Add m-esr52 at 52.6.0

1 files changed, 916 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
+