Check that invalid params will cause an exception/rejection.
+    {
+      for (let p of ["invalid", new Uint8Array(1), NaN]) {
+        try {
+          yield OS.File.setDates(path, p, modDate);
+          do_throw("Invalid access date should have thrown for: " + p);
+        } catch (ex) {
+          let stat = yield OS.File.stat(path);
+          do_check_eq(accDate, stat.lastAccessDate.getTime());
+          do_check_eq(modDate, stat.lastModificationDate.getTime());
+        }
+        try {
+          yield OS.File.setDates(path, accDate, p);
+          do_throw("Invalid modification date should have thrown for: " + p);
+        } catch (ex) {
+          let stat = yield OS.File.stat(path);
+          do_check_eq(accDate, stat.lastAccessDate.getTime());
+          do_check_eq(modDate, stat.lastModificationDate.getTime());
+        }
+        try {
+          yield OS.File.setDates(path, p, p);
+          do_throw("Invalid dates should have thrown for: " + p);
+        } catch (ex) {
+          let stat = yield OS.File.stat(path);
+          do_check_eq(accDate, stat.lastAccessDate.getTime());
+          do_check_eq(modDate, stat.lastModificationDate.getTime());
+        }
+      }
+    }
+  } finally {
+    // Remove the temp file again
+    yield OS.File.remove(path);
+  }
+});
+
+// Prototypical tests, operating on |File| handles.
+add_task(function* test_proto() {
+  if (OS.Constants.Sys.Name == "Android" || OS.Constants.Sys.Name == "Gonk") {
+    do_print("File.prototype.setDates is not implemented for Android/B2G");
+    do_check_eq(OS.File.prototype.setDates, undefined);
+    return;
+  }
+
+  // First, create a file we can mess with.
+  let path = OS.Path.join(OS.Constants.Path.tmpDir,
+                              "test_osfile_async_setDates_proto.tmp");
+  yield OS.File.writeAtomic(path, new Uint8Array(1));
+
+  try {
+    let fd = yield OS.File.open(path, {write: true});
+
+    try {
+      // 1. Try to set some well known dates.
+      // We choose multiples of 2000ms, because the time stamp resolution of
+      // the underlying OS might not support something more precise.
+      const accDate = 2000;
+      const modDate = 4000;
+      {
+        yield fd.setDates(accDate, modDate);
+        let stat = yield fd.stat();
+        do_check_eq(accDate, stat.lastAccessDate.getTime());
+        do_check_eq(modDate, stat.lastModificationDate.getTime());
+      }
+
+      // 2.1 Try to omit modificationDate (which should then default to
+      // |Date.now()|, expect for resolution differences).
+      {
+        yield fd.setDates(accDate);
+        let stat = yield fd.stat();
+        do_check_eq(accDate, stat.lastAccessDate.getTime());
+        do_check_neq(modDate, stat.lastModificationDate.getTime());
+      }
+
+      // 2.2 Try to omit accessDate as well (which should then default to
+      // |Date.now()|, expect for resolution differences).
+      {
+        yield fd.setDates();
+        let stat = yield fd.stat();
+        do_check_neq(accDate, stat.lastAccessDate.getTime());
+        do_check_neq(modDate, stat.lastModificationDate.getTime());
+      }
+
+      // 3. Repeat 1., but with Date objects this time
+      {
+        yield fd.setDates(new Date(accDate), new Date(modDate));
+        let stat = yield fd.stat();
+        do_check_eq(accDate, stat.lastAccessDate.getTime());
+        do_check_eq(modDate, stat.lastModificationDate.getTime());
+      }
+
+      // 4. Check that invalid params will cause an exception/rejection.
+      {
+        for (let p of ["invalid", new Uint8Array(1), NaN]) {
+          try {
+            yield fd.setDates(p, modDate);
+            do_throw("Invalid access date should have thrown for: " + p);
+          } catch (ex) {
+            let stat = yield fd.stat();
+            do_check_eq(accDate, stat.lastAccessDate.getTime());
+            do_check_eq(modDate, stat.lastModificationDate.getTime());
+          }
+          try {
+            yield fd.setDates(accDate, p);
+            do_throw("Invalid modification date should have thrown for: " + p);
+          } catch (ex) {
+            let stat = yield fd.stat();
+            do_check_eq(accDate, stat.lastAccessDate.getTime());
+            do_check_eq(modDate, stat.lastModificationDate.getTime());
+          }
+          try {
+            yield fd.setDates(p, p);
+            do_throw("Invalid dates should have thrown for: " + p);
+          } catch (ex) {
+            let stat = yield fd.stat();
+            do_check_eq(accDate, stat.lastAccessDate.getTime());
+            do_check_eq(modDate, stat.lastModificationDate.getTime());
+          }
+        }
+      }
+    } finally {
+      yield fd.close();
+    }
+  } finally {
+    // Remove the temp file again
+    yield OS.File.remove(path);
+  }
+});
+
+// Tests setting dates on directories.
+add_task(function* test_dirs() {
+  let path = OS.Path.join(OS.Constants.Path.tmpDir,
+                              "test_osfile_async_setDates_dir");
+  yield OS.File.makeDir(path);
+
+  try {
+    // 1. Try to set some well known dates.
+    // We choose multiples of 2000ms, because the time stamp resolution of
+    // the underlying OS might not support something more precise.
+    const accDate = 2000;
+    const modDate = 4000;
+    {
+      yield OS.File.setDates(path, accDate, modDate);
+      let stat = yield OS.File.stat(path);
+      do_check_eq(accDate, stat.lastAccessDate.getTime());
+      do_check_eq(modDate, stat.lastModificationDate.getTime());
+    }
+  } finally {
+    yield OS.File.removeEmptyDir(path);
+  }
+});
diff --git a/toolkit/components/osfile/tests/xpcshell/test_osfile_async_setPermissions.js b/toolkit/components/osfile/tests/xpcshell/test_osfile_async_setPermissions.js
new file mode 100644
index 000000000..ab8bf7dd9
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_osfile_async_setPermissions.js
@@ -0,0 +1,103 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * A test to ensure that OS.File.setPermissions and
+ * OS.File.prototype.setPermissions are all working correctly.
+ * (see bug 1001849)
+ * These functions are currently Unix-specific.  The manifest skips
+ * the test on Windows.
+ */
+
+/**
+ * Helper function for test logging: prints a POSIX file permission mode as an
+ * octal number, with a leading '0' per C (not JS) convention.  When the
+ * numeric value is 0o777 or lower, it is padded on the left with zeroes to
+ * four digits wide.
+ * Sample outputs:  0022, 0644, 04755.
+ */
+function format_mode(mode) {
+  if (mode <= 0o777) {
+    return ("0000" + mode.toString(8)).slice(-4);
+  } else {
+    return "0" + mode.toString(8);
+  }
+}
+
+const _umask = OS.Constants.Sys.umask;
+do_print("umask: " + format_mode(_umask));
+
+/**
+ * Compute the mode that a file should have after applying the umask,
+ * whatever it happens to be.
+ */
+function apply_umask(mode) {
+  return mode & ~_umask;
+}
+
+// Sequence of setPermission parameters and expected file mode.  The first test
+// checks the permissions when the file is first created.
+var testSequence = [
+  [null,                                        apply_umask(0o600)],
+  [{ unixMode: 0o4777 },                        apply_umask(0o4777)],
+  [{ unixMode: 0o4777, unixHonorUmask: false }, 0o4777],
+  [{ unixMode: 0o4777, unixHonorUmask: true },  apply_umask(0o4777)],
+  [undefined,                                   apply_umask(0o600)],
+  [{ unixMode: 0o666 },                         apply_umask(0o666)],
+  [{ unixMode: 0o600 },                         apply_umask(0o600)],
+  [{ unixMode: 0 },                             0],
+  [{},                                          apply_umask(0o600)],
+];
+
+// Test application to paths.
+add_task(function* test_path_setPermissions() {
+  let path = OS.Path.join(OS.Constants.Path.tmpDir,
+                          "test_osfile_async_setPermissions_path.tmp");
+  yield OS.File.writeAtomic(path, new Uint8Array(1));
+
+  try {
+    for (let [options, expectedMode] of testSequence) {
+      if (options !== null) {
+        do_print("Setting permissions to " + JSON.stringify(options));
+        yield OS.File.setPermissions(path, options);
+      }
+
+      let stat = yield OS.File.stat(path);
+      do_check_eq(format_mode(stat.unixMode), format_mode(expectedMode));
+    }
+  } finally {
+    yield OS.File.remove(path);
+  }
+});
+
+// Test application to open files.
+add_task(function* test_file_setPermissions() {
+  let path = OS.Path.join(OS.Constants.Path.tmpDir,
+                              "test_osfile_async_setPermissions_file.tmp");
+  yield OS.File.writeAtomic(path, new Uint8Array(1));
+
+  try {
+    let fd = yield OS.File.open(path, { write: true });
+    try {
+      for (let [options, expectedMode] of testSequence) {
+        if (options !== null) {
+          do_print("Setting permissions to " + JSON.stringify(options));
+          yield fd.setPermissions(options);
+        }
+
+        let stat = yield fd.stat();
+        do_check_eq(format_mode(stat.unixMode), format_mode(expectedMode));
+      }
+    } finally {
+      yield fd.close();
+    }
+  } finally {
+    yield OS.File.remove(path);
+  }
+});
+
+function run_test() {
+  run_next_test();
+}
diff --git a/toolkit/components/osfile/tests/xpcshell/test_osfile_closed.js b/toolkit/components/osfile/tests/xpcshell/test_osfile_closed.js
new file mode 100644
index 000000000..5740f7f1a
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_osfile_closed.js
@@ -0,0 +1,48 @@
+"use strict";
+
+Components.utils.import("resource://gre/modules/osfile.jsm");
+Components.utils.import("resource://gre/modules/Task.jsm");
+
+function run_test() {
+  do_test_pending();
+  run_next_test();
+}
+
+add_task(function test_closed() {
+  OS.Shared.DEBUG = true;
+  let currentDir = yield OS.File.getCurrentDirectory();
+  do_print("Open a file, ensure that we can call stat()");
+  let path = OS.Path.join(currentDir, "test_osfile_closed.js");
+  let file = yield OS.File.open(path);
+  yield file.stat();
+  do_check_true(true);
+
+  yield file.close();
+
+  do_print("Ensure that we cannot stat() on closed file");
+  let exn;
+  try {
+    yield file.stat();
+  } catch (ex) {
+    exn = ex;
+  }
+  do_print("Ensure that this raises the correct error");
+  do_check_true(!!exn);
+  do_check_true(exn instanceof OS.File.Error);
+  do_check_true(exn.becauseClosed);
+
+  do_print("Ensure that we cannot read() on closed file");
+  exn = null;
+  try {
+    yield file.read();
+  } catch (ex) {
+    exn = ex;
+  }
+  do_print("Ensure that this raises the correct error");
+  do_check_true(!!exn);
+  do_check_true(exn instanceof OS.File.Error);
+  do_check_true(exn.becauseClosed);
+
+});
+
+add_task(do_test_finished);
diff --git a/toolkit/components/osfile/tests/xpcshell/test_osfile_error.js b/toolkit/components/osfile/tests/xpcshell/test_osfile_error.js
new file mode 100644
index 000000000..a1c319eca
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_osfile_error.js
@@ -0,0 +1,63 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+var {OS: {File, Path, Constants}} = Components.utils.import("resource://gre/modules/osfile.jsm", {});
+Components.utils.import("resource://gre/modules/Task.jsm");
+
+function run_test() {
+  run_next_test();
+}
+
+add_task(function* testFileError_with_writeAtomic() {
+  let DEFAULT_CONTENTS = "default contents" + Math.random();
+  let path = Path.join(Constants.Path.tmpDir,
+                       "testFileError.tmp");
+  yield File.remove(path);
+  yield File.writeAtomic(path, DEFAULT_CONTENTS);
+  let exception;
+  try {
+    yield File.writeAtomic(path, DEFAULT_CONTENTS, { noOverwrite: true });
+  } catch (ex) {
+    exception = ex;
+  }
+  do_check_true(exception instanceof File.Error);
+  do_check_true(exception.path == path);
+});
+
+add_task(function* testFileError_with_makeDir() {
+  let path = Path.join(Constants.Path.tmpDir,
+                       "directory");
+  yield File.removeDir(path);
+  yield File.makeDir(path);
+  let exception;
+  try {
+    yield File.makeDir(path, { ignoreExisting: false });
+  } catch (ex) {
+    exception = ex;
+  }
+  do_check_true(exception instanceof File.Error);
+  do_check_true(exception.path == path);
+});
+
+add_task(function* testFileError_with_move() {
+  let DEFAULT_CONTENTS = "default contents" + Math.random();
+  let sourcePath = Path.join(Constants.Path.tmpDir,
+                             "src.tmp");
+  let destPath = Path.join(Constants.Path.tmpDir,
+                           "dest.tmp");
+  yield File.remove(sourcePath);
+  yield File.remove(destPath);
+  yield File.writeAtomic(sourcePath, DEFAULT_CONTENTS);
+  yield File.writeAtomic(destPath, DEFAULT_CONTENTS);
+  let exception;
+  try {
+    yield File.move(sourcePath, destPath, { noOverwrite: true });
+  } catch (ex) {
+    exception = ex;
+  }
+  do_print(exception);
+  do_check_true(exception instanceof File.Error);
+  do_check_true(exception.path == sourcePath);
+});
diff --git a/toolkit/components/osfile/tests/xpcshell/test_osfile_kill.js b/toolkit/components/osfile/tests/xpcshell/test_osfile_kill.js
new file mode 100644
index 000000000..e32c37224
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_osfile_kill.js
@@ -0,0 +1,100 @@
+"use strict";
+
+Components.utils.import("resource://gre/modules/osfile.jsm");
+
+// We want the actual global to get at the internals since Scheduler is not
+// exported.
+var AsyncFrontGlobal = Components.utils.import(
+                         "resource://gre/modules/osfile/osfile_async_front.jsm",
+                          null);
+var Scheduler = AsyncFrontGlobal.Scheduler;
+
+/**
+ * Verify that Scheduler.kill() interacts with other OS.File requests correctly,
+ * and that no requests are lost.  This is relevant because on B2G we
+ * auto-kill the worker periodically, making it very possible for valid requests
+ * to be interleaved with the automatic kill().
+ *
+ * This test is being created with the fix for Bug 1125989 where `kill` queue
+ * management was found to be buggy.  It is a glass-box test that explicitly
+ * re-creates the observed failure situation; it is not guaranteed to prevent
+ * all future regressions.  The following is a detailed explanation of the test
+ * for your benefit if this test ever breaks or you are wondering what was the
+ * point of all this.  You might want to skim the code below first.
+ *
+ * OS.File maintains a `queue` of operations to be performed.  This queue is
+ * nominally implemented as a chain of promises.  Every time a new job is
+ * OS.File.push()ed, it effectively becomes the new `queue` promise.  (An
+ * extra promise is interposed with a rejection handler to avoid the rejection
+ * cascading, but that does not matter for our purposes.)
+ *
+ * The flaw in `kill` was that it would wait for the `queue` to complete before
+ * replacing `queue`.  As a result, another OS.File operation could use `push`
+ * (by way of OS.File.post()) to also use .then() on the same `queue` promise.
+ * Accordingly, assuming that promise was not yet resolved (due to a pending
+ * OS.File request), when it was resolved, both the task scheduled in `kill`
+ * and in `post` would be triggered.  Both of those tasks would run until
+ * encountering a call to worker.post().
+ *
+ * Re-creating this race is not entirely trivial because of the large number of
+ * promises used by the code causing control flow to repeatedly be deferred.  In
+ * a slightly simpler world we could run the follwing in the same turn of the
+ * event loop and trigger the problem.
+ * - any OS.File request
+ * - Scheduler.kill()
+ * - any OS.File request
+ *
+ * However, we need the Scheduler.kill task to reach the point where it is
+ * waiting on the same `queue` that another task has been scheduled against.
+ * Since the `kill` task yields on the `killQueue` promise prior to yielding
+ * on `queue`, however, some turns of the event loop are required.  Happily,
+ * for us, as discussed above, the problem triggers when we have two promises
+ * scheduled on the `queue`, so we can just wait to schedule the second OS.File
+ * request on the queue.  (Note that because of the additional then() added to
+ * eat rejections, there is an important difference between the value of
+ * `queue` and the value returned by the first OS.File request.)
+ */
+add_task(function* test_kill_race() {
+  // Ensure the worker has been created and that SET_DEBUG has taken effect.
+  // We have chosen OS.File.exists for our tests because it does not trigger
+  // a rejection and we absolutely do not care what the operation is other
+  // than it does not invoke a native fast-path.
+  yield OS.File.exists('foo.foo');
+
+  do_print('issuing first request');
+  let firstRequest = OS.File.exists('foo.bar');
+  let secondRequest;
+  let secondResolved = false;
+
+  // As noted in our big block comment, we want to wait to schedule the
+  // second request so that it races `kill`'s call to `worker.post`.  Having
+  // ourselves wait on the same promise, `queue`, and registering ourselves
+  // before we issue the kill request means we will get run before the `kill`
+  // task resumes and allow us to precisely create the desired race.
+  Scheduler.queue.then(function() {
+    do_print('issuing second request');
+    secondRequest = OS.File.exists('foo.baz');
+    secondRequest.then(function() {
+      secondResolved = true;
+    });
+  });
+
+  do_print('issuing kill request');
+  let killRequest = Scheduler.kill({ reset: true, shutdown: false });
+
+  // Wait on the killRequest so that we can schedule a new OS.File request
+  // after it completes...
+  yield killRequest;
+  // ...because our ordering guarantee ensures that there is at most one
+  // worker (and this usage here should not be vulnerable even with the
+  // bug present), so when this completes the secondRequest has either been
+  // resolved or lost.
+  yield OS.File.exists('foo.goz');
+
+  ok(secondResolved,
+     'The second request was resolved so we avoided the bug. Victory!');
+});
+
+function run_test() {
+  run_next_test();
+}
diff --git a/toolkit/components/osfile/tests/xpcshell/test_osfile_win_async_setPermissions.js b/toolkit/components/osfile/tests/xpcshell/test_osfile_win_async_setPermissions.js
new file mode 100644
index 000000000..990d722f5
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_osfile_win_async_setPermissions.js
@@ -0,0 +1,114 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * A test to ensure that OS.File.setPermissions and
+ * OS.File.prototype.setPermissions are all working correctly.
+ * (see bug 1022816)
+ * The manifest tests on Windows.
+ */
+
+// Sequence of setPermission parameters.
+var testSequence = [
+  [ { winAttributes: { readOnly: true, system: true, hidden: true } },
+    { readOnly: true, system: true, hidden: true } ],
+  [ { winAttributes: { readOnly: false } },
+    { readOnly: false, system: true, hidden: true } ],
+  [ { winAttributes: { system: false } },
+    { readOnly: false, system: false, hidden: true } ],
+  [ { winAttributes: { hidden: false } },
+    { readOnly: false, system: false, hidden: false } ],
+  [ { winAttributes: {readOnly: true, system: false, hidden: false} },
+    { readOnly: true, system: false, hidden: false } ],
+  [ { winAttributes: {readOnly: false, system: true, hidden: false} },
+    { readOnly: false, system: true, hidden: false } ],
+  [ { winAttributes: {readOnly: false, system: false, hidden: true} },
+    { readOnly: false, system: false, hidden: true } ],
+];
+
+// Test application to paths.
+add_task(function* test_path_setPermissions() {
+  let path = OS.Path.join(OS.Constants.Path.tmpDir,
+                          "test_osfile_win_async_setPermissions_path.tmp");
+  yield OS.File.writeAtomic(path, new Uint8Array(1));
+
+  try {
+    for (let [options, attributesExpected] of testSequence) {
+      if (options !== null) {
+        do_print("Setting permissions to " + JSON.stringify(options));
+        yield OS.File.setPermissions(path, options);
+      }
+
+      let stat = yield OS.File.stat(path);
+      do_print("Got stat winAttributes: " + JSON.stringify(stat.winAttributes));
+
+      do_check_eq(stat.winAttributes.readOnly, attributesExpected.readOnly);
+      do_check_eq(stat.winAttributes.system, attributesExpected.system);
+      do_check_eq(stat.winAttributes.hidden, attributesExpected.hidden);
+
+    }
+  } finally {
+    yield OS.File.remove(path);
+  }
+});
+
+// Test application to open files.
+add_task(function* test_file_setPermissions() {
+  let path = OS.Path.join(OS.Constants.Path.tmpDir,
+                              "test_osfile_win_async_setPermissions_file.tmp");
+  yield OS.File.writeAtomic(path, new Uint8Array(1));
+
+  try {
+    let fd = yield OS.File.open(path, { write: true });
+    try {
+      for (let [options, attributesExpected] of testSequence) {
+        if (options !== null) {
+          do_print("Setting permissions to " + JSON.stringify(options));
+          yield fd.setPermissions(options);
+        }
+
+        let stat = yield fd.stat();
+        do_print("Got stat winAttributes: " + JSON.stringify(stat.winAttributes));
+        do_check_eq(stat.winAttributes.readOnly, attributesExpected.readOnly);
+        do_check_eq(stat.winAttributes.system, attributesExpected.system);
+        do_check_eq(stat.winAttributes.hidden, attributesExpected.hidden);
+      }
+    } finally {
+      yield fd.close();
+    }
+  } finally {
+    yield OS.File.remove(path);
+  }
+});
+
+// Test application to Check setPermissions on a non-existant file path.
+add_task(function* test_non_existant_file_path_setPermissions() {
+  let path = OS.Path.join(OS.Constants.Path.tmpDir,
+                          "test_osfile_win_async_setPermissions_path.tmp");
+  Assert.rejects(OS.File.setPermissions(path, {winAttributes: {readOnly: true}}),
+                 /The system cannot find the file specified/,
+                 "setPermissions failed as expected on a non-existant file path");
+});
+
+// Test application to Check setPermissions on a invalid file handle.
+add_task(function* test_closed_file_handle_setPermissions() {
+  let path = OS.Path.join(OS.Constants.Path.tmpDir,
+                          "test_osfile_win_async_setPermissions_path.tmp");
+  yield OS.File.writeAtomic(path, new Uint8Array(1));
+
+  try {
+    let fd = yield OS.File.open(path, { write: true });
+    yield fd.close();
+    Assert.rejects(fd.setPermissions(path, {winAttributes: {readOnly: true}}),
+                   /The handle is invalid/,
+                   "setPermissions failed as expected on a invalid file handle");
+  } finally {
+    yield OS.File.remove(path);
+  }
+});
+
+function run_test() {
+  run_next_test();
+}
diff --git a/toolkit/components/osfile/tests/xpcshell/test_osfile_writeAtomic_backupTo_option.js b/toolkit/components/osfile/tests/xpcshell/test_osfile_writeAtomic_backupTo_option.js
new file mode 100644
index 000000000..adf345b0c
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_osfile_writeAtomic_backupTo_option.js
@@ -0,0 +1,143 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+var {OS: {File, Path, Constants}} = Components.utils.import("resource://gre/modules/osfile.jsm", {});
+Components.utils.import("resource://gre/modules/Task.jsm");
+
+/**
+ * Remove all temporary files and back up files, including
+ * test_backupTo_option_with_tmpPath.tmp
+ * test_backupTo_option_with_tmpPath.tmp.backup
+ * test_backupTo_option_without_tmpPath.tmp
+ * test_backupTo_option_without_tmpPath.tmp.backup
+ * test_non_backupTo_option.tmp
+ * test_non_backupTo_option.tmp.backup
+ * test_backupTo_option_without_destination_file.tmp
+ * test_backupTo_option_without_destination_file.tmp.backup
+ * test_backupTo_option_with_backup_file.tmp
+ * test_backupTo_option_with_backup_file.tmp.backup
+ */
+function clearFiles() {
+  return Task.spawn(function () {
+    let files = ["test_backupTo_option_with_tmpPath.tmp",
+                  "test_backupTo_option_without_tmpPath.tmp",
+                  "test_non_backupTo_option.tmp",
+                  "test_backupTo_option_without_destination_file.tmp",
+                  "test_backupTo_option_with_backup_file.tmp"];
+    for (let file of files) {
+      let path = Path.join(Constants.Path.tmpDir, file);
+      yield File.remove(path);
+      yield File.remove(path + ".backup");
+    }
+  });
+}
+
+function run_test() {
+  run_next_test();
+}
+
+add_task(function* init() {
+  yield clearFiles();
+});
+
+/**
+ * test when
+ * |backupTo| specified
+ * |tmpPath| specified
+ * destination file exists
+ * @result destination file will be backed up
+ */
+add_task(function* test_backupTo_option_with_tmpPath() {
+  let DEFAULT_CONTENTS = "default contents" + Math.random();
+  let WRITE_CONTENTS = "abc" + Math.random();
+  let path = Path.join(Constants.Path.tmpDir,
+                       "test_backupTo_option_with_tmpPath.tmp");
+  yield File.writeAtomic(path, DEFAULT_CONTENTS);
+  yield File.writeAtomic(path, WRITE_CONTENTS, { tmpPath: path + ".tmp",
+                                        backupTo: path + ".backup" });
+  do_check_true((yield File.exists(path + ".backup")));
+  let contents = yield File.read(path + ".backup");
+  do_check_eq(DEFAULT_CONTENTS, (new TextDecoder()).decode(contents));
+});
+
+/**
+ * test when
+ * |backupTo| specified
+ * |tmpPath| not specified
+ * destination file exists
+ * @result destination file will be backed up
+ */
+add_task(function* test_backupTo_option_without_tmpPath() {
+  let DEFAULT_CONTENTS = "default contents" + Math.random();
+  let WRITE_CONTENTS = "abc" + Math.random();
+  let path = Path.join(Constants.Path.tmpDir,
+                       "test_backupTo_option_without_tmpPath.tmp");
+  yield File.writeAtomic(path, DEFAULT_CONTENTS);
+  yield File.writeAtomic(path, WRITE_CONTENTS, { backupTo: path + ".backup" });
+  do_check_true((yield File.exists(path + ".backup")));
+  let contents = yield File.read(path + ".backup");
+  do_check_eq(DEFAULT_CONTENTS, (new TextDecoder()).decode(contents));
+});
+
+/**
+ * test when
+ * |backupTo| not specified
+ * |tmpPath| not specified
+ * destination file exists
+ * @result destination file will not be backed up
+ */
+add_task(function* test_non_backupTo_option() {
+  let DEFAULT_CONTENTS = "default contents" + Math.random();
+  let WRITE_CONTENTS = "abc" + Math.random();
+  let path = Path.join(Constants.Path.tmpDir,
+                       "test_non_backupTo_option.tmp");
+  yield File.writeAtomic(path, DEFAULT_CONTENTS);
+  yield File.writeAtomic(path, WRITE_CONTENTS);
+  do_check_false((yield File.exists(path + ".backup")));
+});
+
+/**
+ * test when
+ * |backupTo| specified
+ * |tmpPath| not specified
+ * destination file not exists
+ * @result no back up file exists
+ */
+add_task(function* test_backupTo_option_without_destination_file() {
+  let DEFAULT_CONTENTS = "default contents" + Math.random();
+  let WRITE_CONTENTS = "abc" + Math.random();
+  let path = Path.join(Constants.Path.tmpDir,
+                       "test_backupTo_option_without_destination_file.tmp");
+  yield File.remove(path);
+  yield File.writeAtomic(path, WRITE_CONTENTS, { backupTo: path + ".backup" });
+  do_check_false((yield File.exists(path + ".backup")));
+});
+
+/**
+ * test when
+ * |backupTo| specified
+ * |tmpPath| not specified
+ * backup file exists
+ * destination file exists
+ * @result destination file will be backed up
+ */
+add_task(function* test_backupTo_option_with_backup_file() {
+  let DEFAULT_CONTENTS = "default contents" + Math.random();
+  let WRITE_CONTENTS = "abc" + Math.random();
+  let path = Path.join(Constants.Path.tmpDir,
+                       "test_backupTo_option_with_backup_file.tmp");
+  yield File.writeAtomic(path, DEFAULT_CONTENTS);
+
+  yield File.writeAtomic(path + ".backup", new Uint8Array(1000));
+
+  yield File.writeAtomic(path, WRITE_CONTENTS, { backupTo: path + ".backup" });
+  do_check_true((yield File.exists(path + ".backup")));
+  let contents = yield File.read(path + ".backup");
+  do_check_eq(DEFAULT_CONTENTS, (new TextDecoder()).decode(contents));
+});
+
+add_task(function* cleanup() {
+  yield clearFiles();
+});
diff --git a/toolkit/components/osfile/tests/xpcshell/test_osfile_writeAtomic_zerobytes.js b/toolkit/components/osfile/tests/xpcshell/test_osfile_writeAtomic_zerobytes.js
new file mode 100644
index 000000000..a32a690e6
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_osfile_writeAtomic_zerobytes.js
@@ -0,0 +1,27 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+var SHARED_PATH;
+
+add_task(function* init() {
+  do_get_profile();
+  SHARED_PATH = OS.Path.join(OS.Constants.Path.profileDir, "test_osfile_write_zerobytes.tmp");
+});
+
+add_test_pair(function* test_osfile_writeAtomic_zerobytes() {
+  let encoder = new TextEncoder();
+  let string1 = "";
+  let outbin = encoder.encode(string1);
+  yield OS.File.writeAtomic(SHARED_PATH, outbin);
+
+  let decoder = new TextDecoder();
+  let bin = yield OS.File.read(SHARED_PATH);
+  let string2 = decoder.decode(bin);
+  // Checking if writeAtomic supports writing encoded zero-byte strings
+  Assert.equal(string2, string1, "Read the expected (empty) string.");
+});
+
+function run_test() {
+  run_next_test();
+}
\ No newline at end of file
diff --git a/toolkit/components/osfile/tests/xpcshell/test_path.js b/toolkit/components/osfile/tests/xpcshell/test_path.js
new file mode 100644
index 000000000..76a507ee3
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_path.js
@@ -0,0 +1,159 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+Components.utils.import("resource://gre/modules/Services.jsm", this);
+Services.prefs.setBoolPref("toolkit.osfile.test.syslib_necessary", false);
+  // We don't need libc/kernel32.dll for this test
+
+var ImportWin = {};
+var ImportUnix = {};
+Components.utils.import("resource://gre/modules/osfile/ospath_win.jsm", ImportWin);
+Components.utils.import("resource://gre/modules/osfile/ospath_unix.jsm", ImportUnix);
+
+var Win = ImportWin;
+var Unix = ImportUnix;
+
+function do_check_fail(f)
+{
+  try {
+    let result = f();
+    do_print("Failed do_check_fail: " + result);
+    do_check_true(false);
+  } catch (ex) {
+    do_check_true(true);
+  }
+};
+
+function run_test()
+{
+  do_print("Testing Windows paths");
+
+  do_print("Backslash-separated, no drive");
+  do_check_eq(Win.basename("a\\b"), "b");
+  do_check_eq(Win.basename("a\\b\\"), "");
+  do_check_eq(Win.basename("abc"), "abc");
+  do_check_eq(Win.dirname("a\\b"), "a");
+  do_check_eq(Win.dirname("a\\b\\"), "a\\b");
+  do_check_eq(Win.dirname("a\\\\\\\\b"), "a");
+  do_check_eq(Win.dirname("abc"), ".");
+  do_check_eq(Win.normalize("\\a\\b\\c"), "\\a\\b\\c");
+  do_check_eq(Win.normalize("\\a\\b\\\\\\\\c"), "\\a\\b\\c");
+  do_check_eq(Win.normalize("\\a\\b\\c\\\\\\"), "\\a\\b\\c");
+  do_check_eq(Win.normalize("\\a\\b\\c\\..\\..\\..\\d\\e\\f"), "\\d\\e\\f");
+  do_check_eq(Win.normalize("a\\b\\c\\..\\..\\..\\d\\e\\f"), "d\\e\\f");
+  do_check_fail(() => Win.normalize("\\a\\b\\c\\..\\..\\..\\..\\d\\e\\f"));
+
+  do_check_eq(Win.join("\\tmp", "foo", "bar"), "\\tmp\\foo\\bar", "join \\tmp,foo,bar");
+  do_check_eq(Win.join("\\tmp", "\\foo", "bar"), "\\foo\\bar", "join \\tmp,\\foo,bar");
+  do_check_eq(Win.winGetDrive("\\tmp"), null);
+  do_check_eq(Win.winGetDrive("\\tmp\\a\\b\\c\\d\\e"), null);
+  do_check_eq(Win.winGetDrive("\\"), null);
+
+
+  do_print("Backslash-separated, with a drive");
+  do_check_eq(Win.basename("c:a\\b"), "b");
+  do_check_eq(Win.basename("c:a\\b\\"), "");
+  do_check_eq(Win.basename("c:abc"), "abc");
+  do_check_eq(Win.dirname("c:a\\b"), "c:a");
+  do_check_eq(Win.dirname("c:a\\b\\"), "c:a\\b");
+  do_check_eq(Win.dirname("c:a\\\\\\\\b"), "c:a");
+  do_check_eq(Win.dirname("c:abc"), "c:");
+  let options = {
+    winNoDrive: true
+  };
+  do_check_eq(Win.dirname("c:a\\b", options), "a");
+  do_check_eq(Win.dirname("c:a\\b\\", options), "a\\b");
+  do_check_eq(Win.dirname("c:a\\\\\\\\b", options), "a");
+  do_check_eq(Win.dirname("c:abc", options), ".");
+  do_check_eq(Win.join("c:", "abc"), "c:\\abc", "join c:,abc");
+
+  do_check_eq(Win.normalize("c:"), "c:\\");
+  do_check_eq(Win.normalize("c:\\"), "c:\\");
+  do_check_eq(Win.normalize("c:\\a\\b\\c"), "c:\\a\\b\\c");
+  do_check_eq(Win.normalize("c:\\a\\b\\\\\\\\c"), "c:\\a\\b\\c");
+  do_check_eq(Win.normalize("c:\\\\\\\\a\\b\\c"), "c:\\a\\b\\c");
+  do_check_eq(Win.normalize("c:\\a\\b\\c\\\\\\"), "c:\\a\\b\\c");
+  do_check_eq(Win.normalize("c:\\a\\b\\c\\..\\..\\..\\d\\e\\f"), "c:\\d\\e\\f");
+  do_check_eq(Win.normalize("c:a\\b\\c\\..\\..\\..\\d\\e\\f"), "c:\\d\\e\\f");
+  do_check_fail(() => Win.normalize("c:\\a\\b\\c\\..\\..\\..\\..\\d\\e\\f"));
+
+  do_check_eq(Win.join("c:\\", "foo"), "c:\\foo", "join c:\,foo");
+  do_check_eq(Win.join("c:\\tmp", "foo", "bar"), "c:\\tmp\\foo\\bar", "join c:\\tmp,foo,bar");
+  do_check_eq(Win.join("c:\\tmp", "\\foo", "bar"), "c:\\foo\\bar", "join c:\\tmp,\\foo,bar");
+  do_check_eq(Win.join("c:\\tmp", "c:\\foo", "bar"), "c:\\foo\\bar", "join c:\\tmp,c:\\foo,bar");
+  do_check_eq(Win.join("c:\\tmp", "c:foo", "bar"), "c:\\foo\\bar", "join c:\\tmp,c:foo,bar");
+  do_check_eq(Win.winGetDrive("c:"), "c:");
+  do_check_eq(Win.winGetDrive("c:\\"), "c:");
+  do_check_eq(Win.winGetDrive("c:abc"), "c:");
+  do_check_eq(Win.winGetDrive("c:abc\\d\\e\\f\\g"), "c:");
+  do_check_eq(Win.winGetDrive("c:\\abc"), "c:");
+  do_check_eq(Win.winGetDrive("c:\\abc\\d\\e\\f\\g"), "c:");
+
+  do_print("Forwardslash-separated, no drive");
+  do_check_eq(Win.normalize("/a/b/c"), "\\a\\b\\c");
+  do_check_eq(Win.normalize("/a/b////c"), "\\a\\b\\c");
+  do_check_eq(Win.normalize("/a/b/c///"), "\\a\\b\\c");
+  do_check_eq(Win.normalize("/a/b/c/../../../d/e/f"), "\\d\\e\\f");
+  do_check_eq(Win.normalize("a/b/c/../../../d/e/f"), "d\\e\\f");
+
+  do_print("Forwardslash-separated, with a drive");
+  do_check_eq(Win.normalize("c:/"), "c:\\");
+  do_check_eq(Win.normalize("c:/a/b/c"), "c:\\a\\b\\c");
+  do_check_eq(Win.normalize("c:/a/b////c"), "c:\\a\\b\\c");
+  do_check_eq(Win.normalize("c:////a/b/c"), "c:\\a\\b\\c");
+  do_check_eq(Win.normalize("c:/a/b/c///"), "c:\\a\\b\\c");
+  do_check_eq(Win.normalize("c:/a/b/c/../../../d/e/f"), "c:\\d\\e\\f");
+  do_check_eq(Win.normalize("c:a/b/c/../../../d/e/f"), "c:\\d\\e\\f");
+
+  do_print("Backslash-separated, UNC-style");
+  do_check_eq(Win.basename("\\\\a\\b"), "b");
+  do_check_eq(Win.basename("\\\\a\\b\\"), "");
+  do_check_eq(Win.basename("\\\\abc"), "");
+  do_check_eq(Win.dirname("\\\\a\\b"), "\\\\a");
+  do_check_eq(Win.dirname("\\\\a\\b\\"), "\\\\a\\b");
+  do_check_eq(Win.dirname("\\\\a\\\\\\\\b"), "\\\\a");
+  do_check_eq(Win.dirname("\\\\abc"), "\\\\abc");
+  do_check_eq(Win.normalize("\\\\a\\b\\c"), "\\\\a\\b\\c");
+  do_check_eq(Win.normalize("\\\\a\\b\\\\\\\\c"), "\\\\a\\b\\c");
+  do_check_eq(Win.normalize("\\\\a\\b\\c\\\\\\"), "\\\\a\\b\\c");
+  do_check_eq(Win.normalize("\\\\a\\b\\c\\..\\..\\d\\e\\f"), "\\\\a\\d\\e\\f");
+  do_check_fail(() => Win.normalize("\\\\a\\b\\c\\..\\..\\..\\d\\e\\f"));
+
+  do_check_eq(Win.join("\\\\a\\tmp", "foo", "bar"), "\\\\a\\tmp\\foo\\bar");
+  do_check_eq(Win.join("\\\\a\\tmp", "\\foo", "bar"), "\\\\a\\foo\\bar");
+  do_check_eq(Win.join("\\\\a\\tmp", "\\\\foo\\", "bar"), "\\\\foo\\bar");
+  do_check_eq(Win.winGetDrive("\\\\"), null);
+  do_check_eq(Win.winGetDrive("\\\\c"), "\\\\c");
+  do_check_eq(Win.winGetDrive("\\\\c\\abc"), "\\\\c");
+
+  do_print("Testing unix paths");
+  do_check_eq(Unix.basename("a/b"), "b");
+  do_check_eq(Unix.basename("a/b/"), "");
+  do_check_eq(Unix.basename("abc"), "abc");
+  do_check_eq(Unix.dirname("a/b"), "a");
+  do_check_eq(Unix.dirname("a/b/"), "a/b");
+  do_check_eq(Unix.dirname("a////b"), "a");
+  do_check_eq(Unix.dirname("abc"), ".");
+  do_check_eq(Unix.normalize("/a/b/c"), "/a/b/c");
+  do_check_eq(Unix.normalize("/a/b////c"), "/a/b/c");
+  do_check_eq(Unix.normalize("////a/b/c"), "/a/b/c");
+  do_check_eq(Unix.normalize("/a/b/c///"), "/a/b/c");
+  do_check_eq(Unix.normalize("/a/b/c/../../../d/e/f"), "/d/e/f");
+  do_check_eq(Unix.normalize("a/b/c/../../../d/e/f"), "d/e/f");
+  do_check_fail(() => Unix.normalize("/a/b/c/../../../../d/e/f"));
+
+  do_check_eq(Unix.join("/tmp", "foo", "bar"), "/tmp/foo/bar", "join /tmp,foo,bar");
+  do_check_eq(Unix.join("/tmp", "/foo", "bar"), "/foo/bar", "join /tmp,/foo,bar");
+
+  do_print("Testing the presence of ospath.jsm");
+  let Scope = {};
+  try {
+    Components.utils.import("resource://gre/modules/osfile/ospath.jsm", Scope);
+  } catch (ex) {
+    // Can't load ospath
+  }
+  do_check_true(!!Scope.basename);
+}
diff --git a/toolkit/components/osfile/tests/xpcshell/test_path_constants.js b/toolkit/components/osfile/tests/xpcshell/test_path_constants.js
new file mode 100644
index 000000000..c0057c750
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_path_constants.js
@@ -0,0 +1,83 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+Cu.import("resource://gre/modules/ctypes.jsm", this);
+Cu.import("resource://testing-common/AppData.jsm", this);
+
+
+function run_test() {
+  run_next_test();
+}
+
+function compare_paths(ospath, key) {
+  let file;
+  try {
+    file = Services.dirsvc.get(key, Components.interfaces.nsIFile);
+  } catch(ex) {}
+
+  if (file) {
+    do_check_true(!!ospath);
+    do_check_eq(ospath, file.path);
+  } else {
+    do_print("WARNING: " + key + " is not defined. Test may not be testing anything!");
+    do_check_false(!!ospath);
+  }
+}
+
+// Some path constants aren't set up until the profile is available. This
+// test verifies that behavior.
+add_task(function* test_before_after_profile() {
+  do_check_null(OS.Constants.Path.profileDir);
+  do_check_null(OS.Constants.Path.localProfileDir);
+  do_check_null(OS.Constants.Path.userApplicationDataDir);
+
+  do_get_profile();
+  do_check_true(!!OS.Constants.Path.profileDir);
+  do_check_true(!!OS.Constants.Path.localProfileDir);
+
+  // UAppData is still null because the xpcshell profile doesn't set it up.
+  // This test is mostly here to fail in case behavior of do_get_profile() ever
+  // changes. We want to know if our assumptions no longer hold!
+  do_check_null(OS.Constants.Path.userApplicationDataDir);
+
+  yield makeFakeAppDir();
+  do_check_true(!!OS.Constants.Path.userApplicationDataDir);
+
+  // FUTURE: verify AppData too (bug 964291).
+});
+
+// Test simple paths
+add_task(function* test_simple_paths() {
+  do_check_true(!!OS.Constants.Path.tmpDir);
+  compare_paths(OS.Constants.Path.tmpDir, "TmpD");
+
+});
+
+// Test presence of paths that only exist on Desktop platforms
+add_task(function* test_desktop_paths() {
+  if (OS.Constants.Sys.Name == "Android" || OS.Constants.Sys.Name == "Gonk") {
+    return;
+  }
+  do_check_true(!!OS.Constants.Path.desktopDir);
+  do_check_true(!!OS.Constants.Path.homeDir);
+
+  compare_paths(OS.Constants.Path.homeDir, "Home");
+  compare_paths(OS.Constants.Path.desktopDir, "Desk");
+  compare_paths(OS.Constants.Path.userApplicationDataDir, "UAppData");
+
+  compare_paths(OS.Constants.Path.winAppDataDir, "AppData");
+  compare_paths(OS.Constants.Path.winStartMenuProgsDir, "Progs");
+
+  compare_paths(OS.Constants.Path.macUserLibDir, "ULibDir");
+  compare_paths(OS.Constants.Path.macLocalApplicationsDir, "LocApp");
+  compare_paths(OS.Constants.Path.macTrashDir, "Trsh");
+});
+
+// Open libxul
+add_task(function* test_libxul() {
+  ctypes.open(OS.Constants.Path.libxul);
+  do_print("Linked to libxul");
+});
diff --git a/toolkit/components/osfile/tests/xpcshell/test_queue.js b/toolkit/components/osfile/tests/xpcshell/test_queue.js
new file mode 100644
index 000000000..c9d23eabc
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_queue.js
@@ -0,0 +1,38 @@
+"use strict";
+
+Components.utils.import("resource://gre/modules/osfile.jsm");
+
+function run_test() {
+  run_next_test();
+}
+
+// Check if Scheduler.queue returned by OS.File.queue is resolved initially.
+add_task(function* check_init() {
+  yield OS.File.queue;
+  do_print("Function resolved");
+});
+
+// Check if Scheduler.queue returned by OS.File.queue is resolved
+// after an operation is successful.
+add_task(function* check_success() {
+  do_print("Attempting to open a file correctly");
+  let openedFile = yield OS.File.open(OS.Path.join(do_get_cwd().path, "test_queue.js"));
+  do_print("File opened correctly");
+  yield OS.File.queue;
+  do_print("Function resolved");
+});
+
+// Check if Scheduler.queue returned by OS.File.queue is resolved
+// after an operation fails.
+add_task(function* check_failure() {
+  let exception;
+  try {
+    do_print("Attempting to open a non existing file");
+    yield OS.File.open(OS.Path.join(".", "Bigfoot"));
+  } catch (err) {
+    exception = err;
+    yield OS.File.queue;
+  }  
+  do_check_true(exception!=null);
+  do_print("Function resolved");
+});
\ No newline at end of file
diff --git a/toolkit/components/osfile/tests/xpcshell/test_read_write.js b/toolkit/components/osfile/tests/xpcshell/test_read_write.js
new file mode 100644
index 000000000..00235ed8c
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_read_write.js
@@ -0,0 +1,103 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+var {utils: Cu} = Components;
+
+var SHARED_PATH;
+
+var EXISTING_FILE = do_get_file("xpcshell.ini").path;
+
+add_task(function* init() {
+  do_get_profile();
+  SHARED_PATH = OS.Path.join(OS.Constants.Path.profileDir, "test_osfile_read.tmp");
+});
+
+
+// Check that OS.File.read() is executed after the previous operation
+add_test_pair(function* ordering() {
+  let string1 = "Initial state " + Math.random();
+  let string2 = "After writing " + Math.random();
+  yield OS.File.writeAtomic(SHARED_PATH, string1);
+  OS.File.writeAtomic(SHARED_PATH, string2);
+  let string3 = yield OS.File.read(SHARED_PATH, { encoding: "utf-8" });
+  do_check_eq(string3, string2);
+});
+
+add_test_pair(function* read_write_all() {
+  let DEST_PATH = SHARED_PATH + Math.random();
+  let TMP_PATH = DEST_PATH + ".tmp";
+
+  let test_with_options = function(options, suffix) {
+    return Task.spawn(function*() {
+      do_print("Running test read_write_all with options " + JSON.stringify(options));
+      let TEST = "read_write_all " + suffix;
+
+      let optionsBackup = JSON.parse(JSON.stringify(options));
+
+      // Check that read + writeAtomic performs a correct copy
+      let currentDir = yield OS.File.getCurrentDirectory();
+      let pathSource = OS.Path.join(currentDir, EXISTING_FILE);
+      let contents = yield OS.File.read(pathSource);
+      do_check_true(!!contents); // Content is not empty
+      let bytesRead = contents.byteLength;
+
+      let bytesWritten = yield OS.File.writeAtomic(DEST_PATH, contents, options);
+      do_check_eq(bytesRead, bytesWritten); // Correct number of bytes written
+
+      // Check that options are not altered
+      do_check_eq(JSON.stringify(options), JSON.stringify(optionsBackup));
+      yield reference_compare_files(pathSource, DEST_PATH, TEST);
+
+      // Check that temporary file was removed or never created exist
+      do_check_false(new FileUtils.File(TMP_PATH).exists());
+
+      // Check that writeAtomic fails if noOverwrite is true and the destination
+      // file already exists!
+      contents = new Uint8Array(300);
+      let view = new Uint8Array(contents.buffer, 10, 200);
+      try {
+        let opt = JSON.parse(JSON.stringify(options));
+        opt.noOverwrite = true;
+        yield OS.File.writeAtomic(DEST_PATH, view, opt);
+        do_throw("With noOverwrite, writeAtomic should have refused to overwrite file (" + suffix + ")");
+      } catch (err if err instanceof OS.File.Error && err.becauseExists) {
+        do_print("With noOverwrite, writeAtomic correctly failed (" + suffix + ")");
+      }
+      yield reference_compare_files(pathSource, DEST_PATH, TEST);
+
+      // Check that temporary file was removed or never created
+      do_check_false(new FileUtils.File(TMP_PATH).exists());
+
+      // Now write a subset
+      let START = 10;
+      let LENGTH = 100;
+      contents = new Uint8Array(300);
+      for (var i = 0; i < contents.byteLength; i++)
+        contents[i] = i % 256;
+      view = new Uint8Array(contents.buffer, START, LENGTH);
+      bytesWritten = yield OS.File.writeAtomic(DEST_PATH, view, options);
+      do_check_eq(bytesWritten, LENGTH);
+
+      let array2 = yield OS.File.read(DEST_PATH);
+      do_check_eq(LENGTH, array2.length);
+      for (var i = 0; i < LENGTH; i++)
+        do_check_eq(array2[i], (i + START) % 256);
+
+      // Cleanup.
+      yield OS.File.remove(DEST_PATH);
+      yield OS.File.remove(TMP_PATH);
+    });
+  };
+
+  yield test_with_options({tmpPath: TMP_PATH}, "Renaming, not flushing");
+  yield test_with_options({tmpPath: TMP_PATH, flush: true}, "Renaming, flushing");
+  yield test_with_options({}, "Not renaming, not flushing");
+  yield test_with_options({flush: true}, "Not renaming, flushing");
+});
+
+
+function run_test() {
+  run_next_test();
+}
diff --git a/toolkit/components/osfile/tests/xpcshell/test_remove.js b/toolkit/components/osfile/tests/xpcshell/test_remove.js
new file mode 100644
index 000000000..c8dc33054
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_remove.js
@@ -0,0 +1,56 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+Components.utils.import("resource://gre/modules/osfile.jsm");
+Components.utils.import("resource://gre/modules/Task.jsm");
+Components.utils.import("resource://gre/modules/Services.jsm");
+
+do_register_cleanup(function() {
+  Services.prefs.setBoolPref("toolkit.osfile.log", false);
+});
+
+function run_test() {
+  Services.prefs.setBoolPref("toolkit.osfile.log", true);
+  run_next_test();
+}
+
+add_task(function* test_ignoreAbsent() {
+  let absent_file_name = "test_osfile_front_absent.tmp";
+
+  // Removing absent files should throw if "ignoreAbsent" is true.
+  yield Assert.rejects(OS.File.remove(absent_file_name, {ignoreAbsent: false}),
+                       "OS.File.remove throws if there is no such file.");
+
+  // Removing absent files should not throw if "ignoreAbsent" is true or not
+  // defined.
+  let exception = null;
+  try {
+    yield OS.File.remove(absent_file_name, {ignoreAbsent: true});
+    yield OS.File.remove(absent_file_name);
+  } catch (ex) {
+    exception = ex;
+  }
+  Assert.ok(!exception, "OS.File.remove should not throw when not requested.");
+});
+
+add_task(function* test_ignoreAbsent_directory_missing() {
+  let absent_file_name = OS.Path.join("absent_parent", "test.tmp");
+
+  // Removing absent files should throw if "ignoreAbsent" is true.
+  yield Assert.rejects(OS.File.remove(absent_file_name, {ignoreAbsent: false}),
+                       "OS.File.remove throws if there is no such file.");
+
+  // Removing files from absent directories should not throw if "ignoreAbsent"
+  // is true or not defined.
+  let exception = null;
+  try {
+    yield OS.File.remove(absent_file_name, {ignoreAbsent: true});
+    yield OS.File.remove(absent_file_name);
+  } catch (ex) {
+    exception = ex;
+  }
+  Assert.ok(!exception, "OS.File.remove should not throw when not requested.");
+});
diff --git a/toolkit/components/osfile/tests/xpcshell/test_removeDir.js b/toolkit/components/osfile/tests/xpcshell/test_removeDir.js
new file mode 100644
index 000000000..41ad0eb8c
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_removeDir.js
@@ -0,0 +1,177 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+Components.utils.import("resource://gre/modules/osfile.jsm");
+Components.utils.import("resource://gre/modules/Task.jsm");
+Components.utils.import("resource://gre/modules/Services.jsm");
+
+do_register_cleanup(function() {
+  Services.prefs.setBoolPref("toolkit.osfile.log", false);
+});
+
+function run_test() {
+  Services.prefs.setBoolPref("toolkit.osfile.log", true);
+
+  run_next_test();
+}
+
+add_task(function() {
+  // Set up profile. We create the directory in the profile, because the profile
+  // is removed after every test run.
+  do_get_profile();
+
+  let file = OS.Path.join(OS.Constants.Path.profileDir, "file");
+  let dir = OS.Path.join(OS.Constants.Path.profileDir, "directory");
+  let file1 = OS.Path.join(dir, "file1");
+  let file2 = OS.Path.join(dir, "file2");
+  let subDir = OS.Path.join(dir, "subdir");
+  let fileInSubDir = OS.Path.join(subDir, "file");
+
+  // Sanity checking for the test
+  do_check_false((yield OS.File.exists(dir)));
+
+  // Remove non-existent directory
+  let exception = null;
+  try {
+    yield OS.File.removeDir(dir, {ignoreAbsent: false});
+  } catch (ex) {
+    exception = ex;
+  }
+
+  do_check_true(!!exception);
+  do_check_true(exception instanceof OS.File.Error);
+
+  // Remove non-existent directory with ignoreAbsent
+  yield OS.File.removeDir(dir, {ignoreAbsent: true});
+  yield OS.File.removeDir(dir);
+
+  // Remove file with ignoreAbsent: false
+  yield OS.File.writeAtomic(file, "content", { tmpPath: file + ".tmp" });
+  exception = null;
+  try {
+    yield OS.File.removeDir(file, {ignoreAbsent: false});
+  } catch (ex) {
+    exception = ex;
+  }
+
+  do_check_true(!!exception);
+  do_check_true(exception instanceof OS.File.Error);
+
+  // Remove empty directory
+  yield OS.File.makeDir(dir);
+  yield OS.File.removeDir(dir);
+  do_check_false((yield OS.File.exists(dir)));
+
+  // Remove directory that contains one file
+  yield OS.File.makeDir(dir);
+  yield OS.File.writeAtomic(file1, "content", { tmpPath: file1 + ".tmp" });
+  yield OS.File.removeDir(dir);
+  do_check_false((yield OS.File.exists(dir)));
+
+  // Remove directory that contains multiple files
+  yield OS.File.makeDir(dir);
+  yield OS.File.writeAtomic(file1, "content", { tmpPath: file1 + ".tmp" });
+  yield OS.File.writeAtomic(file2, "content", { tmpPath: file2 + ".tmp" });
+  yield OS.File.removeDir(dir);
+  do_check_false((yield OS.File.exists(dir)));
+
+  // Remove directory that contains a file and a directory
+  yield OS.File.makeDir(dir);
+  yield OS.File.writeAtomic(file1, "content", { tmpPath: file1 + ".tmp" });
+  yield OS.File.makeDir(subDir);
+  yield OS.File.writeAtomic(fileInSubDir, "content", { tmpPath: fileInSubDir + ".tmp" });
+  yield OS.File.removeDir(dir);
+  do_check_false((yield OS.File.exists(dir)));
+});
+
+add_task(function* test_unix_symlink() {
+  // Windows does not implement OS.File.unixSymLink()
+  if (OS.Constants.Win) {
+    return;
+  }
+
+  // Android / B2G file systems typically don't support symlinks.
+  if (OS.Constants.Sys.Name == "Android") {
+    return;
+  }
+
+  let file = OS.Path.join(OS.Constants.Path.profileDir, "file");
+  let dir = OS.Path.join(OS.Constants.Path.profileDir, "directory");
+  let file1 = OS.Path.join(dir, "file1");
+
+  // This test will create the following directory structure:
+  // <profileDir>/file                             (regular file)
+  // <profileDir>/file.link => file                (symlink)
+  // <profileDir>/directory                        (directory)
+  // <profileDir>/linkdir => directory             (directory)
+  // <profileDir>/directory/file1                  (regular file)
+  // <profileDir>/directory3                       (directory)
+  // <profileDir>/directory3/file3                 (directory)
+  // <profileDir>/directory/link2 => ../directory3 (regular file)
+
+  // Sanity checking for the test
+  do_check_false((yield OS.File.exists(dir)));
+
+  yield OS.File.writeAtomic(file, "content", { tmpPath: file + ".tmp" });
+  do_check_true((yield OS.File.exists(file)));
+  let info = yield OS.File.stat(file, {unixNoFollowingLinks: true});
+  do_check_false(info.isDir);
+  do_check_false(info.isSymLink);
+
+  yield OS.File.unixSymLink(file, file + ".link");
+  do_check_true((yield OS.File.exists(file + ".link")));
+  info = yield OS.File.stat(file + ".link", {unixNoFollowingLinks: true});
+  do_check_false(info.isDir);
+  do_check_true(info.isSymLink);
+  info = yield OS.File.stat(file + ".link");
+  do_check_false(info.isDir);
+  do_check_false(info.isSymLink);
+  yield OS.File.remove(file + ".link");
+  do_check_false((yield OS.File.exists(file + ".link")));
+
+  yield OS.File.makeDir(dir);
+  do_check_true((yield OS.File.exists(dir)));
+  info = yield OS.File.stat(dir, {unixNoFollowingLinks: true});
+  do_check_true(info.isDir);
+  do_check_false(info.isSymLink);
+
+  let link = OS.Path.join(OS.Constants.Path.profileDir, "linkdir");
+
+  yield OS.File.unixSymLink(dir, link);
+  do_check_true((yield OS.File.exists(link)));
+  info = yield OS.File.stat(link, {unixNoFollowingLinks: true});
+  do_check_false(info.isDir);
+  do_check_true(info.isSymLink);
+  info = yield OS.File.stat(link);
+  do_check_true(info.isDir);
+  do_check_false(info.isSymLink);
+
+  let dir3 = OS.Path.join(OS.Constants.Path.profileDir, "directory3");
+  let file3 = OS.Path.join(dir3, "file3");
+  let link2 = OS.Path.join(dir, "link2");
+
+  yield OS.File.writeAtomic(file1, "content", { tmpPath: file1 + ".tmp" });
+  do_check_true((yield OS.File.exists(file1)));
+  yield OS.File.makeDir(dir3);
+  do_check_true((yield OS.File.exists(dir3)));
+  yield OS.File.writeAtomic(file3, "content", { tmpPath: file3 + ".tmp" });
+  do_check_true((yield OS.File.exists(file3)));
+  yield OS.File.unixSymLink("../directory3", link2);
+  do_check_true((yield OS.File.exists(link2)));
+
+  yield OS.File.removeDir(link);
+  do_check_false((yield OS.File.exists(link)));
+  do_check_true((yield OS.File.exists(file1)));
+  yield OS.File.removeDir(dir);
+  do_check_false((yield OS.File.exists(dir)));
+  do_check_true((yield OS.File.exists(file3)));
+  yield OS.File.removeDir(dir3);
+  do_check_false((yield OS.File.exists(dir3)));
+
+  // This task will be executed only on Unix-like systems.
+  // Please do not add tests independent to operating systems here
+  // or implement symlink() on Windows.
+});
diff --git a/toolkit/components/osfile/tests/xpcshell/test_removeEmptyDir.js b/toolkit/components/osfile/tests/xpcshell/test_removeEmptyDir.js
new file mode 100644
index 000000000..95f8d5cd1
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_removeEmptyDir.js
@@ -0,0 +1,55 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+Components.utils.import("resource://gre/modules/osfile.jsm");
+Components.utils.import("resource://gre/modules/Services.jsm");
+
+do_register_cleanup(function() {
+  Services.prefs.setBoolPref("toolkit.osfile.log", false);
+});
+
+function run_test() {
+  Services.prefs.setBoolPref("toolkit.osfile.log", true);
+
+  run_next_test();
+}
+
+/**
+ * Test OS.File.removeEmptyDir
+ */
+add_task(function() {
+  // Set up profile. We create the directory in the profile, because the profile
+  // is removed after every test run.
+  do_get_profile();
+
+  let dir = OS.Path.join(OS.Constants.Path.profileDir, "directory");
+
+  // Sanity checking for the test
+  do_check_false((yield OS.File.exists(dir)));
+
+  // Remove non-existent directory
+  yield OS.File.removeEmptyDir(dir);
+
+  // Remove non-existent directory with ignoreAbsent
+  yield OS.File.removeEmptyDir(dir, {ignoreAbsent: true});
+
+  // Remove non-existent directory with ignoreAbsent false
+  let exception = null;
+  try {
+    yield OS.File.removeEmptyDir(dir, {ignoreAbsent: false});
+  } catch (ex) {
+    exception = ex;
+  }
+
+  do_check_true(!!exception);
+  do_check_true(exception instanceof OS.File.Error);
+  do_check_true(exception.becauseNoSuchFile);
+
+  // Remove empty directory
+  yield OS.File.makeDir(dir);
+  yield OS.File.removeEmptyDir(dir);
+  do_check_false((yield OS.File.exists(dir)));
+});
diff --git a/toolkit/components/osfile/tests/xpcshell/test_reset.js b/toolkit/components/osfile/tests/xpcshell/test_reset.js
new file mode 100644
index 000000000..f1e1b14d1
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_reset.js
@@ -0,0 +1,95 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+var Path = OS.Constants.Path;
+
+add_task(function* init() {
+  do_get_profile();
+});
+
+add_task(function* reset_before_launching() {
+  do_print("Reset without launching OS.File, it shouldn't break");
+  yield OS.File.resetWorker();
+});
+
+add_task(function* transparent_reset() {
+  for (let i = 1; i < 3; ++i) {
+    do_print("Do stome stuff before and after " + i + " reset(s), " +
+             "it shouldn't break");
+    let CONTENT = "some content " + i;
+    let path = OS.Path.join(Path.profileDir, "tmp");
+    yield OS.File.writeAtomic(path, CONTENT);
+    for (let j = 0; j < i; ++j) {
+      yield OS.File.resetWorker();
+    }
+    let data = yield OS.File.read(path);
+    let string = (new TextDecoder()).decode(data);
+    do_check_eq(string, CONTENT);
+  }
+});
+
+add_task(function* file_open_cannot_reset() {
+  let TEST_FILE = OS.Path.join(Path.profileDir, "tmp-" + Math.random());
+  do_print("Leaking file descriptor " + TEST_FILE + ", we shouldn't be able to reset");
+  let openedFile = yield OS.File.open(TEST_FILE, { create: true} );
+  let thrown = false;
+  try {
+    yield OS.File.resetWorker();
+  } catch (ex if ex.message.indexOf(OS.Path.basename(TEST_FILE)) != -1 ) {
+    thrown = true;
+  }
+  do_check_true(thrown);
+
+  do_print("Closing the file, we should now be able to reset");
+  yield openedFile.close();
+  yield OS.File.resetWorker();
+});
+
+add_task(function* dir_open_cannot_reset() {
+  let TEST_DIR = yield OS.File.getCurrentDirectory();
+  do_print("Leaking directory " + TEST_DIR + ", we shouldn't be able to reset");
+  let iterator = new OS.File.DirectoryIterator(TEST_DIR);
+  let thrown = false;
+  try {
+    yield OS.File.resetWorker();
+  } catch (ex if ex.message.indexOf(OS.Path.basename(TEST_DIR)) != -1 ) {
+    thrown = true;
+  }
+  do_check_true(thrown);
+
+  do_print("Closing the directory, we should now be able to reset");
+  yield iterator.close();
+  yield OS.File.resetWorker();
+});
+
+add_task(function* race_against_itself() {
+  do_print("Attempt to get resetWorker() to race against itself");
+  // Arbitrary operation, just to wake up the worker
+  try {
+    yield OS.File.read("/foo");
+  } catch (ex) {
+  }
+
+  let all = [];
+  for (let i = 0; i < 100; ++i) {
+    all.push(OS.File.resetWorker());
+  }
+
+  yield Promise.all(all);
+});
+
+
+add_task(function* finish_with_a_reset() {
+  do_print("Reset without waiting for the result");
+  // Arbitrary operation, just to wake up the worker
+  try {
+    yield OS.File.read("/foo");
+  } catch (ex) {
+  }
+  // Now reset
+  /*don't yield*/ OS.File.resetWorker();
+});
+
+function run_test() {
+  run_next_test();
+}
diff --git a/toolkit/components/osfile/tests/xpcshell/test_shutdown.js b/toolkit/components/osfile/tests/xpcshell/test_shutdown.js
new file mode 100644
index 000000000..667965d9e
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_shutdown.js
@@ -0,0 +1,98 @@
+Components.utils.import("resource://gre/modules/Services.jsm", this);
+Components.utils.import("resource://gre/modules/Promise.jsm", this);
+Components.utils.import("resource://gre/modules/Task.jsm", this);
+Components.utils.import("resource://gre/modules/osfile.jsm", this);
+
+add_task(function init() {
+  do_get_profile();
+});
+
+/**
+ * Test logging of file descriptors leaks.
+ */
+add_task(function system_shutdown() {
+
+  // Test that unclosed files cause warnings
+  // Test that unclosed directories cause warnings
+  // Test that closed files do not cause warnings
+  // Test that closed directories do not cause warnings
+  function testLeaksOf(resource, topic) {
+    return Task.spawn(function() {
+      let deferred = Promise.defer();
+
+      // Register observer
+      Services.prefs.setBoolPref("toolkit.asyncshutdown.testing", true);
+      Services.prefs.setBoolPref("toolkit.osfile.log", true);
+      Services.prefs.setBoolPref("toolkit.osfile.log.redirect", true);
+      Services.prefs.setCharPref("toolkit.osfile.test.shutdown.observer", topic);
+
+      let observer = function(aMessage) {
+        try {
+          do_print("Got message: " + aMessage);
+          if (!(aMessage instanceof Components.interfaces.nsIConsoleMessage)) {
+            return;
+          }
+          let message = aMessage.message;
+          do_print("Got message: " + message);
+          if (message.indexOf("TEST OS Controller WARNING") < 0) {
+            return;
+          }
+          do_print("Got message: " + message + ", looking for resource " + resource);
+          if (message.indexOf(resource) < 0) {
+            return;
+          }
+          do_print("Resource: " + resource + " found");
+          do_execute_soon(deferred.resolve);
+        } catch (ex) {
+          do_execute_soon(function() {
+            deferred.reject(ex);
+          });
+        }
+      };
+      Services.console.registerListener(observer);
+      Services.obs.notifyObservers(null, topic, null);
+      do_timeout(1000, function() {
+        do_print("Timeout while waiting for resource: " + resource);
+        deferred.reject("timeout");
+      });
+
+      let resolved = false;
+      try {
+        yield deferred.promise;
+        resolved = true;
+      } catch (ex if ex == "timeout") {
+        resolved = false;
+      }
+      Services.console.unregisterListener(observer);
+      Services.prefs.clearUserPref("toolkit.osfile.log");
+      Services.prefs.clearUserPref("toolkit.osfile.log.redirect");
+      Services.prefs.clearUserPref("toolkit.osfile.test.shutdown.observer");
+      Services.prefs.clearUserPref("toolkit.async_shutdown.testing", true);
+
+      throw new Task.Result(resolved);
+    });
+  }
+
+  let TEST_DIR = OS.Path.join((yield OS.File.getCurrentDirectory()), "..");
+  do_print("Testing for leaks of directory iterator " + TEST_DIR);
+  let iterator = new OS.File.DirectoryIterator(TEST_DIR);
+  do_print("At this stage, we leak the directory");
+  do_check_true((yield testLeaksOf(TEST_DIR, "test.shutdown.dir.leak")));
+  yield iterator.close();
+  do_print("At this stage, we don't leak the directory anymore");
+  do_check_false((yield testLeaksOf(TEST_DIR, "test.shutdown.dir.noleak")));
+
+  let TEST_FILE = OS.Path.join(OS.Constants.Path.profileDir, "test");
+  do_print("Testing for leaks of file descriptor: " + TEST_FILE);
+  let openedFile = yield OS.File.open(TEST_FILE, { create: true} );
+  do_print("At this stage, we leak the file");
+  do_check_true((yield testLeaksOf(TEST_FILE, "test.shutdown.file.leak")));
+  yield openedFile.close();
+  do_print("At this stage, we don't leak the file anymore");
+  do_check_false((yield testLeaksOf(TEST_FILE, "test.shutdown.file.leak.2")));
+});
+
+
+function run_test() {
+  run_next_test();
+}
diff --git a/toolkit/components/osfile/tests/xpcshell/test_telemetry.js b/toolkit/components/osfile/tests/xpcshell/test_telemetry.js
new file mode 100644
index 000000000..dc5104443
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_telemetry.js
@@ -0,0 +1,63 @@
+"use strict";
+
+var {OS: {File, Path, Constants}} = Components.utils.import("resource://gre/modules/osfile.jsm", {});
+var {Services} = Components.utils.import("resource://gre/modules/Services.jsm", {});
+
+// Ensure that we have a profile but that the OS.File worker is not launched
+add_task(function* init() {
+  do_get_profile();
+  yield File.resetWorker();
+});
+
+function getCount(histogram) {
+  if (histogram == null) {
+    return 0;
+  }
+
+  let total = 0;
+  for (let i of histogram.counts) {
+    total += i;
+  }
+  return total;
+}
+
+// Ensure that launching the OS.File worker adds data to the relevant
+// histograms
+add_task(function* test_startup() {
+  let LAUNCH = "OSFILE_WORKER_LAUNCH_MS";
+  let READY = "OSFILE_WORKER_READY_MS";
+
+  let before = Services.telemetry.histogramSnapshots;
+
+  // Launch the OS.File worker
+  yield File.getCurrentDirectory();
+
+  let after = Services.telemetry.histogramSnapshots;
+
+
+  do_print("Ensuring that we have recorded measures for histograms");
+  do_check_eq(getCount(after[LAUNCH]), getCount(before[LAUNCH]) + 1);
+  do_check_eq(getCount(after[READY]), getCount(before[READY]) + 1);
+
+  do_print("Ensuring that launh <= ready");
+  do_check_true(after[LAUNCH].sum <= after[READY].sum);
+});
+
+// Ensure that calling writeAtomic adds data to the relevant histograms
+add_task(function* test_writeAtomic() {
+  let LABEL = "OSFILE_WRITEATOMIC_JANK_MS";
+
+  let before = Services.telemetry.histogramSnapshots;
+
+  // Perform a write.
+  let path = Path.join(Constants.Path.profileDir, "test_osfile_telemetry.tmp");
+  yield File.writeAtomic(path, LABEL, { tmpPath: path + ".tmp" } );
+
+  let after = Services.telemetry.histogramSnapshots;
+
+  do_check_eq(getCount(after[LABEL]), getCount(before[LABEL]) + 1);
+});
+
+function run_test() {
+  run_next_test();
+}
diff --git a/toolkit/components/osfile/tests/xpcshell/test_unique.js b/toolkit/components/osfile/tests/xpcshell/test_unique.js
new file mode 100644
index 000000000..8aa81b803
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_unique.js
@@ -0,0 +1,88 @@
+"use strict";
+
+Components.utils.import("resource://gre/modules/osfile.jsm");
+Components.utils.import("resource://gre/modules/Task.jsm");
+
+function run_test() {
+  do_get_profile();
+  run_next_test();
+}
+
+function testFiles(filename) {
+  return Task.spawn(function() {
+    const MAX_TRIES = 10;
+    let profileDir = OS.Constants.Path.profileDir;
+    let path = OS.Path.join(profileDir, filename);
+
+    // Ensure that openUnique() uses the file name if there is no file with that name already.
+    let openedFile = yield OS.File.openUnique(path);
+    do_print("\nCreate new file: " + openedFile.path);
+    yield openedFile.file.close();
+    let exists = yield OS.File.exists(openedFile.path);
+    do_check_true(exists);
+    do_check_eq(path, openedFile.path);
+    let fileInfo = yield OS.File.stat(openedFile.path);
+    do_check_true(fileInfo.size == 0);
+
+    // Ensure that openUnique() creates a new file name using a HEX number, as the original name is already taken.
+    openedFile = yield OS.File.openUnique(path);
+    do_print("\nCreate unique HEX file: " + openedFile.path);
+    yield openedFile.file.close();
+    exists = yield OS.File.exists(openedFile.path);
+    do_check_true(exists);
+    fileInfo = yield OS.File.stat(openedFile.path);
+    do_check_true(fileInfo.size == 0);
+
+    // Ensure that openUnique() generates different file names each time, using the HEX number algorithm
+    let filenames = new Set();
+    for (let i=0; i < MAX_TRIES; i++) {
+      openedFile = yield OS.File.openUnique(path);
+      yield openedFile.file.close();
+      filenames.add(openedFile.path);
+    }
+
+    do_check_eq(filenames.size, MAX_TRIES);
+
+    // Ensure that openUnique() creates a new human readable file name using, as the original name is already taken.
+    openedFile = yield OS.File.openUnique(path, {humanReadable : true});
+    do_print("\nCreate unique Human Readable file: " + openedFile.path);
+    yield openedFile.file.close();
+    exists = yield OS.File.exists(openedFile.path);
+    do_check_true(exists);
+    fileInfo = yield OS.File.stat(openedFile.path);
+    do_check_true(fileInfo.size == 0);
+
+    // Ensure that openUnique() generates different human readable file names each time
+    filenames = new Set();
+    for (let i=0; i < MAX_TRIES; i++) {
+      openedFile = yield OS.File.openUnique(path, {humanReadable : true});
+      yield openedFile.file.close();
+      filenames.add(openedFile.path);
+    }
+
+    do_check_eq(filenames.size, MAX_TRIES);
+
+    let exn;
+    try {
+      for (let i=0; i < 100; i++) {
+        openedFile = yield OS.File.openUnique(path, {humanReadable : true});
+        yield openedFile.file.close();
+      }
+    } catch (ex) {
+      exn = ex;
+    }
+
+    do_print("Ensure that this raises the correct error");
+    do_check_true(!!exn);
+    do_check_true(exn instanceof OS.File.Error);
+    do_check_true(exn.becauseExists);
+  });
+}
+
+add_task(function test_unique() {
+  OS.Shared.DEBUG = true;
+  // Tests files with extension
+  yield testFiles("dummy_unique_file.txt");
+  // Tests files with no extension
+  yield testFiles("dummy_unique_file_no_ext");
+});
diff --git a/toolkit/components/osfile/tests/xpcshell/xpcshell.ini b/toolkit/components/osfile/tests/xpcshell/xpcshell.ini
new file mode 100644
index 000000000..58b106d3d
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/xpcshell.ini
@@ -0,0 +1,51 @@
+[DEFAULT]
+head = head.js
+tail =
+
+support-files =
+  test_loader/module_test_loader.js
+
+[test_available_free_space.js]
+[test_compression.js]
+[test_constants.js]
+[test_creationDate.js]
+[test_duration.js]
+[test_exception.js]
+[test_file_URL_conversion.js]
+[test_loader.js]
+[test_logging.js]
+[test_makeDir.js]
+[test_open.js]
+[test_osfile_async.js]
+[test_osfile_async_append.js]
+[test_osfile_async_bytes.js]
+[test_osfile_async_copy.js]
+[test_osfile_async_flush.js]
+[test_osfile_async_largefiles.js]
+[test_osfile_async_setDates.js]
+# Unimplemented on Windows (bug 1022816).
+# Spurious failure on Android test farm due to non-POSIX behavior of
+# filesystem backing /mnt/sdcard (not worth trying to fix).
+[test_osfile_async_setPermissions.js]
+skip-if = os == "win" || os == "android"
+[test_osfile_closed.js]
+[test_osfile_error.js]
+[test_osfile_kill.js]
+# Windows test
+[test_osfile_win_async_setPermissions.js]
+skip-if = os != "win"
+[test_osfile_writeAtomic_backupTo_option.js]
+[test_osfile_writeAtomic_zerobytes.js]
+[test_path.js]
+[test_path_constants.js]
+[test_queue.js]
+[test_read_write.js]
+requesttimeoutfactor = 4
+[test_remove.js]
+[test_removeDir.js]
+requesttimeoutfactor = 4
+[test_removeEmptyDir.js]
+[test_reset.js]
+[test_shutdown.js]
+[test_telemetry.js]
+[test_unique.js]