Add m-esr52 at 52.6.0

59 files changed, 14655 insertions, 0 deletions

diff --git a/xpcom/threads/AbstractThread.cpp b/xpcom/threads/AbstractThread.cpp
new file mode 100644
index 000000000..451b317d8
--- /dev/null
+++ b/xpcom/threads/AbstractThread.cpp
@@ -0,0 +1,192 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "mozilla/AbstractThread.h"
+
+#include "mozilla/ClearOnShutdown.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/MozPromise.h" // We initialize the MozPromise logging in this file.
+#include "mozilla/StaticPtr.h"
+#include "mozilla/StateWatching.h" // We initialize the StateWatching logging in this file.
+#include "mozilla/TaskQueue.h"
+#include "mozilla/TaskDispatcher.h"
+#include "mozilla/Unused.h"
+
+#include "nsThreadUtils.h"
+#include "nsContentUtils.h"
+#include "nsServiceManagerUtils.h"
+
+
+namespace mozilla {
+
+LazyLogModule gMozPromiseLog("MozPromise");
+LazyLogModule gStateWatchingLog("StateWatching");
+
+StaticRefPtr<AbstractThread> sMainThread;
+MOZ_THREAD_LOCAL(AbstractThread*) AbstractThread::sCurrentThreadTLS;
+
+class XPCOMThreadWrapper : public AbstractThread
+{
+public:
+  explicit XPCOMThreadWrapper(nsIThread* aTarget, bool aRequireTailDispatch)
+    : AbstractThread(aRequireTailDispatch)
+    , mTarget(aTarget)
+  {
+    // Our current mechanism of implementing tail dispatch is appshell-specific.
+    // This is because a very similar mechanism already exists on the main
+    // thread, and we want to avoid making event dispatch on the main thread
+    // more complicated than it already is.
+    //
+    // If you need to use tail dispatch on other XPCOM threads, you'll need to
+    // implement an nsIThreadObserver to fire the tail dispatcher at the
+    // appropriate times.
+    MOZ_ASSERT_IF(aRequireTailDispatch,
+                  NS_IsMainThread() && NS_GetCurrentThread() == aTarget);
+  }
+
+  virtual void Dispatch(already_AddRefed<nsIRunnable> aRunnable,
+                        DispatchFailureHandling aFailureHandling = AssertDispatchSuccess,
+                        DispatchReason aReason = NormalDispatch) override
+  {
+    nsCOMPtr<nsIRunnable> r = aRunnable;
+    AbstractThread* currentThread;
+    if (aReason != TailDispatch && (currentThread = GetCurrent()) && RequiresTailDispatch(currentThread)) {
+      currentThread->TailDispatcher().AddTask(this, r.forget(), aFailureHandling);
+      return;
+    }
+
+    nsresult rv = mTarget->Dispatch(r, NS_DISPATCH_NORMAL);
+    MOZ_DIAGNOSTIC_ASSERT(aFailureHandling == DontAssertDispatchSuccess || NS_SUCCEEDED(rv));
+    Unused << rv;
+  }
+
+  virtual bool IsCurrentThreadIn() override
+  {
+    // Compare NSPR threads so that this works after shutdown when
+    // NS_GetCurrentThread starts returning null.
+    PRThread* thread = nullptr;
+    mTarget->GetPRThread(&thread);
+    bool in = PR_GetCurrentThread() == thread;
+    return in;
+  }
+
+  void FireTailDispatcher()
+  {
+    MOZ_DIAGNOSTIC_ASSERT(mTailDispatcher.isSome());
+    mTailDispatcher.ref().DrainDirectTasks();
+    mTailDispatcher.reset();
+  }
+
+  virtual TaskDispatcher& TailDispatcher() override
+  {
+    MOZ_ASSERT(this == sMainThread); // See the comment in the constructor.
+    MOZ_ASSERT(IsCurrentThreadIn());
+    if (!mTailDispatcher.isSome()) {
+      mTailDispatcher.emplace(/* aIsTailDispatcher = */ true);
+
+      nsCOMPtr<nsIRunnable> event = NewRunnableMethod(this, &XPCOMThreadWrapper::FireTailDispatcher);
+      nsContentUtils::RunInStableState(event.forget());
+    }
+
+    return mTailDispatcher.ref();
+  }
+
+  virtual bool MightHaveTailTasks() override
+  {
+    return mTailDispatcher.isSome();
+  }
+
+  virtual nsIThread* AsXPCOMThread() override { return mTarget; }
+
+private:
+  RefPtr<nsIThread> mTarget;
+  Maybe<AutoTaskDispatcher> mTailDispatcher;
+};
+
+void
+AbstractThread::TailDispatchTasksFor(AbstractThread* aThread)
+{
+  if (MightHaveTailTasks()) {
+    TailDispatcher().DispatchTasksFor(aThread);
+  }
+}
+
+bool
+AbstractThread::HasTailTasksFor(AbstractThread* aThread)
+{
+  if (!MightHaveTailTasks()) {
+    return false;
+  }
+  return TailDispatcher().HasTasksFor(aThread);
+}
+
+bool
+AbstractThread::RequiresTailDispatch(AbstractThread* aThread) const
+{
+  MOZ_ASSERT(aThread);
+  // We require tail dispatch if both the source and destination
+  // threads support it.
+  return SupportsTailDispatch() && aThread->SupportsTailDispatch();
+}
+
+bool
+AbstractThread::RequiresTailDispatchFromCurrentThread() const
+{
+  AbstractThread* current = GetCurrent();
+  return current && RequiresTailDispatch(current);
+}
+
+AbstractThread*
+AbstractThread::MainThread()
+{
+  MOZ_ASSERT(sMainThread);
+  return sMainThread;
+}
+
+void
+AbstractThread::InitStatics()
+{
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(!sMainThread);
+  nsCOMPtr<nsIThread> mainThread;
+  NS_GetMainThread(getter_AddRefs(mainThread));
+  MOZ_DIAGNOSTIC_ASSERT(mainThread);
+  sMainThread = new XPCOMThreadWrapper(mainThread.get(), /* aRequireTailDispatch = */ true);
+  ClearOnShutdown(&sMainThread);
+
+  if (!sCurrentThreadTLS.init()) {
+    MOZ_CRASH();
+  }
+  sCurrentThreadTLS.set(sMainThread);
+}
+
+void
+AbstractThread::DispatchStateChange(already_AddRefed<nsIRunnable> aRunnable)
+{
+  GetCurrent()->TailDispatcher().AddStateChangeTask(this, Move(aRunnable));
+}
+
+/* static */ void
+AbstractThread::DispatchDirectTask(already_AddRefed<nsIRunnable> aRunnable)
+{
+  GetCurrent()->TailDispatcher().AddDirectTask(Move(aRunnable));
+}
+
+/* static */
+already_AddRefed<AbstractThread>
+AbstractThread::CreateXPCOMThreadWrapper(nsIThread* aThread, bool aRequireTailDispatch)
+{
+  RefPtr<XPCOMThreadWrapper> wrapper = new XPCOMThreadWrapper(aThread, aRequireTailDispatch);
+  // Set the thread-local sCurrentThreadTLS to point to the wrapper on the
+  // target thread. This ensures that sCurrentThreadTLS is as expected by
+  // AbstractThread::GetCurrent() on the target thread.
+  nsCOMPtr<nsIRunnable> r =
+    NS_NewRunnableFunction([wrapper]() { sCurrentThreadTLS.set(wrapper); });
+  aThread->Dispatch(r.forget(), NS_DISPATCH_NORMAL);
+  return wrapper.forget();
+}
+
+} // namespace mozilla
diff --git a/xpcom/threads/AbstractThread.h b/xpcom/threads/AbstractThread.h
new file mode 100644
index 000000000..ca6ec1b84
--- /dev/null
+++ b/xpcom/threads/AbstractThread.h
@@ -0,0 +1,111 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 !defined(AbstractThread_h_)
+#define AbstractThread_h_
+
+#include "nscore.h"
+#include "nsIRunnable.h"
+#include "nsISupportsImpl.h"
+#include "nsIThread.h"
+#include "mozilla/RefPtr.h"
+
+#include "mozilla/ThreadLocal.h"
+
+namespace mozilla {
+
+class TaskQueue;
+class TaskDispatcher;
+
+/*
+ * We often want to run tasks on a target that guarantees that events will never
+ * run in parallel. There are various target types that achieve this - namely
+ * nsIThread and TaskQueue. Note that nsIThreadPool (which implements
+ * nsIEventTarget) does not have this property, so we do not want to use
+ * nsIEventTarget for this purpose. This class encapsulates the specifics of
+ * the structures we might use here and provides a consistent interface.
+ *
+ * At present, the supported AbstractThread implementations are TaskQueue
+ * and AbstractThread::MainThread. If you add support for another thread that is
+ * not the MainThread, you'll need to figure out how to make it unique such that
+ * comparing AbstractThread pointers is equivalent to comparing nsIThread pointers.
+ */
+class AbstractThread
+{
+public:
+  // Returns the AbstractThread that the caller is currently running in, or null
+  // if the caller is not running in an AbstractThread.
+  static AbstractThread* GetCurrent() { return sCurrentThreadTLS.get(); }
+
+  AbstractThread(bool aSupportsTailDispatch) : mSupportsTailDispatch(aSupportsTailDispatch) {}
+
+  static already_AddRefed<AbstractThread>
+  CreateXPCOMThreadWrapper(nsIThread* aThread, bool aRequireTailDispatch);
+
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(AbstractThread);
+
+  enum DispatchFailureHandling { AssertDispatchSuccess, DontAssertDispatchSuccess };
+  enum DispatchReason { NormalDispatch, TailDispatch };
+  virtual void Dispatch(already_AddRefed<nsIRunnable> aRunnable,
+                        DispatchFailureHandling aHandling = AssertDispatchSuccess,
+                        DispatchReason aReason = NormalDispatch) = 0;
+
+  virtual bool IsCurrentThreadIn() = 0;
+
+  // Returns true if dispatch is generally reliable. This is used to guard
+  // against FlushableTaskQueues, which should go away.
+  virtual bool IsDispatchReliable() { return true; }
+
+  // Returns a TaskDispatcher that will dispatch its tasks when the currently-
+  // running tasks pops off the stack.
+  //
+  // May only be called when running within the it is invoked up, and only on
+  // threads which support it.
+  virtual TaskDispatcher& TailDispatcher() = 0;
+
+  // Returns true if we have tail tasks scheduled, or if this isn't known.
+  // Returns false if we definitely don't have any tail tasks.
+  virtual bool MightHaveTailTasks() { return true; }
+
+  // Helper functions for methods on the tail TasklDispatcher. These check
+  // HasTailTasks to avoid allocating a TailDispatcher if it isn't
+  // needed.
+  void TailDispatchTasksFor(AbstractThread* aThread);
+  bool HasTailTasksFor(AbstractThread* aThread);
+
+  // Returns true if this supports the tail dispatcher.
+  bool SupportsTailDispatch() const { return mSupportsTailDispatch; }
+
+  // Returns true if this thread requires all dispatches originating from
+  // aThread go through the tail dispatcher.
+  bool RequiresTailDispatch(AbstractThread* aThread) const;
+  bool RequiresTailDispatchFromCurrentThread() const;
+
+  virtual TaskQueue* AsTaskQueue() { MOZ_CRASH("Not a task queue!"); }
+  virtual nsIThread* AsXPCOMThread() { MOZ_CRASH("Not an XPCOM thread!"); }
+
+  // Convenience method for getting an AbstractThread for the main thread.
+  static AbstractThread* MainThread();
+
+  // Must be called exactly once during startup.
+  static void InitStatics();
+
+  void DispatchStateChange(already_AddRefed<nsIRunnable> aRunnable);
+
+  static void DispatchDirectTask(already_AddRefed<nsIRunnable> aRunnable);
+
+protected:
+  virtual ~AbstractThread() {}
+  static MOZ_THREAD_LOCAL(AbstractThread*) sCurrentThreadTLS;
+
+  // True if we want to require that every task dispatched from tasks running in
+  // this queue go through our queue's tail dispatcher.
+  const bool mSupportsTailDispatch;
+};
+
+} // namespace mozilla
+
+#endif
diff --git a/xpcom/threads/BackgroundHangMonitor.cpp b/xpcom/threads/BackgroundHangMonitor.cpp
new file mode 100644
index 000000000..ac65d9f37
--- /dev/null
+++ b/xpcom/threads/BackgroundHangMonitor.cpp
@@ -0,0 +1,734 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "mozilla/ArrayUtils.h"
+#include "mozilla/BackgroundHangMonitor.h"
+#include "mozilla/LinkedList.h"
+#include "mozilla/Monitor.h"
+#include "mozilla/Move.h"
+#include "mozilla/Preferences.h"
+#include "mozilla/StaticPtr.h"
+#include "mozilla/Telemetry.h"
+#include "mozilla/ThreadHangStats.h"
+#include "mozilla/ThreadLocal.h"
+
+#include "prinrval.h"
+#include "prthread.h"
+#include "ThreadStackHelper.h"
+#include "nsIObserverService.h"
+#include "nsIObserver.h"
+#include "mozilla/Services.h"
+#include "nsXULAppAPI.h"
+
+#include <algorithm>
+
+// Activate BHR only for one every BHR_BETA_MOD users.
+// This is now 100% of Beta population for the Beta 45/46 e10s A/B trials
+// It can be scaled back again in the future
+#define BHR_BETA_MOD 1;
+
+// Maximum depth of the call stack in the reported thread hangs. This value represents
+// the 99.9th percentile of the thread hangs stack depths reported by Telemetry.
+static const size_t kMaxThreadHangStackDepth = 30;
+
+// An utility comparator function used by std::unique to collapse "(* script)" entries in
+// a vector representing a call stack.
+bool StackScriptEntriesCollapser(const char* aStackEntry, const char *aAnotherStackEntry)
+{
+  return !strcmp(aStackEntry, aAnotherStackEntry) &&
+         (!strcmp(aStackEntry, "(chrome script)") || !strcmp(aStackEntry, "(content script)"));
+}
+
+namespace mozilla {
+
+/**
+ * BackgroundHangManager is the global object that
+ * manages all instances of BackgroundHangThread.
+ */
+class BackgroundHangManager : public nsIObserver
+{
+private:
+  // Background hang monitor thread function
+  static void MonitorThread(void* aData)
+  {
+    PR_SetCurrentThreadName("BgHangManager");
+
+    /* We do not hold a reference to BackgroundHangManager here
+       because the monitor thread only exists as long as the
+       BackgroundHangManager instance exists. We stop the monitor
+       thread in the BackgroundHangManager destructor, and we can
+       only get to the destructor if we don't hold a reference here. */
+    static_cast<BackgroundHangManager*>(aData)->RunMonitorThread();
+  }
+
+  // Hang monitor thread
+  PRThread* mHangMonitorThread;
+  // Stop hang monitoring
+  bool mShutdown;
+
+  BackgroundHangManager(const BackgroundHangManager&);
+  BackgroundHangManager& operator=(const BackgroundHangManager&);
+  void RunMonitorThread();
+
+public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIOBSERVER
+  static StaticRefPtr<BackgroundHangManager> sInstance;
+  static bool sDisabled;
+
+  // Lock for access to members of this class
+  Monitor mLock;
+  // Current time as seen by hang monitors
+  PRIntervalTime mIntervalNow;
+  // List of BackgroundHangThread instances associated with each thread
+  LinkedList<BackgroundHangThread> mHangThreads;
+
+  void Shutdown()
+  {
+    MonitorAutoLock autoLock(mLock);
+    mShutdown = true;
+    autoLock.Notify();
+  }
+
+  void Wakeup()
+  {
+    // PR_CreateThread could have failed earlier
+    if (mHangMonitorThread) {
+      // Use PR_Interrupt to avoid potentially taking a lock
+      PR_Interrupt(mHangMonitorThread);
+    }
+  }
+
+  BackgroundHangManager();
+private:
+  virtual ~BackgroundHangManager();
+};
+
+NS_IMPL_ISUPPORTS(BackgroundHangManager, nsIObserver)
+
+NS_IMETHODIMP
+BackgroundHangManager::Observe(nsISupports* aSubject, const char* aTopic, const char16_t* aData) {
+  NS_ENSURE_TRUE(!strcmp(aTopic, "profile-after-change"), NS_ERROR_UNEXPECTED);
+  BackgroundHangMonitor::DisableOnBeta();
+
+  nsCOMPtr<nsIObserverService> observerService = mozilla::services::GetObserverService();
+  MOZ_ASSERT(observerService);
+  observerService->RemoveObserver(this, "profile-after-change");
+
+  return NS_OK;
+}
+
+/**
+ * BackgroundHangThread is a per-thread object that is used
+ * by all instances of BackgroundHangMonitor to monitor hangs.
+ */
+class BackgroundHangThread : public LinkedListElement<BackgroundHangThread>
+{
+private:
+  static MOZ_THREAD_LOCAL(BackgroundHangThread*) sTlsKey;
+  static bool sTlsKeyInitialized;
+
+  BackgroundHangThread(const BackgroundHangThread&);
+  BackgroundHangThread& operator=(const BackgroundHangThread&);
+  ~BackgroundHangThread();
+
+  /* Keep a reference to the manager, so we can keep going even
+     after BackgroundHangManager::Shutdown is called. */
+  const RefPtr<BackgroundHangManager> mManager;
+  // Unique thread ID for identification
+  const PRThread* mThreadID;
+
+  void Update();
+
+public:
+  NS_INLINE_DECL_REFCOUNTING(BackgroundHangThread)
+  /**
+   * Returns the BackgroundHangThread associated with the
+   * running thread. Note that this will not find private
+   * BackgroundHangThread threads.
+   *
+   * @return BackgroundHangThread*, or nullptr if no thread
+   *         is found.
+   */
+  static BackgroundHangThread* FindThread();
+
+  static void Startup()
+  {
+    /* We can tolerate init() failing. */
+    sTlsKeyInitialized = sTlsKey.init();
+  }
+
+  // Hang timeout in ticks
+  const PRIntervalTime mTimeout;
+  // PermaHang timeout in ticks
+  const PRIntervalTime mMaxTimeout;
+  // Time at last activity
+  PRIntervalTime mInterval;
+  // Time when a hang started
+  PRIntervalTime mHangStart;
+  // Is the thread in a hang
+  bool mHanging;
+  // Is the thread in a waiting state
+  bool mWaiting;
+  // Is the thread dedicated to a single BackgroundHangMonitor
+  BackgroundHangMonitor::ThreadType mThreadType;
+  // Platform-specific helper to get hang stacks
+  ThreadStackHelper mStackHelper;
+  // Stack of current hang
+  Telemetry::HangStack mHangStack;
+  // Statistics for telemetry
+  Telemetry::ThreadHangStats mStats;
+  // Annotations for the current hang
+  UniquePtr<HangMonitor::HangAnnotations> mAnnotations;
+  // Annotators registered for this thread
+  HangMonitor::Observer::Annotators mAnnotators;
+
+  BackgroundHangThread(const char* aName,
+                       uint32_t aTimeoutMs,
+                       uint32_t aMaxTimeoutMs,
+                       BackgroundHangMonitor::ThreadType aThreadType = BackgroundHangMonitor::THREAD_SHARED);
+
+  // Report a hang; aManager->mLock IS locked
+  Telemetry::HangHistogram& ReportHang(PRIntervalTime aHangTime);
+  // Report a permanent hang; aManager->mLock IS locked
+  void ReportPermaHang();
+  // Called by BackgroundHangMonitor::NotifyActivity
+  void NotifyActivity()
+  {
+    MonitorAutoLock autoLock(mManager->mLock);
+    Update();
+  }
+  // Called by BackgroundHangMonitor::NotifyWait
+  void NotifyWait()
+  {
+    MonitorAutoLock autoLock(mManager->mLock);
+
+    if (mWaiting) {
+      return;
+    }
+
+    Update();
+    mWaiting = true;
+  }
+
+  // Returns true if this thread is (or might be) shared between other
+  // BackgroundHangMonitors for the monitored thread.
+  bool IsShared() {
+    return mThreadType == BackgroundHangMonitor::THREAD_SHARED;
+  }
+};
+
+
+StaticRefPtr<BackgroundHangManager> BackgroundHangManager::sInstance;
+bool BackgroundHangManager::sDisabled = false;
+
+MOZ_THREAD_LOCAL(BackgroundHangThread*) BackgroundHangThread::sTlsKey;
+bool BackgroundHangThread::sTlsKeyInitialized;
+
+BackgroundHangManager::BackgroundHangManager()
+  : mShutdown(false)
+  , mLock("BackgroundHangManager")
+  , mIntervalNow(0)
+{
+  // Lock so we don't race against the new monitor thread
+  MonitorAutoLock autoLock(mLock);
+  mHangMonitorThread = PR_CreateThread(
+    PR_USER_THREAD, MonitorThread, this,
+    PR_PRIORITY_LOW, PR_GLOBAL_THREAD, PR_JOINABLE_THREAD, 0);
+
+  MOZ_ASSERT(mHangMonitorThread, "Failed to create monitor thread");
+}
+
+BackgroundHangManager::~BackgroundHangManager()
+{
+  MOZ_ASSERT(mShutdown, "Destruction without Shutdown call");
+  MOZ_ASSERT(mHangThreads.isEmpty(), "Destruction with outstanding monitors");
+  MOZ_ASSERT(mHangMonitorThread, "No monitor thread");
+
+  // PR_CreateThread could have failed above due to resource limitation
+  if (mHangMonitorThread) {
+    // The monitor thread can only live as long as the instance lives
+    PR_JoinThread(mHangMonitorThread);
+  }
+}
+
+void
+BackgroundHangManager::RunMonitorThread()
+{
+  // Keep us locked except when waiting
+  MonitorAutoLock autoLock(mLock);
+
+  /* mIntervalNow is updated at various intervals determined by waitTime.
+     However, if an update latency is too long (due to CPU scheduling, system
+     sleep, etc.), we don't update mIntervalNow at all. This is done so that
+     long latencies in our timing are not detected as hangs. systemTime is
+     used to track PR_IntervalNow() and determine our latency. */
+
+  PRIntervalTime systemTime = PR_IntervalNow();
+  // Default values for the first iteration of thread loop
+  PRIntervalTime waitTime = PR_INTERVAL_NO_WAIT;
+  PRIntervalTime recheckTimeout = PR_INTERVAL_NO_WAIT;
+
+  while (!mShutdown) {
+
+    PR_ClearInterrupt();
+    nsresult rv = autoLock.Wait(waitTime);
+
+    PRIntervalTime newTime = PR_IntervalNow();
+    PRIntervalTime systemInterval = newTime - systemTime;
+    systemTime = newTime;
+
+    /* waitTime is a quarter of the shortest timeout value; If our timing
+       latency is low enough (less than half the shortest timeout value),
+       we can update mIntervalNow. */
+    if (MOZ_LIKELY(waitTime != PR_INTERVAL_NO_TIMEOUT &&
+                   systemInterval < 2 * waitTime)) {
+      mIntervalNow += systemInterval;
+    }
+
+    /* If it's before the next recheck timeout, and our wait did not
+       get interrupted (either through Notify or PR_Interrupt), we can
+       keep the current waitTime and skip iterating through hang monitors. */
+    if (MOZ_LIKELY(systemInterval < recheckTimeout &&
+                   systemInterval >= waitTime &&
+                   rv == NS_OK)) {
+      recheckTimeout -= systemInterval;
+      continue;
+    }
+
+    /* We are in one of the following scenarios,
+     - Hang or permahang recheck timeout
+     - Thread added/removed
+     - Thread wait or hang ended
+       In all cases, we want to go through our list of hang
+       monitors and update waitTime and recheckTimeout. */
+    waitTime = PR_INTERVAL_NO_TIMEOUT;
+    recheckTimeout = PR_INTERVAL_NO_TIMEOUT;
+
+    // Locally hold mIntervalNow
+    PRIntervalTime intervalNow = mIntervalNow;
+
+    // iterate through hang monitors
+    for (BackgroundHangThread* currentThread = mHangThreads.getFirst();
+         currentThread; currentThread = currentThread->getNext()) {
+
+      if (currentThread->mWaiting) {
+        // Thread is waiting, not hanging
+        continue;
+      }
+      PRIntervalTime interval = currentThread->mInterval;
+      PRIntervalTime hangTime = intervalNow - interval;
+      if (MOZ_UNLIKELY(hangTime >= currentThread->mMaxTimeout)) {
+        // A permahang started
+        // Skip subsequent iterations and tolerate a race on mWaiting here
+        currentThread->mWaiting = true;
+        currentThread->mHanging = false;
+        currentThread->ReportPermaHang();
+        continue;
+      }
+
+      if (MOZ_LIKELY(!currentThread->mHanging)) {
+        if (MOZ_UNLIKELY(hangTime >= currentThread->mTimeout)) {
+          // A hang started
+          currentThread->mStackHelper.GetStack(currentThread->mHangStack);
+          currentThread->mHangStart = interval;
+          currentThread->mHanging = true;
+          currentThread->mAnnotations =
+            currentThread->mAnnotators.GatherAnnotations();
+        }
+      } else {
+        if (MOZ_LIKELY(interval != currentThread->mHangStart)) {
+          // A hang ended
+          currentThread->ReportHang(intervalNow - currentThread->mHangStart);
+          currentThread->mHanging = false;
+        }
+      }
+
+      /* If we are hanging, the next time we check for hang status is when
+         the hang turns into a permahang. If we're not hanging, the next
+         recheck timeout is when we may be entering a hang. */
+      PRIntervalTime nextRecheck;
+      if (currentThread->mHanging) {
+        nextRecheck = currentThread->mMaxTimeout;
+      } else {
+        nextRecheck = currentThread->mTimeout;
+      }
+      recheckTimeout = std::min(recheckTimeout, nextRecheck - hangTime);
+
+      if (currentThread->mTimeout != PR_INTERVAL_NO_TIMEOUT) {
+        /* We wait for a quarter of the shortest timeout
+           value to give mIntervalNow enough granularity. */
+        waitTime = std::min(waitTime, currentThread->mTimeout / 4);
+      }
+    }
+  }
+
+  /* We are shutting down now.
+     Wait for all outstanding monitors to unregister. */
+  while (!mHangThreads.isEmpty()) {
+    autoLock.Wait(PR_INTERVAL_NO_TIMEOUT);
+  }
+}
+
+
+BackgroundHangThread::BackgroundHangThread(const char* aName,
+                                           uint32_t aTimeoutMs,
+                                           uint32_t aMaxTimeoutMs,
+                                           BackgroundHangMonitor::ThreadType aThreadType)
+  : mManager(BackgroundHangManager::sInstance)
+  , mThreadID(PR_GetCurrentThread())
+  , mTimeout(aTimeoutMs == BackgroundHangMonitor::kNoTimeout
+             ? PR_INTERVAL_NO_TIMEOUT
+             : PR_MillisecondsToInterval(aTimeoutMs))
+  , mMaxTimeout(aMaxTimeoutMs == BackgroundHangMonitor::kNoTimeout
+                ? PR_INTERVAL_NO_TIMEOUT
+                : PR_MillisecondsToInterval(aMaxTimeoutMs))
+  , mInterval(mManager->mIntervalNow)
+  , mHangStart(mInterval)
+  , mHanging(false)
+  , mWaiting(true)
+  , mThreadType(aThreadType)
+  , mStats(aName)
+{
+  if (sTlsKeyInitialized && IsShared()) {
+    sTlsKey.set(this);
+  }
+  // Lock here because LinkedList is not thread-safe
+  MonitorAutoLock autoLock(mManager->mLock);
+  // Add to thread list
+  mManager->mHangThreads.insertBack(this);
+  // Wake up monitor thread to process new thread
+  autoLock.Notify();
+}
+
+BackgroundHangThread::~BackgroundHangThread()
+{
+  // Lock here because LinkedList is not thread-safe
+  MonitorAutoLock autoLock(mManager->mLock);
+  // Remove from thread list
+  remove();
+  // Wake up monitor thread to process removed thread
+  autoLock.Notify();
+
+  // We no longer have a thread
+  if (sTlsKeyInitialized && IsShared()) {
+    sTlsKey.set(nullptr);
+  }
+
+  // Move our copy of ThreadHangStats to Telemetry storage
+  Telemetry::RecordThreadHangStats(mStats);
+}
+
+Telemetry::HangHistogram&
+BackgroundHangThread::ReportHang(PRIntervalTime aHangTime)
+{
+  // Recovered from a hang; called on the monitor thread
+  // mManager->mLock IS locked
+
+  // Remove unwanted "js::RunScript" frame from the stack
+  for (size_t i = 0; i < mHangStack.length(); ) {
+    const char** f = mHangStack.begin() + i;
+    if (!mHangStack.IsInBuffer(*f) && !strcmp(*f, "js::RunScript")) {
+      mHangStack.erase(f);
+    } else {
+      i++;
+    }
+  }
+
+  // Collapse duplicated "(chrome script)" and "(content script)" entries in the stack.
+  auto it = std::unique(mHangStack.begin(), mHangStack.end(), StackScriptEntriesCollapser);
+  mHangStack.erase(it, mHangStack.end());
+
+  // Limit the depth of the reported stack if greater than our limit. Only keep its
+  // last entries, since the most recent frames are at the end of the vector.
+  if (mHangStack.length() > kMaxThreadHangStackDepth) {
+    const int elementsToRemove = mHangStack.length() - kMaxThreadHangStackDepth;
+    // Replace the oldest frame with a known label so that we can tell this stack
+    // was limited.
+    mHangStack[0] = "(reduced stack)";
+    mHangStack.erase(mHangStack.begin() + 1, mHangStack.begin() + elementsToRemove);
+  }
+
+  Telemetry::HangHistogram newHistogram(Move(mHangStack));
+  for (Telemetry::HangHistogram* oldHistogram = mStats.mHangs.begin();
+       oldHistogram != mStats.mHangs.end(); oldHistogram++) {
+    if (newHistogram == *oldHistogram) {
+      // New histogram matches old one
+      oldHistogram->Add(aHangTime, Move(mAnnotations));
+      return *oldHistogram;
+    }
+  }
+  // Add new histogram
+  newHistogram.Add(aHangTime, Move(mAnnotations));
+  if (!mStats.mHangs.append(Move(newHistogram))) {
+    MOZ_CRASH();
+  }
+  return mStats.mHangs.back();
+}
+
+void
+BackgroundHangThread::ReportPermaHang()
+{
+  // Permanently hanged; called on the monitor thread
+  // mManager->mLock IS locked
+
+  Telemetry::HangHistogram& hang = ReportHang(mMaxTimeout);
+  Telemetry::HangStack& stack = hang.GetNativeStack();
+  if (stack.empty()) {
+    mStackHelper.GetNativeStack(stack);
+  }
+}
+
+MOZ_ALWAYS_INLINE void
+BackgroundHangThread::Update()
+{
+  PRIntervalTime intervalNow = mManager->mIntervalNow;
+  if (mWaiting) {
+    mInterval = intervalNow;
+    mWaiting = false;
+    /* We have to wake up the manager thread because when all threads
+       are waiting, the manager thread waits indefinitely as well. */
+    mManager->Wakeup();
+  } else {
+    PRIntervalTime duration = intervalNow - mInterval;
+    mStats.mActivity.Add(duration);
+    if (MOZ_UNLIKELY(duration >= mTimeout)) {
+      /* Wake up the manager thread to tell it that a hang ended */
+      mManager->Wakeup();
+    }
+    mInterval = intervalNow;
+  }
+}
+
+BackgroundHangThread*
+BackgroundHangThread::FindThread()
+{
+#ifdef MOZ_ENABLE_BACKGROUND_HANG_MONITOR
+  if (BackgroundHangManager::sInstance == nullptr) {
+    MOZ_ASSERT(BackgroundHangManager::sDisabled,
+               "BackgroundHandleManager is not initialized");
+    return nullptr;
+  }
+
+  if (sTlsKeyInitialized) {
+    // Use TLS if available
+    return sTlsKey.get();
+  }
+  // If TLS is unavailable, we can search through the thread list
+  RefPtr<BackgroundHangManager> manager(BackgroundHangManager::sInstance);
+  MOZ_ASSERT(manager, "Creating BackgroundHangMonitor after shutdown");
+
+  PRThread* threadID = PR_GetCurrentThread();
+  // Lock thread list for traversal
+  MonitorAutoLock autoLock(manager->mLock);
+  for (BackgroundHangThread* thread = manager->mHangThreads.getFirst();
+       thread; thread = thread->getNext()) {
+    if (thread->mThreadID == threadID && thread->IsShared()) {
+      return thread;
+    }
+  }
+#endif
+  // Current thread is not initialized
+  return nullptr;
+}
+
+bool
+BackgroundHangMonitor::ShouldDisableOnBeta(const nsCString &clientID) {
+  MOZ_ASSERT(clientID.Length() == 36, "clientID is invalid");
+  const char *suffix = clientID.get() + clientID.Length() - 4;
+  return strtol(suffix, NULL, 16) % BHR_BETA_MOD;
+}
+
+bool
+BackgroundHangMonitor::IsDisabled() {
+#ifdef MOZ_ENABLE_BACKGROUND_HANG_MONITOR
+  return BackgroundHangManager::sDisabled;
+#else
+  return true;
+#endif
+}
+
+bool
+BackgroundHangMonitor::DisableOnBeta() {
+  nsAdoptingCString clientID = Preferences::GetCString("toolkit.telemetry.cachedClientID");
+  bool telemetryEnabled = Preferences::GetBool("toolkit.telemetry.enabled");
+
+  if (!telemetryEnabled || !clientID || BackgroundHangMonitor::ShouldDisableOnBeta(clientID)) {
+    if (XRE_IsParentProcess()) {
+      BackgroundHangMonitor::Shutdown();
+    } else {
+      BackgroundHangManager::sDisabled = true;
+    }
+    return true;
+  }
+
+  return false;
+}
+
+void
+BackgroundHangMonitor::Startup()
+{
+#ifdef MOZ_ENABLE_BACKGROUND_HANG_MONITOR
+  MOZ_ASSERT(!BackgroundHangManager::sInstance, "Already initialized");
+
+  if (!strcmp(NS_STRINGIFY(MOZ_UPDATE_CHANNEL), "beta")) {
+    if (XRE_IsParentProcess()) { // cached ClientID hasn't been read yet
+      ThreadStackHelper::Startup();
+      BackgroundHangThread::Startup();
+      BackgroundHangManager::sInstance = new BackgroundHangManager();
+
+      nsCOMPtr<nsIObserverService> observerService = mozilla::services::GetObserverService();
+      MOZ_ASSERT(observerService);
+
+      observerService->AddObserver(BackgroundHangManager::sInstance, "profile-after-change", false);
+      return;
+    } else if(DisableOnBeta()){
+      return;
+    }
+  }
+
+  ThreadStackHelper::Startup();
+  BackgroundHangThread::Startup();
+  BackgroundHangManager::sInstance = new BackgroundHangManager();
+#endif
+}
+
+void
+BackgroundHangMonitor::Shutdown()
+{
+#ifdef MOZ_ENABLE_BACKGROUND_HANG_MONITOR
+  if (BackgroundHangManager::sDisabled) {
+    MOZ_ASSERT(!BackgroundHangManager::sInstance, "Initialized");
+    return;
+  }
+
+  MOZ_ASSERT(BackgroundHangManager::sInstance, "Not initialized");
+  /* Scope our lock inside Shutdown() because the sInstance object can
+     be destroyed as soon as we set sInstance to nullptr below, and
+     we don't want to hold the lock when it's being destroyed. */
+  BackgroundHangManager::sInstance->Shutdown();
+  BackgroundHangManager::sInstance = nullptr;
+  ThreadStackHelper::Shutdown();
+  BackgroundHangManager::sDisabled = true;
+#endif
+}
+
+BackgroundHangMonitor::BackgroundHangMonitor(const char* aName,
+                                             uint32_t aTimeoutMs,
+                                             uint32_t aMaxTimeoutMs,
+                                             ThreadType aThreadType)
+  : mThread(aThreadType == THREAD_SHARED ? BackgroundHangThread::FindThread() : nullptr)
+{
+#ifdef MOZ_ENABLE_BACKGROUND_HANG_MONITOR
+  if (!BackgroundHangManager::sDisabled && !mThread) {
+    mThread = new BackgroundHangThread(aName, aTimeoutMs, aMaxTimeoutMs,
+                                       aThreadType);
+  }
+#endif
+}
+
+BackgroundHangMonitor::BackgroundHangMonitor()
+  : mThread(BackgroundHangThread::FindThread())
+{
+#ifdef MOZ_ENABLE_BACKGROUND_HANG_MONITOR
+  if (BackgroundHangManager::sDisabled) {
+    return;
+  }
+#endif
+}
+
+BackgroundHangMonitor::~BackgroundHangMonitor()
+{
+}
+
+void
+BackgroundHangMonitor::NotifyActivity()
+{
+#ifdef MOZ_ENABLE_BACKGROUND_HANG_MONITOR
+  if (mThread == nullptr) {
+    MOZ_ASSERT(BackgroundHangManager::sDisabled,
+               "This thread is not initialized for hang monitoring");
+    return;
+  }
+
+  if (Telemetry::CanRecordExtended()) {
+    mThread->NotifyActivity();
+  }
+#endif
+}
+
+void
+BackgroundHangMonitor::NotifyWait()
+{
+#ifdef MOZ_ENABLE_BACKGROUND_HANG_MONITOR
+  if (mThread == nullptr) {
+    MOZ_ASSERT(BackgroundHangManager::sDisabled,
+               "This thread is not initialized for hang monitoring");
+    return;
+  }
+
+  if (Telemetry::CanRecordExtended()) {
+    mThread->NotifyWait();
+  }
+#endif
+}
+
+bool
+BackgroundHangMonitor::RegisterAnnotator(HangMonitor::Annotator& aAnnotator)
+{
+#ifdef MOZ_ENABLE_BACKGROUND_HANG_MONITOR
+  BackgroundHangThread* thisThread = BackgroundHangThread::FindThread();
+  if (!thisThread) {
+    return false;
+  }
+  return thisThread->mAnnotators.Register(aAnnotator);
+#else
+  return false;
+#endif
+}
+
+bool
+BackgroundHangMonitor::UnregisterAnnotator(HangMonitor::Annotator& aAnnotator)
+{
+#ifdef MOZ_ENABLE_BACKGROUND_HANG_MONITOR
+  BackgroundHangThread* thisThread = BackgroundHangThread::FindThread();
+  if (!thisThread) {
+    return false;
+  }
+  return thisThread->mAnnotators.Unregister(aAnnotator);
+#else
+  return false;
+#endif
+}
+
+/* Because we are iterating through the BackgroundHangThread linked list,
+   we need to take a lock. Using MonitorAutoLock as a base class makes
+   sure all of that is taken care of for us. */
+BackgroundHangMonitor::ThreadHangStatsIterator::ThreadHangStatsIterator()
+  : MonitorAutoLock(BackgroundHangManager::sInstance->mLock)
+  , mThread(BackgroundHangManager::sInstance ?
+            BackgroundHangManager::sInstance->mHangThreads.getFirst() :
+            nullptr)
+{
+#ifdef MOZ_ENABLE_BACKGROUND_HANG_MONITOR
+  MOZ_ASSERT(BackgroundHangManager::sInstance ||
+             BackgroundHangManager::sDisabled,
+             "Inconsistent state");
+#endif
+}
+
+Telemetry::ThreadHangStats*
+BackgroundHangMonitor::ThreadHangStatsIterator::GetNext()
+{
+  if (!mThread) {
+    return nullptr;
+  }
+  Telemetry::ThreadHangStats* stats = &mThread->mStats;
+  mThread = mThread->getNext();
+  return stats;
+}
+
+} // namespace mozilla
diff --git a/xpcom/threads/BackgroundHangMonitor.h b/xpcom/threads/BackgroundHangMonitor.h
new file mode 100644
index 000000000..698cf2305
--- /dev/null
+++ b/xpcom/threads/BackgroundHangMonitor.h
@@ -0,0 +1,246 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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_BackgroundHangMonitor_h
+#define mozilla_BackgroundHangMonitor_h
+
+#include "mozilla/HangAnnotations.h"
+#include "mozilla/Monitor.h"
+#include "mozilla/RefPtr.h"
+
+#include "nsString.h"
+
+#include <stdint.h>
+
+namespace mozilla {
+
+namespace Telemetry {
+class ThreadHangStats;
+} // namespace Telemetry
+
+class BackgroundHangThread;
+class BackgroundHangManager;
+
+/**
+ * The background hang monitor is responsible for detecting and reporting
+ * hangs in main and background threads. A thread registers itself using
+ * the BackgroundHangMonitor object and periodically calls its methods to
+ * inform the hang monitor of the thread's activity. Each thread is given
+ * a thread name, a timeout, and a maximum timeout. If one of the thread's
+ * tasks runs for longer than the timeout duration but shorter than the
+ * maximum timeout, a (transient) hang is reported. On the other hand, if
+ * a task runs for longer than the maximum timeout duration or never
+ * finishes (e.g. in a deadlock), a permahang is reported.
+ *
+ * Tasks are defined arbitrarily, but are typically represented by events
+ * in an event loop -- processing one event is equivalent to running one
+ * task. To ensure responsiveness, tasks in a thread often have a target
+ * running time. This is a good starting point for determining the timeout
+ * and maximum timeout values. For example, the Compositor thread has a
+ * responsiveness goal of 60Hz or 17ms, so a starting timeout could be
+ * 100ms. Considering some platforms (e.g. Android) can terminate the app
+ * when a critical thread hangs for longer than a few seconds, a good
+ * starting maximum timeout is 4 or 5 seconds.
+ *
+ * A thread registers itself through the BackgroundHangMonitor constructor.
+ * Multiple BackgroundHangMonitor objects can be used in one thread. The
+ * constructor without arguments can be used when it is known that the thread
+ * already has a BackgroundHangMonitor registered. When all instances of
+ * BackgroundHangMonitor are destroyed, the thread is unregistered.
+ *
+ * The thread then uses two methods to inform BackgroundHangMonitor of the
+ * thread's activity:
+ *
+ *  > BackgroundHangMonitor::NotifyActivity should be called *before*
+ *    starting a task. The task run time is determined by the interval
+ *    between this call and the next NotifyActivity call.
+ *
+ *  > BackgroundHangMonitor::NotifyWait should be called *before* the
+ *    thread enters a wait state (e.g. to wait for a new event). This
+ *    prevents a waiting thread from being detected as hanging. The wait
+ *    state is automatically cleared at the next NotifyActivity call.
+ *
+ * The following example shows hang monitoring in a simple event loop:
+ *
+ *  void thread_main()
+ *  {
+ *    mozilla::BackgroundHangMonitor hangMonitor("example1", 100, 1000);
+ *    while (!exiting) {
+ *      hangMonitor.NotifyActivity();
+ *      process_next_event();
+ *      hangMonitor.NotifyWait();
+ *      wait_for_next_event();
+ *    }
+ *  }
+ *
+ * The following example shows reentrancy in nested event loops:
+ *
+ *  void thread_main()
+ *  {
+ *    mozilla::BackgroundHangMonitor hangMonitor("example2", 100, 1000);
+ *    while (!exiting) {
+ *      hangMonitor.NotifyActivity();
+ *      process_next_event();
+ *      hangMonitor.NotifyWait();
+ *      wait_for_next_event();
+ *    }
+ *  }
+ *
+ *  void process_next_event()
+ *  {
+ *    mozilla::BackgroundHangMonitor hangMonitor();
+ *    if (is_sync_event) {
+ *      while (!finished_event) {
+ *        hangMonitor.NotifyActivity();
+ *        process_next_event();
+ *        hangMonitor.NotifyWait();
+ *        wait_for_next_event();
+ *      }
+ *    } else {
+ *      process_nonsync_event();
+ *    }
+ *  }
+ */
+class BackgroundHangMonitor
+{
+private:
+  friend BackgroundHangManager;
+
+  RefPtr<BackgroundHangThread> mThread;
+
+  static bool ShouldDisableOnBeta(const nsCString &);
+  static bool DisableOnBeta();
+
+public:
+  static const uint32_t kNoTimeout = 0;
+  enum ThreadType {
+    // For a new BackgroundHangMonitor for thread T, only create a new
+    // monitoring thread for T if one doesn't already exist. If one does,
+    // share that pre-existing monitoring thread.
+    THREAD_SHARED,
+    // For a new BackgroundHangMonitor for thread T, create a new
+    // monitoring thread for T even if there are other, pre-existing
+    // monitoring threads for T.
+    THREAD_PRIVATE
+  };
+
+  /**
+   * ThreadHangStatsIterator is used to iterate through the ThreadHangStats
+   * associated with each active monitored thread. Because of an internal
+   * lock while this object is alive, a thread must use only one instance
+   * of this class at a time and must iterate through the list as fast as
+   * possible. The following example shows using the iterator:
+   *
+   * {
+   *   // Scope the iter variable so it's destroyed as soon as we're done
+   *   BackgroundHangMonitor::ThreadHangStatsIterator iter;
+   *   for (ThreadHangStats* histogram = iter.GetNext();
+   *        histogram; histogram = iter.GetNext()) {
+   *     // Process histogram
+   *   }
+   * }
+   */
+  class ThreadHangStatsIterator : public MonitorAutoLock
+  {
+  private:
+    BackgroundHangThread* mThread;
+
+    ThreadHangStatsIterator(const ThreadHangStatsIterator&);
+    ThreadHangStatsIterator& operator=(const ThreadHangStatsIterator&);
+
+  public:
+    /**
+     * Create an ThreadHangStatsIterator instance and take the internal lock.
+     * Internal lock is released on destruction.
+     */
+    ThreadHangStatsIterator();
+
+    /**
+     * Get the next item in the list; the first call returns the first item.
+     * Returns nullptr at the end of the list.
+     */
+    Telemetry::ThreadHangStats* GetNext();
+  };
+
+  /**
+   * Enable hang monitoring.
+   * Must return before using BackgroundHangMonitor.
+   */
+  static void Startup();
+
+  /**
+   * Disable hang monitoring.
+   * Can be called without destroying all BackgroundHangMonitors first.
+   */
+  static void Shutdown();
+
+  /**
+   * Returns true if BHR is disabled.
+   */
+  static bool IsDisabled();
+
+  /**
+   * Start monitoring hangs for the current thread.
+   *
+   * @param aName Name to identify the thread with
+   * @param aTimeoutMs Amount of time in milliseconds without
+   *  activity before registering a hang
+   * @param aMaxTimeoutMs Amount of time in milliseconds without
+   *  activity before registering a permanent hang
+   * @param aThreadType
+   *  The ThreadType type of monitoring thread that should be created
+   *  for this monitor. See the documentation for ThreadType.
+   */
+  BackgroundHangMonitor(const char* aName,
+                        uint32_t aTimeoutMs,
+                        uint32_t aMaxTimeoutMs,
+                        ThreadType aThreadType = THREAD_SHARED);
+
+  /**
+   * Monitor hangs using an existing monitor
+   * associated with the current thread.
+   */
+  BackgroundHangMonitor();
+
+  /**
+   * Destroys the hang monitor; hang monitoring for a thread stops
+   * when all monitors associated with the thread are destroyed.
+   */
+  ~BackgroundHangMonitor();
+
+  /**
+   * Notify the hang monitor of pending current thread activity.
+   * Call this method before starting an "activity" or after
+   * exiting from a wait state.
+   */
+  void NotifyActivity();
+
+  /**
+   * Notify the hang monitor of current thread wait.
+   * Call this method before entering a wait state; call
+   * NotifyActivity when subsequently exiting the wait state.
+   */
+  void NotifyWait();
+
+  /**
+   * Register an annotator with BHR for the current thread.
+   * @param aAnnotator annotator to register
+   * @return true if the annotator was registered, otherwise false.
+   */
+  static bool RegisterAnnotator(HangMonitor::Annotator& aAnnotator);
+
+  /**
+   * Unregister an annotator that was previously registered via
+   * RegisterAnnotator.
+   * @param aAnnotator annotator to unregister
+   * @return true if there are still remaining annotators registered
+   */
+  static bool UnregisterAnnotator(HangMonitor::Annotator& aAnnotator);
+};
+
+} // namespace mozilla
+
+#endif // mozilla_BackgroundHangMonitor_h
diff --git a/xpcom/threads/HangAnnotations.cpp b/xpcom/threads/HangAnnotations.cpp
new file mode 100644
index 000000000..529b57b8e
--- /dev/null
+++ b/xpcom/threads/HangAnnotations.cpp
@@ -0,0 +1,262 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "mozilla/HangAnnotations.h"
+
+#include <vector>
+
+#include "MainThreadUtils.h"
+#include "mozilla/DebugOnly.h"
+#include "nsXULAppAPI.h"
+
+namespace mozilla {
+namespace HangMonitor {
+
+// Chrome hang annotators. This can go away once BHR has completely replaced
+// ChromeHangs.
+static StaticAutoPtr<Observer::Annotators> gChromehangAnnotators;
+
+class BrowserHangAnnotations : public HangAnnotations
+{
+public:
+  BrowserHangAnnotations();
+  ~BrowserHangAnnotations();
+
+  void AddAnnotation(const nsAString& aName, const int32_t aData) override;
+  void AddAnnotation(const nsAString& aName, const double aData) override;
+  void AddAnnotation(const nsAString& aName, const nsAString& aData) override;
+  void AddAnnotation(const nsAString& aName, const nsACString& aData) override;
+  void AddAnnotation(const nsAString& aName, const bool aData) override;
+
+  size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const override;
+  bool IsEmpty() const override;
+  UniquePtr<Enumerator> GetEnumerator() override;
+
+  typedef std::pair<nsString, nsString> AnnotationType;
+  typedef std::vector<AnnotationType> VectorType;
+  typedef VectorType::const_iterator IteratorType;
+
+private:
+  VectorType  mAnnotations;
+};
+
+BrowserHangAnnotations::BrowserHangAnnotations()
+{
+  MOZ_COUNT_CTOR(BrowserHangAnnotations);
+}
+
+BrowserHangAnnotations::~BrowserHangAnnotations()
+{
+  MOZ_COUNT_DTOR(BrowserHangAnnotations);
+}
+
+void
+BrowserHangAnnotations::AddAnnotation(const nsAString& aName, const int32_t aData)
+{
+  nsString dataString;
+  dataString.AppendInt(aData);
+  AnnotationType annotation = std::make_pair(nsString(aName), dataString);
+  mAnnotations.push_back(annotation);
+}
+
+void
+BrowserHangAnnotations::AddAnnotation(const nsAString& aName, const double aData)
+{
+  nsString dataString;
+  dataString.AppendFloat(aData);
+  AnnotationType annotation = std::make_pair(nsString(aName), dataString);
+  mAnnotations.push_back(annotation);
+}
+
+void
+BrowserHangAnnotations::AddAnnotation(const nsAString& aName, const nsAString& aData)
+{
+  AnnotationType annotation = std::make_pair(nsString(aName), nsString(aData));
+  mAnnotations.push_back(annotation);
+}
+
+void
+BrowserHangAnnotations::AddAnnotation(const nsAString& aName, const nsACString& aData)
+{
+  nsString dataString;
+  AppendUTF8toUTF16(aData, dataString);
+  AnnotationType annotation = std::make_pair(nsString(aName), dataString);
+  mAnnotations.push_back(annotation);
+}
+
+void
+BrowserHangAnnotations::AddAnnotation(const nsAString& aName, const bool aData)
+{
+  nsString dataString;
+  dataString += aData ? NS_LITERAL_STRING("true") : NS_LITERAL_STRING("false");
+  AnnotationType annotation = std::make_pair(nsString(aName), dataString);
+  mAnnotations.push_back(annotation);
+}
+
+/**
+ * This class itself does not use synchronization but it (and its parent object)
+ * should be protected by mutual exclusion in some way. In Telemetry the chrome
+ * hang data is protected via TelemetryImpl::mHangReportsMutex.
+ */
+class ChromeHangAnnotationEnumerator : public HangAnnotations::Enumerator
+{
+public:
+  explicit ChromeHangAnnotationEnumerator(const BrowserHangAnnotations::VectorType& aAnnotations);
+  ~ChromeHangAnnotationEnumerator();
+
+  virtual bool Next(nsAString& aOutName, nsAString& aOutValue);
+
+private:
+  BrowserHangAnnotations::IteratorType mIterator;
+  BrowserHangAnnotations::IteratorType mEnd;
+};
+
+ChromeHangAnnotationEnumerator::ChromeHangAnnotationEnumerator(
+                          const BrowserHangAnnotations::VectorType& aAnnotations)
+  : mIterator(aAnnotations.begin())
+  , mEnd(aAnnotations.end())
+{
+  MOZ_COUNT_CTOR(ChromeHangAnnotationEnumerator);
+}
+
+ChromeHangAnnotationEnumerator::~ChromeHangAnnotationEnumerator()
+{
+  MOZ_COUNT_DTOR(ChromeHangAnnotationEnumerator);
+}
+
+bool
+ChromeHangAnnotationEnumerator::Next(nsAString& aOutName, nsAString& aOutValue)
+{
+  aOutName.Truncate();
+  aOutValue.Truncate();
+  if (mIterator == mEnd) {
+    return false;
+  }
+  aOutName = mIterator->first;
+  aOutValue = mIterator->second;
+  ++mIterator;
+  return true;
+}
+
+bool
+BrowserHangAnnotations::IsEmpty() const
+{
+  return mAnnotations.empty();
+}
+
+size_t
+BrowserHangAnnotations::SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const
+{
+  size_t result = sizeof(mAnnotations) +
+                  mAnnotations.capacity() * sizeof(AnnotationType);
+  for (IteratorType i = mAnnotations.begin(), e = mAnnotations.end(); i != e;
+       ++i) {
+    result += i->first.SizeOfExcludingThisIfUnshared(aMallocSizeOf);
+    result += i->second.SizeOfExcludingThisIfUnshared(aMallocSizeOf);
+  }
+
+  return result;
+}
+
+UniquePtr<HangAnnotations::Enumerator>
+BrowserHangAnnotations::GetEnumerator()
+{
+  if (mAnnotations.empty()) {
+    return nullptr;
+  }
+  return MakeUnique<ChromeHangAnnotationEnumerator>(mAnnotations);
+}
+
+namespace Observer {
+
+Annotators::Annotators()
+  : mMutex("HangMonitor::Annotators::mMutex")
+{
+  MOZ_COUNT_CTOR(Annotators);
+}
+
+Annotators::~Annotators()
+{
+  MOZ_ASSERT(mAnnotators.empty());
+  MOZ_COUNT_DTOR(Annotators);
+}
+
+bool
+Annotators::Register(Annotator& aAnnotator)
+{
+  MutexAutoLock lock(mMutex);
+  auto result = mAnnotators.insert(&aAnnotator);
+  return result.second;
+}
+
+bool
+Annotators::Unregister(Annotator& aAnnotator)
+{
+  MutexAutoLock lock(mMutex);
+  DebugOnly<std::set<Annotator*>::size_type> numErased;
+  numErased = mAnnotators.erase(&aAnnotator);
+  MOZ_ASSERT(numErased == 1);
+  return mAnnotators.empty();
+}
+
+UniquePtr<HangAnnotations>
+Annotators::GatherAnnotations()
+{
+  auto annotations = MakeUnique<BrowserHangAnnotations>();
+  { // Scope for lock
+    MutexAutoLock lock(mMutex);
+    for (std::set<Annotator*>::iterator i = mAnnotators.begin(),
+                                        e = mAnnotators.end();
+         i != e; ++i) {
+      (*i)->AnnotateHang(*annotations);
+    }
+  }
+  if (annotations->IsEmpty()) {
+    return nullptr;
+  }
+  return Move(annotations);
+}
+
+} // namespace Observer
+
+void
+RegisterAnnotator(Annotator& aAnnotator)
+{
+  BackgroundHangMonitor::RegisterAnnotator(aAnnotator);
+  // We still register annotators for ChromeHangs
+  if (NS_IsMainThread() &&
+      GeckoProcessType_Default == XRE_GetProcessType()) {
+    if (!gChromehangAnnotators) {
+      gChromehangAnnotators = new Observer::Annotators();
+    }
+    gChromehangAnnotators->Register(aAnnotator);
+  }
+}
+
+void
+UnregisterAnnotator(Annotator& aAnnotator)
+{
+  BackgroundHangMonitor::UnregisterAnnotator(aAnnotator);
+  // We still register annotators for ChromeHangs
+  if (NS_IsMainThread() &&
+      GeckoProcessType_Default == XRE_GetProcessType()) {
+    if (gChromehangAnnotators->Unregister(aAnnotator)) {
+      gChromehangAnnotators = nullptr;
+    }
+  }
+}
+
+UniquePtr<HangAnnotations>
+ChromeHangAnnotatorCallout()
+{
+  if (!gChromehangAnnotators) {
+    return nullptr;
+  }
+  return gChromehangAnnotators->GatherAnnotations();
+}
+
+} // namespace HangMonitor
+} // namespace mozilla
diff --git a/xpcom/threads/HangAnnotations.h b/xpcom/threads/HangAnnotations.h
new file mode 100644
index 000000000..6dddbf4bb
--- /dev/null
+++ b/xpcom/threads/HangAnnotations.h
@@ -0,0 +1,104 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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_HangAnnotations_h
+#define mozilla_HangAnnotations_h
+
+#include <set>
+
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/UniquePtr.h"
+#include "mozilla/Vector.h"
+#include "nsString.h"
+
+namespace mozilla {
+namespace HangMonitor {
+
+/**
+ * This class declares an abstraction for a data type that encapsulates all
+ * of the annotations being reported by a registered hang Annotator.
+ */
+class HangAnnotations
+{
+public:
+  virtual ~HangAnnotations() {}
+
+  virtual void AddAnnotation(const nsAString& aName, const int32_t aData) = 0;
+  virtual void AddAnnotation(const nsAString& aName, const double aData) = 0;
+  virtual void AddAnnotation(const nsAString& aName, const nsAString& aData) = 0;
+  virtual void AddAnnotation(const nsAString& aName, const nsACString& aData) = 0;
+  virtual void AddAnnotation(const nsAString& aName, const bool aData) = 0;
+
+  class Enumerator
+  {
+  public:
+    virtual ~Enumerator() {}
+    virtual bool Next(nsAString& aOutName, nsAString& aOutValue) = 0;
+  };
+
+  virtual size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const = 0;
+  virtual bool IsEmpty() const = 0;
+  virtual UniquePtr<Enumerator> GetEnumerator() = 0;
+};
+
+typedef UniquePtr<HangAnnotations> HangAnnotationsPtr;
+typedef Vector<HangAnnotationsPtr> HangAnnotationsVector;
+
+class Annotator
+{
+public:
+  /**
+   * NB: This function is always called by the HangMonitor thread.
+   *     Plan accordingly.
+   */
+  virtual void AnnotateHang(HangAnnotations& aAnnotations) = 0;
+};
+
+/**
+ * Registers an Annotator to be called when a hang is detected.
+ * @param aAnnotator Reference to an object that implements the
+ * HangMonitor::Annotator interface.
+ */
+void RegisterAnnotator(Annotator& aAnnotator);
+
+/**
+ * Registers an Annotator that was previously registered via RegisterAnnotator.
+ * @param aAnnotator Reference to an object that implements the
+ * HangMonitor::Annotator interface.
+ */
+void UnregisterAnnotator(Annotator& aAnnotator);
+
+/**
+ * Gathers annotations. This function should be called by ChromeHangs.
+ * @return UniquePtr to HangAnnotations object or nullptr if none.
+ */
+HangAnnotationsPtr ChromeHangAnnotatorCallout();
+
+namespace Observer {
+
+class Annotators
+{
+public:
+  Annotators();
+  ~Annotators();
+
+  bool Register(Annotator& aAnnotator);
+  bool Unregister(Annotator& aAnnotator);
+
+  HangAnnotationsPtr GatherAnnotations();
+
+private:
+  Mutex                mMutex;
+  std::set<Annotator*> mAnnotators;
+};
+
+} // namespace Observer
+
+} // namespace HangMonitor
+} // namespace mozilla
+
+#endif // mozilla_HangAnnotations_h
diff --git a/xpcom/threads/HangMonitor.cpp b/xpcom/threads/HangMonitor.cpp
new file mode 100644
index 000000000..71cc67ca4
--- /dev/null
+++ b/xpcom/threads/HangMonitor.cpp
@@ -0,0 +1,434 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "mozilla/HangMonitor.h"
+
+#include "mozilla/Atomics.h"
+#include "mozilla/BackgroundHangMonitor.h"
+#include "mozilla/Monitor.h"
+#include "mozilla/Preferences.h"
+#include "mozilla/ProcessedStack.h"
+#include "mozilla/Telemetry.h"
+#include "mozilla/StaticPtr.h"
+#include "mozilla/UniquePtr.h"
+#include "nsReadableUtils.h"
+#include "mozilla/StackWalk.h"
+#ifdef _WIN64
+#include "mozilla/StackWalk_windows.h"
+#endif
+#include "nsThreadUtils.h"
+#include "nsXULAppAPI.h"
+
+#ifdef MOZ_CRASHREPORTER
+#include "nsExceptionHandler.h"
+#endif
+
+#ifdef XP_WIN
+#include <windows.h>
+#endif
+
+#if defined(MOZ_ENABLE_PROFILER_SPS) && defined(MOZ_PROFILING) && defined(XP_WIN)
+  #define REPORT_CHROME_HANGS
+#endif
+
+namespace mozilla {
+namespace HangMonitor {
+
+/**
+ * A flag which may be set from within a debugger to disable the hang
+ * monitor.
+ */
+volatile bool gDebugDisableHangMonitor = false;
+
+const char kHangMonitorPrefName[] = "hangmonitor.timeout";
+
+#ifdef REPORT_CHROME_HANGS
+const char kTelemetryPrefName[] = "toolkit.telemetry.enabled";
+#endif
+
+// Monitor protects gShutdown and gTimeout, but not gTimestamp which rely on
+// being atomically set by the processor; synchronization doesn't really matter
+// in this use case.
+Monitor* gMonitor;
+
+// The timeout preference, in seconds.
+int32_t gTimeout;
+
+PRThread* gThread;
+
+// Set when shutdown begins to signal the thread to exit immediately.
+bool gShutdown;
+
+// The timestamp of the last event notification, or PR_INTERVAL_NO_WAIT if
+// we're currently not processing events.
+Atomic<PRIntervalTime> gTimestamp(PR_INTERVAL_NO_WAIT);
+
+#ifdef REPORT_CHROME_HANGS
+// Main thread ID used in reporting chrome hangs under Windows
+static HANDLE winMainThreadHandle = nullptr;
+
+// Default timeout for reporting chrome hangs to Telemetry (5 seconds)
+static const int32_t DEFAULT_CHROME_HANG_INTERVAL = 5;
+
+// Maximum number of PCs to gather from the stack
+static const int32_t MAX_CALL_STACK_PCS = 400;
+#endif
+
+// PrefChangedFunc
+void
+PrefChanged(const char*, void*)
+{
+  int32_t newval = Preferences::GetInt(kHangMonitorPrefName);
+#ifdef REPORT_CHROME_HANGS
+  // Monitor chrome hangs on the profiling branch if Telemetry enabled
+  if (newval == 0) {
+    bool telemetryEnabled = Preferences::GetBool(kTelemetryPrefName);
+    if (telemetryEnabled) {
+      newval = DEFAULT_CHROME_HANG_INTERVAL;
+    }
+  }
+#endif
+  MonitorAutoLock lock(*gMonitor);
+  if (newval != gTimeout) {
+    gTimeout = newval;
+    lock.Notify();
+  }
+}
+
+void
+Crash()
+{
+  if (gDebugDisableHangMonitor) {
+    return;
+  }
+
+#ifdef XP_WIN
+  if (::IsDebuggerPresent()) {
+    return;
+  }
+#endif
+
+#ifdef MOZ_CRASHREPORTER
+  // If you change this, you must also deal with the threadsafety of AnnotateCrashReport in
+  // non-chrome processes!
+  if (GeckoProcessType_Default == XRE_GetProcessType()) {
+    CrashReporter::AnnotateCrashReport(NS_LITERAL_CSTRING("Hang"),
+                                       NS_LITERAL_CSTRING("1"));
+  }
+#endif
+
+  NS_RUNTIMEABORT("HangMonitor triggered");
+}
+
+#ifdef REPORT_CHROME_HANGS
+
+static void
+ChromeStackWalker(uint32_t aFrameNumber, void* aPC, void* aSP, void* aClosure)
+{
+  MOZ_ASSERT(aClosure);
+  std::vector<uintptr_t>* stack =
+    static_cast<std::vector<uintptr_t>*>(aClosure);
+  if (stack->size() == MAX_CALL_STACK_PCS) {
+    return;
+  }
+  MOZ_ASSERT(stack->size() < MAX_CALL_STACK_PCS);
+  stack->push_back(reinterpret_cast<uintptr_t>(aPC));
+}
+
+static void
+GetChromeHangReport(Telemetry::ProcessedStack& aStack,
+                    int32_t& aSystemUptime,
+                    int32_t& aFirefoxUptime)
+{
+  MOZ_ASSERT(winMainThreadHandle);
+
+  // The thread we're about to suspend might have the alloc lock
+  // so allocate ahead of time
+  std::vector<uintptr_t> rawStack;
+  rawStack.reserve(MAX_CALL_STACK_PCS);
+
+  // Workaround possible deadlock where the main thread is running a
+  // long-standing JS job, and happens to be in the JIT allocator when we
+  // suspend it. Since, on win 64, this requires holding a process lock that
+  // MozStackWalk requires, take this "workaround lock" to avoid deadlock.
+#ifdef _WIN64
+  AcquireStackWalkWorkaroundLock();
+#endif
+  DWORD ret = ::SuspendThread(winMainThreadHandle);
+  bool suspended = false;
+  if (ret != -1) {
+    // SuspendThread is asynchronous, so the thread may still be running. Use
+    // GetThreadContext to ensure it's really suspended.
+    // See https://blogs.msdn.microsoft.com/oldnewthing/20150205-00/?p=44743.
+    CONTEXT context;
+    context.ContextFlags = CONTEXT_CONTROL;
+    if (::GetThreadContext(winMainThreadHandle, &context)) {
+      suspended = true;
+    }
+  }
+
+#ifdef _WIN64
+  ReleaseStackWalkWorkaroundLock();
+#endif
+
+  if (!suspended) {
+    if (ret != -1) {
+      MOZ_ALWAYS_TRUE(::ResumeThread(winMainThreadHandle) != DWORD(-1));
+    }
+    return;
+  }
+
+  MozStackWalk(ChromeStackWalker, /* skipFrames */ 0, /* maxFrames */ 0,
+               reinterpret_cast<void*>(&rawStack),
+               reinterpret_cast<uintptr_t>(winMainThreadHandle), nullptr);
+  ret = ::ResumeThread(winMainThreadHandle);
+  if (ret == -1) {
+    return;
+  }
+  aStack = Telemetry::GetStackAndModules(rawStack);
+
+  // Record system uptime (in minutes) at the time of the hang
+  aSystemUptime = ((GetTickCount() / 1000) - (gTimeout * 2)) / 60;
+
+  // Record Firefox uptime (in minutes) at the time of the hang
+  bool error;
+  TimeStamp processCreation = TimeStamp::ProcessCreation(error);
+  if (!error) {
+    TimeDuration td = TimeStamp::Now() - processCreation;
+    aFirefoxUptime = (static_cast<int32_t>(td.ToSeconds()) - (gTimeout * 2)) / 60;
+  } else {
+    aFirefoxUptime = -1;
+  }
+}
+
+#endif
+
+void
+ThreadMain(void*)
+{
+  PR_SetCurrentThreadName("Hang Monitor");
+
+  MonitorAutoLock lock(*gMonitor);
+
+  // In order to avoid issues with the hang monitor incorrectly triggering
+  // during a general system stop such as sleeping, the monitor thread must
+  // run twice to trigger hang protection.
+  PRIntervalTime lastTimestamp = 0;
+  int waitCount = 0;
+
+#ifdef REPORT_CHROME_HANGS
+  Telemetry::ProcessedStack stack;
+  int32_t systemUptime = -1;
+  int32_t firefoxUptime = -1;
+  UniquePtr<HangAnnotations> annotations;
+#endif
+
+  while (true) {
+    if (gShutdown) {
+      return; // Exit the thread
+    }
+
+    // avoid rereading the volatile value in this loop
+    PRIntervalTime timestamp = gTimestamp;
+
+    PRIntervalTime now = PR_IntervalNow();
+
+    if (timestamp != PR_INTERVAL_NO_WAIT &&
+        now < timestamp) {
+      // 32-bit overflow, reset for another waiting period
+      timestamp = 1; // lowest legal PRInterval value
+    }
+
+    if (timestamp != PR_INTERVAL_NO_WAIT &&
+        timestamp == lastTimestamp &&
+        gTimeout > 0) {
+      ++waitCount;
+#ifdef REPORT_CHROME_HANGS
+      // Capture the chrome-hang stack + Firefox & system uptimes after
+      // the minimum hang duration has been reached (not when the hang ends)
+      if (waitCount == 2) {
+        GetChromeHangReport(stack, systemUptime, firefoxUptime);
+        annotations = ChromeHangAnnotatorCallout();
+      }
+#else
+      // This is the crash-on-hang feature.
+      // See bug 867313 for the quirk in the waitCount comparison
+      if (waitCount >= 2) {
+        int32_t delay =
+          int32_t(PR_IntervalToSeconds(now - timestamp));
+        if (delay >= gTimeout) {
+          MonitorAutoUnlock unlock(*gMonitor);
+          Crash();
+        }
+      }
+#endif
+    } else {
+#ifdef REPORT_CHROME_HANGS
+      if (waitCount >= 2) {
+        uint32_t hangDuration = PR_IntervalToSeconds(now - lastTimestamp);
+        Telemetry::RecordChromeHang(hangDuration, stack, systemUptime,
+                                    firefoxUptime, Move(annotations));
+        stack.Clear();
+      }
+#endif
+      lastTimestamp = timestamp;
+      waitCount = 0;
+    }
+
+    PRIntervalTime timeout;
+    if (gTimeout <= 0) {
+      timeout = PR_INTERVAL_NO_TIMEOUT;
+    } else {
+      timeout = PR_MillisecondsToInterval(gTimeout * 500);
+    }
+    lock.Wait(timeout);
+  }
+}
+
+void
+Startup()
+{
+  if (GeckoProcessType_Default != XRE_GetProcessType() &&
+      GeckoProcessType_Content != XRE_GetProcessType()) {
+    return;
+  }
+
+  MOZ_ASSERT(!gMonitor, "Hang monitor already initialized");
+  gMonitor = new Monitor("HangMonitor");
+
+  Preferences::RegisterCallback(PrefChanged, kHangMonitorPrefName, nullptr);
+  PrefChanged(nullptr, nullptr);
+
+#ifdef REPORT_CHROME_HANGS
+  Preferences::RegisterCallback(PrefChanged, kTelemetryPrefName, nullptr);
+  winMainThreadHandle =
+    OpenThread(THREAD_ALL_ACCESS, FALSE, GetCurrentThreadId());
+  if (!winMainThreadHandle) {
+    return;
+  }
+#endif
+
+  // Don't actually start measuring hangs until we hit the main event loop.
+  // This potentially misses a small class of really early startup hangs,
+  // but avoids dealing with some xpcshell tests and other situations which
+  // start XPCOM but don't ever start the event loop.
+  Suspend();
+
+  gThread = PR_CreateThread(PR_USER_THREAD,
+                            ThreadMain,
+                            nullptr, PR_PRIORITY_LOW, PR_GLOBAL_THREAD,
+                            PR_JOINABLE_THREAD, 0);
+}
+
+void
+Shutdown()
+{
+  if (GeckoProcessType_Default != XRE_GetProcessType() &&
+      GeckoProcessType_Content != XRE_GetProcessType()) {
+    return;
+  }
+
+  MOZ_ASSERT(gMonitor, "Hang monitor not started");
+
+  {
+    // Scope the lock we're going to delete later
+    MonitorAutoLock lock(*gMonitor);
+    gShutdown = true;
+    lock.Notify();
+  }
+
+  // thread creation could theoretically fail
+  if (gThread) {
+    PR_JoinThread(gThread);
+    gThread = nullptr;
+  }
+
+  delete gMonitor;
+  gMonitor = nullptr;
+}
+
+static bool
+IsUIMessageWaiting()
+{
+#ifndef XP_WIN
+  return false;
+#else
+#define NS_WM_IMEFIRST WM_IME_SETCONTEXT
+#define NS_WM_IMELAST  WM_IME_KEYUP
+  BOOL haveUIMessageWaiting = FALSE;
+  MSG msg;
+  haveUIMessageWaiting |= ::PeekMessageW(&msg, nullptr, WM_KEYFIRST,
+                                         WM_IME_KEYLAST, PM_NOREMOVE);
+  haveUIMessageWaiting |= ::PeekMessageW(&msg, nullptr, NS_WM_IMEFIRST,
+                                         NS_WM_IMELAST, PM_NOREMOVE);
+  haveUIMessageWaiting |= ::PeekMessageW(&msg, nullptr, WM_MOUSEFIRST,
+                                         WM_MOUSELAST, PM_NOREMOVE);
+  return haveUIMessageWaiting;
+#endif
+}
+
+void
+NotifyActivity(ActivityType aActivityType)
+{
+  MOZ_ASSERT(NS_IsMainThread(),
+             "HangMonitor::Notify called from off the main thread.");
+
+  // Determine the activity type more specifically
+  if (aActivityType == kGeneralActivity) {
+    aActivityType = IsUIMessageWaiting() ? kActivityUIAVail :
+                                           kActivityNoUIAVail;
+  }
+
+  // Calculate the cumulative amount of lag time since the last UI message
+  static uint32_t cumulativeUILagMS = 0;
+  switch (aActivityType) {
+    case kActivityNoUIAVail:
+      cumulativeUILagMS = 0;
+      break;
+    case kActivityUIAVail:
+    case kUIActivity:
+      if (gTimestamp != PR_INTERVAL_NO_WAIT) {
+        cumulativeUILagMS += PR_IntervalToMilliseconds(PR_IntervalNow() -
+                                                       gTimestamp);
+      }
+      break;
+    default:
+      break;
+  }
+
+  // This is not a locked activity because PRTimeStamp is a 32-bit quantity
+  // which can be read/written atomically, and we don't want to pay locking
+  // penalties here.
+  gTimestamp = PR_IntervalNow();
+
+  // If we have UI activity we should reset the timer and report it
+  if (aActivityType == kUIActivity) {
+    mozilla::Telemetry::Accumulate(mozilla::Telemetry::EVENTLOOP_UI_ACTIVITY_EXP_MS,
+                                     cumulativeUILagMS);
+    cumulativeUILagMS = 0;
+  }
+
+  if (gThread && !gShutdown) {
+    mozilla::BackgroundHangMonitor().NotifyActivity();
+  }
+}
+
+void
+Suspend()
+{
+  MOZ_ASSERT(NS_IsMainThread(),
+             "HangMonitor::Suspend called from off the main thread.");
+
+  // Because gTimestamp changes this resets the wait count.
+  gTimestamp = PR_INTERVAL_NO_WAIT;
+
+  if (gThread && !gShutdown) {
+    mozilla::BackgroundHangMonitor().NotifyWait();
+  }
+}
+
+} // namespace HangMonitor
+} // namespace mozilla
diff --git a/xpcom/threads/HangMonitor.h b/xpcom/threads/HangMonitor.h
new file mode 100644
index 000000000..fd0e6ff83
--- /dev/null
+++ b/xpcom/threads/HangMonitor.h
@@ -0,0 +1,58 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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_HangMonitor_h
+#define mozilla_HangMonitor_h
+
+namespace mozilla {
+namespace HangMonitor {
+
+/**
+ * Signifies the type of activity in question
+*/
+enum ActivityType
+{
+  /* There is activity and it is known to be UI related activity. */
+  kUIActivity,
+
+  /* There is non UI activity and no UI activity is pending */
+  kActivityNoUIAVail,
+
+  /* There is non UI activity and UI activity is known to be pending */
+  kActivityUIAVail,
+
+  /* There is non UI activity and UI activity pending is unknown */
+  kGeneralActivity
+};
+
+/**
+ * Start monitoring hangs. Should be called by the XPCOM startup process only.
+ */
+void Startup();
+
+/**
+ * Stop monitoring hangs and join the thread.
+ */
+void Shutdown();
+
+/**
+ * Notify the hang monitor of activity which will reset its internal timer.
+ *
+ * @param activityType The type of activity being reported.
+ * @see ActivityType
+ */
+void NotifyActivity(ActivityType activityType = kGeneralActivity);
+
+/*
+ * Notify the hang monitor that the browser is now idle and no detection should
+ * be done.
+ */
+void Suspend();
+
+} // namespace HangMonitor
+} // namespace mozilla
+
+#endif // mozilla_HangMonitor_h
diff --git a/xpcom/threads/LazyIdleThread.cpp b/xpcom/threads/LazyIdleThread.cpp
new file mode 100644
index 000000000..527cc6819
--- /dev/null
+++ b/xpcom/threads/LazyIdleThread.cpp
@@ -0,0 +1,624 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "LazyIdleThread.h"
+
+#include "nsIObserverService.h"
+
+#include "GeckoProfiler.h"
+#include "nsComponentManagerUtils.h"
+#include "nsIIdlePeriod.h"
+#include "nsServiceManagerUtils.h"
+#include "nsThreadUtils.h"
+#include "mozilla/Services.h"
+
+#ifdef DEBUG
+#define ASSERT_OWNING_THREAD()                                                 \
+  PR_BEGIN_MACRO                                                               \
+    nsIThread* currentThread = NS_GetCurrentThread();                          \
+    if (currentThread) {                                                       \
+      nsCOMPtr<nsISupports> current(do_QueryInterface(currentThread));         \
+      nsCOMPtr<nsISupports> test(do_QueryInterface(mOwningThread));            \
+      MOZ_ASSERT(current == test, "Wrong thread!");                            \
+    }                                                                          \
+  PR_END_MACRO
+#else
+#define ASSERT_OWNING_THREAD() /* nothing */
+#endif
+
+namespace mozilla {
+
+LazyIdleThread::LazyIdleThread(uint32_t aIdleTimeoutMS,
+                               const nsCSubstring& aName,
+                               ShutdownMethod aShutdownMethod,
+                               nsIObserver* aIdleObserver)
+  : mMutex("LazyIdleThread::mMutex")
+  , mOwningThread(NS_GetCurrentThread())
+  , mIdleObserver(aIdleObserver)
+  , mQueuedRunnables(nullptr)
+  , mIdleTimeoutMS(aIdleTimeoutMS)
+  , mPendingEventCount(0)
+  , mIdleNotificationCount(0)
+  , mShutdownMethod(aShutdownMethod)
+  , mShutdown(false)
+  , mThreadIsShuttingDown(false)
+  , mIdleTimeoutEnabled(true)
+  , mName(aName)
+{
+  MOZ_ASSERT(mOwningThread, "Need owning thread!");
+}
+
+LazyIdleThread::~LazyIdleThread()
+{
+  ASSERT_OWNING_THREAD();
+
+  Shutdown();
+}
+
+void
+LazyIdleThread::SetWeakIdleObserver(nsIObserver* aObserver)
+{
+  ASSERT_OWNING_THREAD();
+
+  if (mShutdown) {
+    NS_WARNING_ASSERTION(!aObserver,
+                         "Setting an observer after Shutdown was called!");
+    return;
+  }
+
+  mIdleObserver = aObserver;
+}
+
+void
+LazyIdleThread::DisableIdleTimeout()
+{
+  ASSERT_OWNING_THREAD();
+  if (!mIdleTimeoutEnabled) {
+    return;
+  }
+  mIdleTimeoutEnabled = false;
+
+  if (mIdleTimer && NS_FAILED(mIdleTimer->Cancel())) {
+    NS_WARNING("Failed to cancel timer!");
+  }
+
+  MutexAutoLock lock(mMutex);
+
+  // Pretend we have a pending event to keep the idle timer from firing.
+  MOZ_ASSERT(mPendingEventCount < UINT32_MAX, "Way too many!");
+  mPendingEventCount++;
+}
+
+void
+LazyIdleThread::EnableIdleTimeout()
+{
+  ASSERT_OWNING_THREAD();
+  if (mIdleTimeoutEnabled) {
+    return;
+  }
+  mIdleTimeoutEnabled = true;
+
+  {
+    MutexAutoLock lock(mMutex);
+
+    MOZ_ASSERT(mPendingEventCount, "Mismatched calls to observer methods!");
+    --mPendingEventCount;
+  }
+
+  if (mThread) {
+    nsCOMPtr<nsIRunnable> runnable(new Runnable());
+    if (NS_FAILED(Dispatch(runnable.forget(), NS_DISPATCH_NORMAL))) {
+      NS_WARNING("Failed to dispatch!");
+    }
+  }
+}
+
+void
+LazyIdleThread::PreDispatch()
+{
+  MutexAutoLock lock(mMutex);
+
+  MOZ_ASSERT(mPendingEventCount < UINT32_MAX, "Way too many!");
+  mPendingEventCount++;
+}
+
+nsresult
+LazyIdleThread::EnsureThread()
+{
+  ASSERT_OWNING_THREAD();
+
+  if (mShutdown) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  if (mThread) {
+    return NS_OK;
+  }
+
+  MOZ_ASSERT(!mPendingEventCount, "Shouldn't have events yet!");
+  MOZ_ASSERT(!mIdleNotificationCount, "Shouldn't have idle events yet!");
+  MOZ_ASSERT(!mIdleTimer, "Should have killed this long ago!");
+  MOZ_ASSERT(!mThreadIsShuttingDown, "Should have cleared that!");
+
+  nsresult rv;
+
+  if (mShutdownMethod == AutomaticShutdown && NS_IsMainThread()) {
+    nsCOMPtr<nsIObserverService> obs =
+      do_GetService(NS_OBSERVERSERVICE_CONTRACTID, &rv);
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      return rv;
+    }
+
+    rv = obs->AddObserver(this, "xpcom-shutdown-threads", false);
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      return rv;
+    }
+  }
+
+  mIdleTimer = do_CreateInstance(NS_TIMER_CONTRACTID, &rv);
+  if (NS_WARN_IF(!mIdleTimer)) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  nsCOMPtr<nsIRunnable> runnable =
+    NewRunnableMethod(this, &LazyIdleThread::InitThread);
+  if (NS_WARN_IF(!runnable)) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  rv = NS_NewThread(getter_AddRefs(mThread), runnable);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  return NS_OK;
+}
+
+void
+LazyIdleThread::InitThread()
+{
+  char aLocal;
+  profiler_register_thread(mName.get(), &aLocal);
+
+  PR_SetCurrentThreadName(mName.get());
+
+  // Happens on mThread but mThread may not be set yet...
+
+  nsCOMPtr<nsIThreadInternal> thread(do_QueryInterface(NS_GetCurrentThread()));
+  MOZ_ASSERT(thread, "This should always succeed!");
+
+  if (NS_FAILED(thread->SetObserver(this))) {
+    NS_WARNING("Failed to set thread observer!");
+  }
+}
+
+void
+LazyIdleThread::CleanupThread()
+{
+  nsCOMPtr<nsIThreadInternal> thread(do_QueryInterface(NS_GetCurrentThread()));
+  MOZ_ASSERT(thread, "This should always succeed!");
+
+  if (NS_FAILED(thread->SetObserver(nullptr))) {
+    NS_WARNING("Failed to set thread observer!");
+  }
+
+  {
+    MutexAutoLock lock(mMutex);
+
+    MOZ_ASSERT(!mThreadIsShuttingDown, "Shouldn't be true ever!");
+    mThreadIsShuttingDown = true;
+  }
+
+  profiler_unregister_thread();
+}
+
+void
+LazyIdleThread::ScheduleTimer()
+{
+  ASSERT_OWNING_THREAD();
+
+  bool shouldSchedule;
+  {
+    MutexAutoLock lock(mMutex);
+
+    MOZ_ASSERT(mIdleNotificationCount, "Should have at least one!");
+    --mIdleNotificationCount;
+
+    shouldSchedule = !mIdleNotificationCount && !mPendingEventCount;
+  }
+
+  if (mIdleTimer) {
+    if (NS_FAILED(mIdleTimer->Cancel())) {
+      NS_WARNING("Failed to cancel timer!");
+    }
+
+    if (shouldSchedule &&
+        NS_FAILED(mIdleTimer->InitWithCallback(this, mIdleTimeoutMS,
+                                               nsITimer::TYPE_ONE_SHOT))) {
+      NS_WARNING("Failed to schedule timer!");
+    }
+  }
+}
+
+nsresult
+LazyIdleThread::ShutdownThread()
+{
+  ASSERT_OWNING_THREAD();
+
+  // Before calling Shutdown() on the real thread we need to put a queue in
+  // place in case a runnable is posted to the thread while it's in the
+  // process of shutting down. This will be our queue.
+  AutoTArray<nsCOMPtr<nsIRunnable>, 10> queuedRunnables;
+
+  nsresult rv;
+
+  // Make sure to cancel the shutdown timer before spinning the event loop
+  // during |mThread->Shutdown()| below. Otherwise the timer might fire and we
+  // could reenter here.
+  if (mIdleTimer) {
+    rv = mIdleTimer->Cancel();
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      return rv;
+    }
+
+    mIdleTimer = nullptr;
+  }
+
+  if (mThread) {
+    if (mShutdownMethod == AutomaticShutdown && NS_IsMainThread()) {
+      nsCOMPtr<nsIObserverService> obs =
+        mozilla::services::GetObserverService();
+      NS_WARNING_ASSERTION(obs, "Failed to get observer service!");
+
+      if (obs &&
+          NS_FAILED(obs->RemoveObserver(this, "xpcom-shutdown-threads"))) {
+        NS_WARNING("Failed to remove observer!");
+      }
+    }
+
+    if (mIdleObserver) {
+      mIdleObserver->Observe(static_cast<nsIThread*>(this), IDLE_THREAD_TOPIC,
+                             nullptr);
+    }
+
+#ifdef DEBUG
+    {
+      MutexAutoLock lock(mMutex);
+      MOZ_ASSERT(!mThreadIsShuttingDown, "Huh?!");
+    }
+#endif
+
+    nsCOMPtr<nsIRunnable> runnable =
+      NewRunnableMethod(this, &LazyIdleThread::CleanupThread);
+    if (NS_WARN_IF(!runnable)) {
+      return NS_ERROR_UNEXPECTED;
+    }
+
+    PreDispatch();
+
+    rv = mThread->Dispatch(runnable.forget(), NS_DISPATCH_NORMAL);
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      return rv;
+    }
+
+    // Put the temporary queue in place before calling Shutdown().
+    mQueuedRunnables = &queuedRunnables;
+
+    if (NS_FAILED(mThread->Shutdown())) {
+      NS_ERROR("Failed to shutdown the thread!");
+    }
+
+    // Now unset the queue.
+    mQueuedRunnables = nullptr;
+
+    mThread = nullptr;
+
+    {
+      MutexAutoLock lock(mMutex);
+
+      MOZ_ASSERT(!mPendingEventCount, "Huh?!");
+      MOZ_ASSERT(!mIdleNotificationCount, "Huh?!");
+      MOZ_ASSERT(mThreadIsShuttingDown, "Huh?!");
+      mThreadIsShuttingDown = false;
+    }
+  }
+
+  // If our temporary queue has any runnables then we need to dispatch them.
+  if (queuedRunnables.Length()) {
+    // If the thread manager has gone away then these runnables will never run.
+    if (mShutdown) {
+      NS_ERROR("Runnables dispatched to LazyIdleThread will never run!");
+      return NS_OK;
+    }
+
+    // Re-dispatch the queued runnables.
+    for (uint32_t index = 0; index < queuedRunnables.Length(); index++) {
+      nsCOMPtr<nsIRunnable> runnable;
+      runnable.swap(queuedRunnables[index]);
+      MOZ_ASSERT(runnable, "Null runnable?!");
+
+      if (NS_FAILED(Dispatch(runnable.forget(), NS_DISPATCH_NORMAL))) {
+        NS_ERROR("Failed to re-dispatch queued runnable!");
+      }
+    }
+  }
+
+  return NS_OK;
+}
+
+void
+LazyIdleThread::SelfDestruct()
+{
+  MOZ_ASSERT(mRefCnt == 1, "Bad refcount!");
+  delete this;
+}
+
+NS_IMPL_ADDREF(LazyIdleThread)
+
+NS_IMETHODIMP_(MozExternalRefCountType)
+LazyIdleThread::Release()
+{
+  nsrefcnt count = --mRefCnt;
+  NS_LOG_RELEASE(this, count, "LazyIdleThread");
+
+  if (!count) {
+    // Stabilize refcount.
+    mRefCnt = 1;
+
+    nsCOMPtr<nsIRunnable> runnable =
+      NewNonOwningRunnableMethod(this, &LazyIdleThread::SelfDestruct);
+    NS_WARNING_ASSERTION(runnable, "Couldn't make runnable!");
+
+    if (NS_FAILED(NS_DispatchToCurrentThread(runnable))) {
+      MOZ_ASSERT(NS_IsMainThread(), "Wrong thread!");
+      // The only way this could fail is if we're in shutdown, and in that case
+      // threads should have been joined already. Deleting here isn't dangerous
+      // anymore because we won't spin the event loop waiting to join the
+      // thread.
+      SelfDestruct();
+    }
+  }
+
+  return count;
+}
+
+NS_IMPL_QUERY_INTERFACE(LazyIdleThread, nsIThread,
+                        nsIEventTarget,
+                        nsITimerCallback,
+                        nsIThreadObserver,
+                        nsIObserver)
+
+NS_IMETHODIMP
+LazyIdleThread::DispatchFromScript(nsIRunnable* aEvent, uint32_t aFlags)
+{
+  nsCOMPtr<nsIRunnable> event(aEvent);
+  return Dispatch(event.forget(), aFlags);
+}
+
+NS_IMETHODIMP
+LazyIdleThread::Dispatch(already_AddRefed<nsIRunnable> aEvent,
+                         uint32_t aFlags)
+{
+  ASSERT_OWNING_THREAD();
+  nsCOMPtr<nsIRunnable> event(aEvent); // avoid leaks
+
+  // LazyIdleThread can't always support synchronous dispatch currently.
+  if (NS_WARN_IF(aFlags != NS_DISPATCH_NORMAL)) {
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+
+  if (NS_WARN_IF(mShutdown)) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  // If our thread is shutting down then we can't actually dispatch right now.
+  // Queue this runnable for later.
+  if (UseRunnableQueue()) {
+    mQueuedRunnables->AppendElement(event);
+    return NS_OK;
+  }
+
+  nsresult rv = EnsureThread();
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  PreDispatch();
+
+  return mThread->Dispatch(event.forget(), aFlags);
+}
+
+NS_IMETHODIMP
+LazyIdleThread::DelayedDispatch(already_AddRefed<nsIRunnable>, uint32_t)
+{
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+NS_IMETHODIMP
+LazyIdleThread::IsOnCurrentThread(bool* aIsOnCurrentThread)
+{
+  if (mThread) {
+    return mThread->IsOnCurrentThread(aIsOnCurrentThread);
+  }
+
+  *aIsOnCurrentThread = false;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+LazyIdleThread::GetPRThread(PRThread** aPRThread)
+{
+  if (mThread) {
+    return mThread->GetPRThread(aPRThread);
+  }
+
+  *aPRThread = nullptr;
+  return NS_ERROR_NOT_AVAILABLE;
+}
+
+NS_IMETHODIMP
+LazyIdleThread::GetCanInvokeJS(bool* aCanInvokeJS)
+{
+  *aCanInvokeJS = false;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+LazyIdleThread::SetCanInvokeJS(bool aCanInvokeJS)
+{
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+NS_IMETHODIMP
+LazyIdleThread::AsyncShutdown()
+{
+  ASSERT_OWNING_THREAD();
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+NS_IMETHODIMP
+LazyIdleThread::Shutdown()
+{
+  ASSERT_OWNING_THREAD();
+
+  mShutdown = true;
+
+  nsresult rv = ShutdownThread();
+  MOZ_ASSERT(!mThread, "Should have destroyed this by now!");
+
+  mIdleObserver = nullptr;
+
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+LazyIdleThread::HasPendingEvents(bool* aHasPendingEvents)
+{
+  // This is only supposed to be called from the thread itself so it's not
+  // implemented here.
+  NS_NOTREACHED("Shouldn't ever call this!");
+  return NS_ERROR_UNEXPECTED;
+}
+
+NS_IMETHODIMP
+LazyIdleThread::IdleDispatch(already_AddRefed<nsIRunnable> aEvent)
+{
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+NS_IMETHODIMP
+LazyIdleThread::RegisterIdlePeriod(already_AddRefed<nsIIdlePeriod> aIdlePeriod)
+{
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+NS_IMETHODIMP
+LazyIdleThread::ProcessNextEvent(bool aMayWait,
+                                 bool* aEventWasProcessed)
+{
+  // This is only supposed to be called from the thread itself so it's not
+  // implemented here.
+  NS_NOTREACHED("Shouldn't ever call this!");
+  return NS_ERROR_UNEXPECTED;
+}
+
+NS_IMETHODIMP
+LazyIdleThread::Notify(nsITimer* aTimer)
+{
+  ASSERT_OWNING_THREAD();
+
+  {
+    MutexAutoLock lock(mMutex);
+
+    if (mPendingEventCount || mIdleNotificationCount) {
+      // Another event was scheduled since this timer was set. Don't do
+      // anything and wait for the timer to fire again.
+      return NS_OK;
+    }
+  }
+
+  nsresult rv = ShutdownThread();
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+LazyIdleThread::OnDispatchedEvent(nsIThreadInternal* /*aThread */)
+{
+  MOZ_ASSERT(NS_GetCurrentThread() == mOwningThread, "Wrong thread!");
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+LazyIdleThread::OnProcessNextEvent(nsIThreadInternal* /* aThread */,
+                                   bool /* aMayWait */)
+{
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+LazyIdleThread::AfterProcessNextEvent(nsIThreadInternal* /* aThread */,
+                                      bool aEventWasProcessed)
+{
+  bool shouldNotifyIdle;
+  {
+    MutexAutoLock lock(mMutex);
+
+    if (aEventWasProcessed) {
+      MOZ_ASSERT(mPendingEventCount, "Mismatched calls to observer methods!");
+      --mPendingEventCount;
+    }
+
+    if (mThreadIsShuttingDown) {
+      // We're shutting down, no need to fire any timer.
+      return NS_OK;
+    }
+
+    shouldNotifyIdle = !mPendingEventCount;
+    if (shouldNotifyIdle) {
+      MOZ_ASSERT(mIdleNotificationCount < UINT32_MAX, "Way too many!");
+      mIdleNotificationCount++;
+    }
+  }
+
+  if (shouldNotifyIdle) {
+    nsCOMPtr<nsIRunnable> runnable =
+      NewRunnableMethod(this, &LazyIdleThread::ScheduleTimer);
+    if (NS_WARN_IF(!runnable)) {
+      return NS_ERROR_UNEXPECTED;
+    }
+
+    nsresult rv = mOwningThread->Dispatch(runnable.forget(), NS_DISPATCH_NORMAL);
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      return rv;
+    }
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+LazyIdleThread::Observe(nsISupports* /* aSubject */,
+                        const char*  aTopic,
+                        const char16_t* /* aData */)
+{
+  MOZ_ASSERT(NS_IsMainThread(), "Wrong thread!");
+  MOZ_ASSERT(mShutdownMethod == AutomaticShutdown,
+             "Should not receive notifications if not AutomaticShutdown!");
+  MOZ_ASSERT(!strcmp("xpcom-shutdown-threads", aTopic), "Bad topic!");
+
+  Shutdown();
+  return NS_OK;
+}
+
+} // namespace mozilla
diff --git a/xpcom/threads/LazyIdleThread.h b/xpcom/threads/LazyIdleThread.h
new file mode 100644
index 000000000..6bf8e8e81
--- /dev/null
+++ b/xpcom/threads/LazyIdleThread.h
@@ -0,0 +1,226 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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_lazyidlethread_h__
+#define mozilla_lazyidlethread_h__
+
+#ifndef MOZILLA_INTERNAL_API
+#error "This header is only usable from within libxul (MOZILLA_INTERNAL_API)."
+#endif
+
+#include "nsIObserver.h"
+#include "nsIThreadInternal.h"
+#include "nsITimer.h"
+
+#include "mozilla/Mutex.h"
+#include "nsCOMPtr.h"
+#include "nsTArray.h"
+#include "nsString.h"
+#include "mozilla/Attributes.h"
+
+#define IDLE_THREAD_TOPIC "thread-shutting-down"
+
+namespace mozilla {
+
+/**
+ * This class provides a basic event target that creates its thread lazily and
+ * destroys its thread after a period of inactivity. It may be created on any
+ * thread but it may only be used from the thread on which it is created. If it
+ * is created on the main thread then it will automatically join its thread on
+ * XPCOM shutdown using the Observer Service.
+ */
+class LazyIdleThread final
+  : public nsIThread
+  , public nsITimerCallback
+  , public nsIThreadObserver
+  , public nsIObserver
+{
+public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIEVENTTARGET
+  NS_DECL_NSITHREAD
+  NS_DECL_NSITIMERCALLBACK
+  NS_DECL_NSITHREADOBSERVER
+  NS_DECL_NSIOBSERVER
+  using nsIEventTarget::Dispatch;
+
+  enum ShutdownMethod
+  {
+    AutomaticShutdown = 0,
+    ManualShutdown
+  };
+
+  /**
+   * Create a new LazyIdleThread that will destroy its thread after the given
+   * number of milliseconds.
+   */
+  LazyIdleThread(uint32_t aIdleTimeoutMS,
+                 const nsCSubstring& aName,
+                 ShutdownMethod aShutdownMethod = AutomaticShutdown,
+                 nsIObserver* aIdleObserver = nullptr);
+
+  /**
+   * Add an observer that will be notified when the thread is idle and about to
+   * be shut down. The aSubject argument can be QueryInterface'd to an nsIThread
+   * that can be used to post cleanup events. The aTopic argument will be
+   * IDLE_THREAD_TOPIC, and aData will be null. The LazyIdleThread does not add
+   * a reference to the observer to avoid circular references as it is assumed
+   * to be the owner. It is the caller's responsibility to clear this observer
+   * if the pointer becomes invalid.
+   */
+  void SetWeakIdleObserver(nsIObserver* aObserver);
+
+  /**
+   * Disable the idle timeout for this thread. No effect if the timeout is
+   * already disabled.
+   */
+  void DisableIdleTimeout();
+
+  /**
+   * Enable the idle timeout. No effect if the timeout is already enabled.
+   */
+  void EnableIdleTimeout();
+
+private:
+  /**
+   * Calls Shutdown().
+   */
+  ~LazyIdleThread();
+
+  /**
+   * Called just before dispatching to mThread.
+   */
+  void PreDispatch();
+
+  /**
+   * Makes sure a valid thread lives in mThread.
+   */
+  nsresult EnsureThread();
+
+  /**
+   * Called on mThread to set up the thread observer.
+   */
+  void InitThread();
+
+  /**
+   * Called on mThread to clean up the thread observer.
+   */
+  void CleanupThread();
+
+  /**
+   * Called on the main thread when mThread believes itself to be idle. Sets up
+   * the idle timer.
+   */
+  void ScheduleTimer();
+
+  /**
+   * Called when we are shutting down mThread.
+   */
+  nsresult ShutdownThread();
+
+  /**
+   * Deletes this object. Used to delay calling mThread->Shutdown() during the
+   * final release (during a GC, for instance).
+   */
+  void SelfDestruct();
+
+  /**
+   * Returns true if events should be queued rather than immediately dispatched
+   * to mThread. Currently only happens when the thread is shutting down.
+   */
+  bool UseRunnableQueue()
+  {
+    return !!mQueuedRunnables;
+  }
+
+  /**
+   * Protects data that is accessed on both threads.
+   */
+  mozilla::Mutex mMutex;
+
+  /**
+   * Touched on both threads but set before mThread is created. Used to direct
+   * timer events to the owning thread.
+   */
+  nsCOMPtr<nsIThread> mOwningThread;
+
+  /**
+   * Only accessed on the owning thread. Set by EnsureThread().
+   */
+  nsCOMPtr<nsIThread> mThread;
+
+  /**
+   * Protected by mMutex. Created when mThread has no pending events and fired
+   * at mOwningThread. Any thread that dispatches to mThread will take ownership
+   * of the timer and fire a separate cancel event to the owning thread.
+   */
+  nsCOMPtr<nsITimer> mIdleTimer;
+
+  /**
+   * Idle observer. Called when the thread is about to be shut down. Released
+   * only when Shutdown() is called.
+   */
+  nsIObserver* MOZ_UNSAFE_REF("See the documentation for SetWeakIdleObserver for "
+                              "how the owner of LazyIdleThread should manage the "
+                              "lifetime information of this field") mIdleObserver;
+
+  /**
+   * Temporary storage for events that happen to be dispatched while we're in
+   * the process of shutting down our real thread.
+   */
+  nsTArray<nsCOMPtr<nsIRunnable>>* mQueuedRunnables;
+
+  /**
+   * The number of milliseconds a thread should be idle before dying.
+   */
+  const uint32_t mIdleTimeoutMS;
+
+  /**
+   * The number of events that are pending on mThread. A nonzero value means
+   * that the thread cannot be cleaned up.
+   */
+  uint32_t mPendingEventCount;
+
+  /**
+   * The number of times that mThread has dispatched an idle notification. Any
+   * timer that fires while this count is nonzero can safely be ignored as
+   * another timer will be on the way.
+   */
+  uint32_t mIdleNotificationCount;
+
+  /**
+   * Whether or not the thread should automatically shutdown. If the owner
+   * specified ManualShutdown at construction time then the owner should take
+   * care to call Shutdown() manually when appropriate.
+   */
+  ShutdownMethod mShutdownMethod;
+
+  /**
+   * Only accessed on the owning thread. Set to true when Shutdown() has been
+   * called and prevents EnsureThread() from recreating mThread.
+   */
+  bool mShutdown;
+
+  /**
+   * Set from CleanupThread and lasting until the thread has shut down. Prevents
+   * further idle notifications during the shutdown process.
+   */
+  bool mThreadIsShuttingDown;
+
+  /**
+   * Whether or not the idle timeout is enabled.
+   */
+  bool mIdleTimeoutEnabled;
+
+  /**
+   * Name of the thread, set on the actual thread after it gets created.
+   */
+  nsCString mName;
+};
+
+} // namespace mozilla
+
+#endif // mozilla_lazyidlethread_h__
diff --git a/xpcom/threads/LeakRefPtr.h b/xpcom/threads/LeakRefPtr.h
new file mode 100644
index 000000000..56f5d90af
--- /dev/null
+++ b/xpcom/threads/LeakRefPtr.h
@@ -0,0 +1,52 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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/. */
+
+/* Smart pointer which leaks its owning refcounted object by default. */
+
+#ifndef LeakRefPtr_h
+#define LeakRefPtr_h
+
+#include "mozilla/AlreadyAddRefed.h"
+
+namespace mozilla {
+
+/**
+ * Instance of this class behaves like a raw pointer which leaks the
+ * resource it's owning if not explicitly released.
+ */
+template<class T>
+class LeakRefPtr
+{
+public:
+  explicit LeakRefPtr(already_AddRefed<T>&& aPtr)
+    : mRawPtr(aPtr.take()) { }
+
+  explicit operator bool() const { return !!mRawPtr; }
+
+  LeakRefPtr<T>& operator=(already_AddRefed<T>&& aPtr)
+  {
+    mRawPtr = aPtr.take();
+    return *this;
+  }
+
+  T* get() const { return mRawPtr; }
+
+  already_AddRefed<T> take()
+  {
+    T* rawPtr = mRawPtr;
+    mRawPtr = nullptr;
+    return already_AddRefed<T>(rawPtr);
+  }
+
+  void release() { NS_RELEASE(mRawPtr); }
+
+private:
+  T* MOZ_OWNING_REF mRawPtr;
+};
+
+} // namespace mozilla
+
+#endif // LeakRefPtr_h
diff --git a/xpcom/threads/MainThreadIdlePeriod.cpp b/xpcom/threads/MainThreadIdlePeriod.cpp
new file mode 100644
index 000000000..4a5f99dd7
--- /dev/null
+++ b/xpcom/threads/MainThreadIdlePeriod.cpp
@@ -0,0 +1,76 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "MainThreadIdlePeriod.h"
+
+#include "mozilla/Maybe.h"
+#include "mozilla/Preferences.h"
+#include "nsRefreshDriver.h"
+
+#define DEFAULT_LONG_IDLE_PERIOD 50.0f
+#define DEFAULT_MIN_IDLE_PERIOD 3.0f
+
+namespace mozilla {
+
+NS_IMETHODIMP
+MainThreadIdlePeriod::GetIdlePeriodHint(TimeStamp* aIdleDeadline)
+{
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(aIdleDeadline);
+
+  Maybe<TimeStamp> deadline = nsRefreshDriver::GetIdleDeadlineHint();
+
+  if (deadline.isSome()) {
+    // If the idle period is too small, then just return a null time
+    // to indicate we are busy. Otherwise return the actual deadline.
+    TimeDuration minIdlePeriod =
+      TimeDuration::FromMilliseconds(GetMinIdlePeriod());
+    bool busySoon = deadline.value().IsNull() ||
+                    (TimeStamp::Now() >= (deadline.value() - minIdlePeriod));
+    *aIdleDeadline = busySoon ? TimeStamp() : deadline.value();
+  } else {
+    *aIdleDeadline =
+      TimeStamp::Now() + TimeDuration::FromMilliseconds(GetLongIdlePeriod());
+  }
+
+  return NS_OK;
+}
+
+/* static */ float
+MainThreadIdlePeriod::GetLongIdlePeriod()
+{
+  MOZ_ASSERT(NS_IsMainThread());
+
+  static float sLongIdlePeriod = DEFAULT_LONG_IDLE_PERIOD;
+  static bool sInitialized = false;
+
+  if (!sInitialized && Preferences::IsServiceAvailable()) {
+    sInitialized = true;
+    Preferences::AddFloatVarCache(&sLongIdlePeriod, "idle_queue.long_period",
+                                  DEFAULT_LONG_IDLE_PERIOD);
+  }
+
+  return sLongIdlePeriod;
+}
+
+/* static */ float
+MainThreadIdlePeriod::GetMinIdlePeriod()
+{
+  MOZ_ASSERT(NS_IsMainThread());
+
+  static float sMinIdlePeriod = DEFAULT_MIN_IDLE_PERIOD;
+  static bool sInitialized = false;
+
+  if (!sInitialized && Preferences::IsServiceAvailable()) {
+    sInitialized = true;
+    Preferences::AddFloatVarCache(&sMinIdlePeriod, "idle_queue.min_period",
+                                  DEFAULT_MIN_IDLE_PERIOD);
+  }
+
+  return sMinIdlePeriod;
+}
+
+} // namespace mozilla
diff --git a/xpcom/threads/MainThreadIdlePeriod.h b/xpcom/threads/MainThreadIdlePeriod.h
new file mode 100644
index 000000000..2b773551c
--- /dev/null
+++ b/xpcom/threads/MainThreadIdlePeriod.h
@@ -0,0 +1,28 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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_dom_mainthreadidleperiod_h
+#define mozilla_dom_mainthreadidleperiod_h
+
+#include "mozilla/TimeStamp.h"
+#include "nsThreadUtils.h"
+
+namespace mozilla {
+
+class MainThreadIdlePeriod final : public IdlePeriod
+{
+public:
+  NS_DECL_NSIIDLEPERIOD
+
+  static float GetLongIdlePeriod();
+  static float GetMinIdlePeriod();
+private:
+  virtual ~MainThreadIdlePeriod() {}
+};
+
+} // namespace mozilla
+
+#endif // mozilla_dom_mainthreadidleperiod_h
diff --git a/xpcom/threads/MozPromise.h b/xpcom/threads/MozPromise.h
new file mode 100644
index 000000000..7a2921d2a
--- /dev/null
+++ b/xpcom/threads/MozPromise.h
@@ -0,0 +1,1067 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 !defined(MozPromise_h_)
+#define MozPromise_h_
+
+#include "mozilla/AbstractThread.h"
+#include "mozilla/IndexSequence.h"
+#include "mozilla/Logging.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/Monitor.h"
+#include "mozilla/Tuple.h"
+#include "mozilla/TypeTraits.h"
+
+#include "nsTArray.h"
+#include "nsThreadUtils.h"
+
+#if defined(DEBUG) || !defined(RELEASE_OR_BETA)
+#define PROMISE_DEBUG
+#endif
+
+#ifdef PROMISE_DEBUG
+#define PROMISE_ASSERT MOZ_RELEASE_ASSERT
+#else
+#define PROMISE_ASSERT(...) do { } while (0)
+#endif
+
+namespace mozilla {
+
+extern LazyLogModule gMozPromiseLog;
+
+#define PROMISE_LOG(x, ...) \
+  MOZ_LOG(gMozPromiseLog, mozilla::LogLevel::Debug, (x, ##__VA_ARGS__))
+
+namespace detail {
+template<typename ThisType, typename Ret, typename ArgType>
+static TrueType TakesArgumentHelper(Ret (ThisType::*)(ArgType));
+template<typename ThisType, typename Ret, typename ArgType>
+static TrueType TakesArgumentHelper(Ret (ThisType::*)(ArgType) const);
+template<typename ThisType, typename Ret>
+static FalseType TakesArgumentHelper(Ret (ThisType::*)());
+template<typename ThisType, typename Ret>
+static FalseType TakesArgumentHelper(Ret (ThisType::*)() const);
+
+template<typename ThisType, typename Ret, typename ArgType>
+static Ret ReturnTypeHelper(Ret (ThisType::*)(ArgType));
+template<typename ThisType, typename Ret, typename ArgType>
+static Ret ReturnTypeHelper(Ret (ThisType::*)(ArgType) const);
+template<typename ThisType, typename Ret>
+static Ret ReturnTypeHelper(Ret (ThisType::*)());
+template<typename ThisType, typename Ret>
+static Ret ReturnTypeHelper(Ret (ThisType::*)() const);
+
+template<typename MethodType>
+struct ReturnType {
+  typedef decltype(detail::ReturnTypeHelper(DeclVal<MethodType>())) Type;
+};
+
+} // namespace detail
+
+template<typename MethodType>
+struct TakesArgument {
+  static const bool value = decltype(detail::TakesArgumentHelper(DeclVal<MethodType>()))::value;
+};
+
+template<typename MethodType, typename TargetType>
+struct ReturnTypeIs {
+  static const bool value = IsConvertible<typename detail::ReturnType<MethodType>::Type, TargetType>::value;
+};
+
+/*
+ * A promise manages an asynchronous request that may or may not be able to be
+ * fulfilled immediately. When an API returns a promise, the consumer may attach
+ * callbacks to be invoked (asynchronously, on a specified thread) when the
+ * request is either completed (resolved) or cannot be completed (rejected).
+ * Whereas JS promise callbacks are dispatched from Microtask checkpoints,
+ * MozPromises resolution/rejection make a normal round-trip through the event
+ * loop, which simplifies their ordering semantics relative to other native code.
+ *
+ * MozPromises attempt to mirror the spirit of JS Promises to the extent that
+ * is possible (and desirable) in C++. While the intent is that MozPromises
+ * feel familiar to programmers who are accustomed to their JS-implemented cousin,
+ * we don't shy away from imposing restrictions and adding features that make
+ * sense for the use cases we encounter.
+ *
+ * A MozPromise is ThreadSafe, and may be ->Then()ed on any thread. The Then()
+ * call accepts resolve and reject callbacks, and returns a MozPromise::Request.
+ * The Request object serves several purposes for the consumer.
+ *
+ *   (1) It allows the caller to cancel the delivery of the resolve/reject value
+ *       if it has not already occurred, via Disconnect() (this must be done on
+ *       the target thread to avoid racing).
+ *
+ *   (2) It provides access to a "Completion Promise", which is roughly analagous
+ *       to the Promise returned directly by ->then() calls on JS promises. If
+ *       the resolve/reject callback returns a new MozPromise, that promise is
+ *       chained to the completion promise, such that its resolve/reject value
+ *       will be forwarded along when it arrives. If the resolve/reject callback
+ *       returns void, the completion promise is resolved/rejected with the same
+ *       value that was passed to the callback.
+ *
+ * The MozPromise APIs skirt traditional XPCOM convention by returning nsRefPtrs
+ * (rather than already_AddRefed) from various methods. This is done to allow elegant
+ * chaining of calls without cluttering up the code with intermediate variables, and
+ * without introducing separate API variants for callers that want a return value
+ * (from, say, ->Then()) from those that don't.
+ *
+ * When IsExclusive is true, the MozPromise does a release-mode assertion that
+ * there is at most one call to either Then(...) or ChainTo(...).
+ */
+
+class MozPromiseRefcountable
+{
+public:
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(MozPromiseRefcountable)
+protected:
+  virtual ~MozPromiseRefcountable() {}
+};
+
+template<typename T> class MozPromiseHolder;
+template<typename ResolveValueT, typename RejectValueT, bool IsExclusive>
+class MozPromise : public MozPromiseRefcountable
+{
+  static const uint32_t sMagic = 0xcecace11;
+
+public:
+  typedef ResolveValueT ResolveValueType;
+  typedef RejectValueT RejectValueType;
+  class ResolveOrRejectValue
+  {
+  public:
+    template<typename ResolveValueType_>
+    void SetResolve(ResolveValueType_&& aResolveValue)
+    {
+      MOZ_ASSERT(IsNothing());
+      mResolveValue.emplace(Forward<ResolveValueType_>(aResolveValue));
+    }
+
+    template<typename RejectValueType_>
+    void SetReject(RejectValueType_&& aRejectValue)
+    {
+      MOZ_ASSERT(IsNothing());
+      mRejectValue.emplace(Forward<RejectValueType_>(aRejectValue));
+    }
+
+    template<typename ResolveValueType_>
+    static ResolveOrRejectValue MakeResolve(ResolveValueType_&& aResolveValue)
+    {
+      ResolveOrRejectValue val;
+      val.SetResolve(Forward<ResolveValueType_>(aResolveValue));
+      return val;
+    }
+
+    template<typename RejectValueType_>
+    static ResolveOrRejectValue MakeReject(RejectValueType_&& aRejectValue)
+    {
+      ResolveOrRejectValue val;
+      val.SetReject(Forward<RejectValueType_>(aRejectValue));
+      return val;
+    }
+
+    bool IsResolve() const { return mResolveValue.isSome(); }
+    bool IsReject() const { return mRejectValue.isSome(); }
+    bool IsNothing() const { return mResolveValue.isNothing() && mRejectValue.isNothing(); }
+
+    const ResolveValueType& ResolveValue() const { return mResolveValue.ref(); }
+    const RejectValueType& RejectValue() const { return mRejectValue.ref(); }
+
+  private:
+    Maybe<ResolveValueType> mResolveValue;
+    Maybe<RejectValueType> mRejectValue;
+  };
+
+protected:
+  // MozPromise is the public type, and never constructed directly. Construct
+  // a MozPromise::Private, defined below.
+  MozPromise(const char* aCreationSite, bool aIsCompletionPromise)
+    : mCreationSite(aCreationSite)
+    , mMutex("MozPromise Mutex")
+    , mHaveRequest(false)
+    , mIsCompletionPromise(aIsCompletionPromise)
+#ifdef PROMISE_DEBUG
+    , mMagic4(mMutex.mLock)
+#endif
+  {
+    PROMISE_LOG("%s creating MozPromise (%p)", mCreationSite, this);
+  }
+
+public:
+  // MozPromise::Private allows us to separate the public interface (upon which
+  // consumers of the promise may invoke methods like Then()) from the private
+  // interface (upon which the creator of the promise may invoke Resolve() or
+  // Reject()). APIs should create and store a MozPromise::Private (usually
+  // via a MozPromiseHolder), and return a MozPromise to consumers.
+  //
+  // NB: We can include the definition of this class inline once B2G ICS is gone.
+  class Private;
+
+  template<typename ResolveValueType_>
+  static RefPtr<MozPromise>
+  CreateAndResolve(ResolveValueType_&& aResolveValue, const char* aResolveSite)
+  {
+    RefPtr<typename MozPromise::Private> p = new MozPromise::Private(aResolveSite);
+    p->Resolve(Forward<ResolveValueType_>(aResolveValue), aResolveSite);
+    return p.forget();
+  }
+
+  template<typename RejectValueType_>
+  static RefPtr<MozPromise>
+  CreateAndReject(RejectValueType_&& aRejectValue, const char* aRejectSite)
+  {
+    RefPtr<typename MozPromise::Private> p = new MozPromise::Private(aRejectSite);
+    p->Reject(Forward<RejectValueType_>(aRejectValue), aRejectSite);
+    return p.forget();
+  }
+
+  typedef MozPromise<nsTArray<ResolveValueType>, RejectValueType, IsExclusive> AllPromiseType;
+private:
+  class AllPromiseHolder : public MozPromiseRefcountable
+  {
+  public:
+    explicit AllPromiseHolder(size_t aDependentPromises)
+      : mPromise(new typename AllPromiseType::Private(__func__))
+      , mOutstandingPromises(aDependentPromises)
+    {
+      mResolveValues.SetLength(aDependentPromises);
+    }
+
+    void Resolve(size_t aIndex, const ResolveValueType& aResolveValue)
+    {
+      if (!mPromise) {
+        // Already rejected.
+        return;
+      }
+
+      mResolveValues[aIndex].emplace(aResolveValue);
+      if (--mOutstandingPromises == 0) {
+        nsTArray<ResolveValueType> resolveValues;
+        resolveValues.SetCapacity(mResolveValues.Length());
+        for (size_t i = 0; i < mResolveValues.Length(); ++i) {
+          resolveValues.AppendElement(mResolveValues[i].ref());
+        }
+
+        mPromise->Resolve(resolveValues, __func__);
+        mPromise = nullptr;
+        mResolveValues.Clear();
+      }
+    }
+
+    void Reject(const RejectValueType& aRejectValue)
+    {
+      if (!mPromise) {
+        // Already rejected.
+        return;
+      }
+
+      mPromise->Reject(aRejectValue, __func__);
+      mPromise = nullptr;
+      mResolveValues.Clear();
+    }
+
+    AllPromiseType* Promise() { return mPromise; }
+
+  private:
+    nsTArray<Maybe<ResolveValueType>> mResolveValues;
+    RefPtr<typename AllPromiseType::Private> mPromise;
+    size_t mOutstandingPromises;
+  };
+public:
+
+  static RefPtr<AllPromiseType> All(AbstractThread* aProcessingThread, nsTArray<RefPtr<MozPromise>>& aPromises)
+  {
+    RefPtr<AllPromiseHolder> holder = new AllPromiseHolder(aPromises.Length());
+    for (size_t i = 0; i < aPromises.Length(); ++i) {
+      aPromises[i]->Then(aProcessingThread, __func__,
+        [holder, i] (ResolveValueType aResolveValue) -> void { holder->Resolve(i, aResolveValue); },
+        [holder] (RejectValueType aRejectValue) -> void { holder->Reject(aRejectValue); }
+      );
+    }
+    return holder->Promise();
+  }
+
+  class Request : public MozPromiseRefcountable
+  {
+  public:
+    virtual void Disconnect() = 0;
+
+    virtual MozPromise* CompletionPromise() = 0;
+
+    virtual void AssertIsDead() = 0;
+
+  protected:
+    Request() : mComplete(false), mDisconnected(false) {}
+    virtual ~Request() {}
+
+    bool mComplete;
+    bool mDisconnected;
+  };
+
+protected:
+
+  /*
+   * A ThenValue tracks a single consumer waiting on the promise. When a consumer
+   * invokes promise->Then(...), a ThenValue is created. Once the Promise is
+   * resolved or rejected, a {Resolve,Reject}Runnable is dispatched, which
+   * invokes the resolve/reject method and then deletes the ThenValue.
+   */
+  class ThenValueBase : public Request
+  {
+    static const uint32_t sMagic = 0xfadece11;
+
+  public:
+    class ResolveOrRejectRunnable : public Runnable
+    {
+    public:
+      ResolveOrRejectRunnable(ThenValueBase* aThenValue, MozPromise* aPromise)
+        : mThenValue(aThenValue)
+        , mPromise(aPromise)
+      {
+        MOZ_DIAGNOSTIC_ASSERT(!mPromise->IsPending());
+      }
+
+      ~ResolveOrRejectRunnable()
+      {
+        if (mThenValue) {
+          mThenValue->AssertIsDead();
+        }
+      }
+
+      NS_IMETHOD Run() override
+      {
+        PROMISE_LOG("ResolveOrRejectRunnable::Run() [this=%p]", this);
+        mThenValue->DoResolveOrReject(mPromise->Value());
+        mThenValue = nullptr;
+        mPromise = nullptr;
+        return NS_OK;
+      }
+
+    private:
+      RefPtr<ThenValueBase> mThenValue;
+      RefPtr<MozPromise> mPromise;
+    };
+
+    explicit ThenValueBase(AbstractThread* aResponseTarget, const char* aCallSite)
+      : mResponseTarget(aResponseTarget), mCallSite(aCallSite) {}
+
+#ifdef PROMISE_DEBUG
+    ~ThenValueBase()
+    {
+      mMagic1 = 0;
+      mMagic2 = 0;
+    }
+#endif
+
+    MozPromise* CompletionPromise() override
+    {
+      MOZ_DIAGNOSTIC_ASSERT(mResponseTarget->IsCurrentThreadIn());
+      MOZ_DIAGNOSTIC_ASSERT(!Request::mComplete);
+      if (!mCompletionPromise) {
+        mCompletionPromise = new MozPromise::Private(
+          "<completion promise>", true /* aIsCompletionPromise */);
+      }
+      return mCompletionPromise;
+    }
+
+    void AssertIsDead() override
+    {
+      PROMISE_ASSERT(mMagic1 == sMagic && mMagic2 == sMagic);
+      // We want to assert that this ThenValues is dead - that is to say, that
+      // there are no consumers waiting for the result. In the case of a normal
+      // ThenValue, we check that it has been disconnected, which is the way
+      // that the consumer signals that it no longer wishes to hear about the
+      // result. If this ThenValue has a completion promise (which is mutually
+      // exclusive with being disconnectable), we recursively assert that every
+      // ThenValue associated with the completion promise is dead.
+      if (mCompletionPromise) {
+        mCompletionPromise->AssertIsDead();
+      } else {
+        MOZ_DIAGNOSTIC_ASSERT(Request::mDisconnected);
+      }
+    }
+
+    void Dispatch(MozPromise *aPromise)
+    {
+      PROMISE_ASSERT(mMagic1 == sMagic && mMagic2 == sMagic);
+      aPromise->mMutex.AssertCurrentThreadOwns();
+      MOZ_ASSERT(!aPromise->IsPending());
+
+      RefPtr<Runnable> runnable =
+        static_cast<Runnable*>(new (typename ThenValueBase::ResolveOrRejectRunnable)(this, aPromise));
+      PROMISE_LOG("%s Then() call made from %s [Runnable=%p, Promise=%p, ThenValue=%p]",
+                  aPromise->mValue.IsResolve() ? "Resolving" : "Rejecting", ThenValueBase::mCallSite,
+                  runnable.get(), aPromise, this);
+
+      // Promise consumers are allowed to disconnect the Request object and
+      // then shut down the thread or task queue that the promise result would
+      // be dispatched on. So we unfortunately can't assert that promise
+      // dispatch succeeds. :-(
+      mResponseTarget->Dispatch(runnable.forget(), AbstractThread::DontAssertDispatchSuccess);
+    }
+
+    virtual void Disconnect() override
+    {
+      MOZ_DIAGNOSTIC_ASSERT(ThenValueBase::mResponseTarget->IsCurrentThreadIn());
+      MOZ_DIAGNOSTIC_ASSERT(!Request::mComplete);
+      Request::mDisconnected = true;
+
+      // We could support rejecting the completion promise on disconnection, but
+      // then we'd need to have some sort of default reject value. The use cases
+      // of disconnection and completion promise chaining seem pretty orthogonal,
+      // so let's use assert against it.
+      MOZ_DIAGNOSTIC_ASSERT(!mCompletionPromise);
+    }
+
+  protected:
+    virtual already_AddRefed<MozPromise> DoResolveOrRejectInternal(const ResolveOrRejectValue& aValue) = 0;
+
+    void DoResolveOrReject(const ResolveOrRejectValue& aValue)
+    {
+      PROMISE_ASSERT(mMagic1 == sMagic && mMagic2 == sMagic);
+      MOZ_DIAGNOSTIC_ASSERT(mResponseTarget->IsCurrentThreadIn());
+      Request::mComplete = true;
+      if (Request::mDisconnected) {
+        PROMISE_LOG("ThenValue::DoResolveOrReject disconnected - bailing out [this=%p]", this);
+        return;
+      }
+
+      // Invoke the resolve or reject method.
+      RefPtr<MozPromise> p = DoResolveOrRejectInternal(aValue);
+
+      // If there's a completion promise, resolve it appropriately with the
+      // result of the method.
+      //
+      // We jump through some hoops to cast to MozPromise::Private here. This
+      // can go away when we can just declare mCompletionPromise as
+      // MozPromise::Private. See the declaration below.
+      RefPtr<MozPromise::Private> completionPromise =
+        dont_AddRef(static_cast<MozPromise::Private*>(mCompletionPromise.forget().take()));
+      if (completionPromise) {
+        if (p) {
+          p->ChainTo(completionPromise.forget(), "<chained completion promise>");
+        } else {
+          completionPromise->ResolveOrReject(aValue, "<completion of non-promise-returning method>");
+        }
+      }
+    }
+
+    RefPtr<AbstractThread> mResponseTarget; // May be released on any thread.
+#ifdef PROMISE_DEBUG
+    uint32_t mMagic1 = sMagic;
+#endif
+    // Declaring RefPtr<MozPromise::Private> here causes build failures
+    // on MSVC because MozPromise::Private is only forward-declared at this
+    // point. This hack can go away when we inline-declare MozPromise::Private,
+    // which is blocked on the B2G ICS compiler being too old.
+    RefPtr<MozPromise> mCompletionPromise;
+#ifdef PROMISE_DEBUG
+    uint32_t mMagic2 = sMagic;
+#endif
+    const char* mCallSite;
+  };
+
+  /*
+   * We create two overloads for invoking Resolve/Reject Methods so as to
+   * make the resolve/reject value argument "optional".
+   */
+
+  template<typename ThisType, typename MethodType, typename ValueType>
+  static typename EnableIf<ReturnTypeIs<MethodType, RefPtr<MozPromise>>::value &&
+                           TakesArgument<MethodType>::value,
+                           already_AddRefed<MozPromise>>::Type
+  InvokeCallbackMethod(ThisType* aThisVal, MethodType aMethod, ValueType&& aValue)
+  {
+    return ((*aThisVal).*aMethod)(Forward<ValueType>(aValue)).forget();
+  }
+
+  template<typename ThisType, typename MethodType, typename ValueType>
+  static typename EnableIf<ReturnTypeIs<MethodType, void>::value &&
+                           TakesArgument<MethodType>::value,
+                           already_AddRefed<MozPromise>>::Type
+  InvokeCallbackMethod(ThisType* aThisVal, MethodType aMethod, ValueType&& aValue)
+  {
+    ((*aThisVal).*aMethod)(Forward<ValueType>(aValue));
+    return nullptr;
+  }
+
+  template<typename ThisType, typename MethodType, typename ValueType>
+  static typename EnableIf<ReturnTypeIs<MethodType, RefPtr<MozPromise>>::value &&
+                           !TakesArgument<MethodType>::value,
+                           already_AddRefed<MozPromise>>::Type
+  InvokeCallbackMethod(ThisType* aThisVal, MethodType aMethod, ValueType&& aValue)
+  {
+    return ((*aThisVal).*aMethod)().forget();
+  }
+
+  template<typename ThisType, typename MethodType, typename ValueType>
+  static typename EnableIf<ReturnTypeIs<MethodType, void>::value &&
+                           !TakesArgument<MethodType>::value,
+                           already_AddRefed<MozPromise>>::Type
+  InvokeCallbackMethod(ThisType* aThisVal, MethodType aMethod, ValueType&& aValue)
+  {
+    ((*aThisVal).*aMethod)();
+    return nullptr;
+  }
+
+  template<typename ThisType, typename ResolveMethodType, typename RejectMethodType>
+  class MethodThenValue : public ThenValueBase
+  {
+  public:
+    MethodThenValue(AbstractThread* aResponseTarget, ThisType* aThisVal,
+                    ResolveMethodType aResolveMethod, RejectMethodType aRejectMethod,
+                    const char* aCallSite)
+      : ThenValueBase(aResponseTarget, aCallSite)
+      , mThisVal(aThisVal)
+      , mResolveMethod(aResolveMethod)
+      , mRejectMethod(aRejectMethod) {}
+
+  virtual void Disconnect() override
+  {
+    ThenValueBase::Disconnect();
+
+    // If a Request has been disconnected, we don't guarantee that the
+    // resolve/reject runnable will be dispatched. Null out our refcounted
+    // this-value now so that it's released predictably on the dispatch thread.
+    mThisVal = nullptr;
+  }
+
+  protected:
+    virtual already_AddRefed<MozPromise> DoResolveOrRejectInternal(const ResolveOrRejectValue& aValue) override
+    {
+      RefPtr<MozPromise> completion;
+      if (aValue.IsResolve()) {
+        completion = InvokeCallbackMethod(mThisVal.get(), mResolveMethod, aValue.ResolveValue());
+      } else {
+        completion = InvokeCallbackMethod(mThisVal.get(), mRejectMethod, aValue.RejectValue());
+      }
+
+      // Null out mThisVal after invoking the callback so that any references are
+      // released predictably on the dispatch thread. Otherwise, it would be
+      // released on whatever thread last drops its reference to the ThenValue,
+      // which may or may not be ok.
+      mThisVal = nullptr;
+
+      return completion.forget();
+    }
+
+  private:
+    RefPtr<ThisType> mThisVal; // Only accessed and refcounted on dispatch thread.
+    ResolveMethodType mResolveMethod;
+    RejectMethodType mRejectMethod;
+  };
+
+  // NB: We could use std::function here instead of a template if it were supported. :-(
+  template<typename ResolveFunction, typename RejectFunction>
+  class FunctionThenValue : public ThenValueBase
+  {
+  public:
+    FunctionThenValue(AbstractThread* aResponseTarget,
+                      ResolveFunction&& aResolveFunction,
+                      RejectFunction&& aRejectFunction,
+                      const char* aCallSite)
+      : ThenValueBase(aResponseTarget, aCallSite)
+    {
+      mResolveFunction.emplace(Move(aResolveFunction));
+      mRejectFunction.emplace(Move(aRejectFunction));
+    }
+
+  virtual void Disconnect() override
+  {
+    ThenValueBase::Disconnect();
+
+    // If a Request has been disconnected, we don't guarantee that the
+    // resolve/reject runnable will be dispatched. Destroy our callbacks
+    // now so that any references in closures are released predictable on
+    // the dispatch thread.
+    mResolveFunction.reset();
+    mRejectFunction.reset();
+  }
+
+  protected:
+    virtual already_AddRefed<MozPromise> DoResolveOrRejectInternal(const ResolveOrRejectValue& aValue) override
+    {
+      // Note: The usage of InvokeCallbackMethod here requires that
+      // ResolveFunction/RejectFunction are capture-lambdas (i.e. anonymous
+      // classes with ::operator()), since it allows us to share code more easily.
+      // We could fix this if need be, though it's quite easy to work around by
+      // just capturing something.
+      RefPtr<MozPromise> completion;
+      if (aValue.IsResolve()) {
+        completion = InvokeCallbackMethod(mResolveFunction.ptr(), &ResolveFunction::operator(), aValue.ResolveValue());
+      } else {
+        completion = InvokeCallbackMethod(mRejectFunction.ptr(), &RejectFunction::operator(), aValue.RejectValue());
+      }
+
+      // Destroy callbacks after invocation so that any references in closures are
+      // released predictably on the dispatch thread. Otherwise, they would be
+      // released on whatever thread last drops its reference to the ThenValue,
+      // which may or may not be ok.
+      mResolveFunction.reset();
+      mRejectFunction.reset();
+
+      return completion.forget();
+    }
+
+  private:
+    Maybe<ResolveFunction> mResolveFunction; // Only accessed and deleted on dispatch thread.
+    Maybe<RejectFunction> mRejectFunction; // Only accessed and deleted on dispatch thread.
+  };
+
+public:
+  void ThenInternal(AbstractThread* aResponseThread, ThenValueBase* aThenValue,
+                    const char* aCallSite)
+  {
+    PROMISE_ASSERT(mMagic1 == sMagic && mMagic2 == sMagic && mMagic3 == sMagic && mMagic4 == mMutex.mLock);
+    MutexAutoLock lock(mMutex);
+    MOZ_ASSERT(aResponseThread->IsDispatchReliable());
+    MOZ_DIAGNOSTIC_ASSERT(!IsExclusive || !mHaveRequest);
+    mHaveRequest = true;
+    PROMISE_LOG("%s invoking Then() [this=%p, aThenValue=%p, isPending=%d]",
+                aCallSite, this, aThenValue, (int) IsPending());
+    if (!IsPending()) {
+      aThenValue->Dispatch(this);
+    } else {
+      mThenValues.AppendElement(aThenValue);
+    }
+  }
+
+public:
+
+  template<typename ThisType, typename ResolveMethodType, typename RejectMethodType>
+  RefPtr<Request> Then(AbstractThread* aResponseThread, const char* aCallSite, ThisType* aThisVal,
+                         ResolveMethodType aResolveMethod, RejectMethodType aRejectMethod)
+  {
+    RefPtr<ThenValueBase> thenValue = new MethodThenValue<ThisType, ResolveMethodType, RejectMethodType>(
+                                              aResponseThread, aThisVal, aResolveMethod, aRejectMethod, aCallSite);
+    ThenInternal(aResponseThread, thenValue, aCallSite);
+    return thenValue.forget(); // Implicit conversion from already_AddRefed<ThenValueBase> to RefPtr<Request>.
+  }
+
+  template<typename ResolveFunction, typename RejectFunction>
+  RefPtr<Request> Then(AbstractThread* aResponseThread, const char* aCallSite,
+                         ResolveFunction&& aResolveFunction, RejectFunction&& aRejectFunction)
+  {
+    RefPtr<ThenValueBase> thenValue = new FunctionThenValue<ResolveFunction, RejectFunction>(aResponseThread,
+                                              Move(aResolveFunction), Move(aRejectFunction), aCallSite);
+    ThenInternal(aResponseThread, thenValue, aCallSite);
+    return thenValue.forget(); // Implicit conversion from already_AddRefed<ThenValueBase> to RefPtr<Request>.
+  }
+
+  void ChainTo(already_AddRefed<Private> aChainedPromise, const char* aCallSite)
+  {
+    MutexAutoLock lock(mMutex);
+    MOZ_DIAGNOSTIC_ASSERT(!IsExclusive || !mHaveRequest);
+    mHaveRequest = true;
+    RefPtr<Private> chainedPromise = aChainedPromise;
+    PROMISE_LOG("%s invoking Chain() [this=%p, chainedPromise=%p, isPending=%d]",
+                aCallSite, this, chainedPromise.get(), (int) IsPending());
+    if (!IsPending()) {
+      ForwardTo(chainedPromise);
+    } else {
+      mChainedPromises.AppendElement(chainedPromise);
+    }
+  }
+
+  // Note we expose the function AssertIsDead() instead of IsDead() since
+  // checking IsDead() is a data race in the situation where the request is not
+  // dead. Therefore we enforce the form |Assert(IsDead())| by exposing
+  // AssertIsDead() only.
+  void AssertIsDead()
+  {
+    PROMISE_ASSERT(mMagic1 == sMagic && mMagic2 == sMagic && mMagic3 == sMagic && mMagic4 == mMutex.mLock);
+    MutexAutoLock lock(mMutex);
+    for (auto&& then : mThenValues) {
+      then->AssertIsDead();
+    }
+    for (auto&& chained : mChainedPromises) {
+      chained->AssertIsDead();
+    }
+  }
+
+protected:
+  bool IsPending() const { return mValue.IsNothing(); }
+  const ResolveOrRejectValue& Value() const
+  {
+    // This method should only be called once the value has stabilized. As
+    // such, we don't need to acquire the lock here.
+    MOZ_DIAGNOSTIC_ASSERT(!IsPending());
+    return mValue;
+  }
+
+  void DispatchAll()
+  {
+    mMutex.AssertCurrentThreadOwns();
+    for (size_t i = 0; i < mThenValues.Length(); ++i) {
+      mThenValues[i]->Dispatch(this);
+    }
+    mThenValues.Clear();
+
+    for (size_t i = 0; i < mChainedPromises.Length(); ++i) {
+      ForwardTo(mChainedPromises[i]);
+    }
+    mChainedPromises.Clear();
+  }
+
+  void ForwardTo(Private* aOther)
+  {
+    MOZ_ASSERT(!IsPending());
+    if (mValue.IsResolve()) {
+      aOther->Resolve(mValue.ResolveValue(), "<chained promise>");
+    } else {
+      aOther->Reject(mValue.RejectValue(), "<chained promise>");
+    }
+  }
+
+  virtual ~MozPromise()
+  {
+    PROMISE_LOG("MozPromise::~MozPromise [this=%p]", this);
+    AssertIsDead();
+    // We can't guarantee a completion promise will always be revolved or
+    // rejected since ResolveOrRejectRunnable might not run when dispatch fails.
+    if (!mIsCompletionPromise) {
+      MOZ_ASSERT(!IsPending());
+      MOZ_ASSERT(mThenValues.IsEmpty());
+      MOZ_ASSERT(mChainedPromises.IsEmpty());
+    }
+#ifdef PROMISE_DEBUG
+    mMagic1 = 0;
+    mMagic2 = 0;
+    mMagic3 = 0;
+    mMagic4 = nullptr;
+#endif
+  };
+
+  const char* mCreationSite; // For logging
+  Mutex mMutex;
+  ResolveOrRejectValue mValue;
+#ifdef PROMISE_DEBUG
+  uint32_t mMagic1 = sMagic;
+#endif
+  nsTArray<RefPtr<ThenValueBase>> mThenValues;
+#ifdef PROMISE_DEBUG
+  uint32_t mMagic2 = sMagic;
+#endif
+  nsTArray<RefPtr<Private>> mChainedPromises;
+#ifdef PROMISE_DEBUG
+  uint32_t mMagic3 = sMagic;
+#endif
+  bool mHaveRequest;
+  const bool mIsCompletionPromise;
+#ifdef PROMISE_DEBUG
+  void* mMagic4;
+#endif
+};
+
+template<typename ResolveValueT, typename RejectValueT, bool IsExclusive>
+class MozPromise<ResolveValueT, RejectValueT, IsExclusive>::Private
+  : public MozPromise<ResolveValueT, RejectValueT, IsExclusive>
+{
+public:
+  explicit Private(const char* aCreationSite, bool aIsCompletionPromise = false)
+    : MozPromise(aCreationSite, aIsCompletionPromise) {}
+
+  template<typename ResolveValueT_>
+  void Resolve(ResolveValueT_&& aResolveValue, const char* aResolveSite)
+  {
+    PROMISE_ASSERT(mMagic1 == sMagic && mMagic2 == sMagic && mMagic3 == sMagic && mMagic4 == mMutex.mLock);
+    MutexAutoLock lock(mMutex);
+    MOZ_ASSERT(IsPending());
+    PROMISE_LOG("%s resolving MozPromise (%p created at %s)", aResolveSite, this, mCreationSite);
+    mValue.SetResolve(Forward<ResolveValueT_>(aResolveValue));
+    DispatchAll();
+  }
+
+  template<typename RejectValueT_>
+  void Reject(RejectValueT_&& aRejectValue, const char* aRejectSite)
+  {
+    PROMISE_ASSERT(mMagic1 == sMagic && mMagic2 == sMagic && mMagic3 == sMagic && mMagic4 == mMutex.mLock);
+    MutexAutoLock lock(mMutex);
+    MOZ_ASSERT(IsPending());
+    PROMISE_LOG("%s rejecting MozPromise (%p created at %s)", aRejectSite, this, mCreationSite);
+    mValue.SetReject(Forward<RejectValueT_>(aRejectValue));
+    DispatchAll();
+  }
+
+  template<typename ResolveOrRejectValue_>
+  void ResolveOrReject(ResolveOrRejectValue_&& aValue, const char* aSite)
+  {
+    PROMISE_ASSERT(mMagic1 == sMagic && mMagic2 == sMagic && mMagic3 == sMagic && mMagic4 == mMutex.mLock);
+    MutexAutoLock lock(mMutex);
+    MOZ_ASSERT(IsPending());
+    PROMISE_LOG("%s resolveOrRejecting MozPromise (%p created at %s)", aSite, this, mCreationSite);
+    mValue = Forward<ResolveOrRejectValue_>(aValue);
+    DispatchAll();
+  }
+};
+
+// A generic promise type that does the trick for simple use cases.
+typedef MozPromise<bool, nsresult, /* IsExclusive = */ false> GenericPromise;
+
+/*
+ * Class to encapsulate a promise for a particular role. Use this as the member
+ * variable for a class whose method returns a promise.
+ */
+template<typename PromiseType>
+class MozPromiseHolder
+{
+public:
+  MozPromiseHolder()
+    : mMonitor(nullptr) {}
+
+  // Move semantics.
+  MozPromiseHolder& operator=(MozPromiseHolder&& aOther)
+  {
+    MOZ_ASSERT(!mMonitor && !aOther.mMonitor);
+    MOZ_DIAGNOSTIC_ASSERT(!mPromise);
+    mPromise = aOther.mPromise;
+    aOther.mPromise = nullptr;
+    return *this;
+  }
+
+  ~MozPromiseHolder() { MOZ_ASSERT(!mPromise); }
+
+  already_AddRefed<PromiseType> Ensure(const char* aMethodName) {
+    if (mMonitor) {
+      mMonitor->AssertCurrentThreadOwns();
+    }
+    if (!mPromise) {
+      mPromise = new (typename PromiseType::Private)(aMethodName);
+    }
+    RefPtr<PromiseType> p = mPromise.get();
+    return p.forget();
+  }
+
+  // Provide a Monitor that should always be held when accessing this instance.
+  void SetMonitor(Monitor* aMonitor) { mMonitor = aMonitor; }
+
+  bool IsEmpty() const
+  {
+    if (mMonitor) {
+      mMonitor->AssertCurrentThreadOwns();
+    }
+    return !mPromise;
+  }
+
+  already_AddRefed<typename PromiseType::Private> Steal()
+  {
+    if (mMonitor) {
+      mMonitor->AssertCurrentThreadOwns();
+    }
+
+    RefPtr<typename PromiseType::Private> p = mPromise;
+    mPromise = nullptr;
+    return p.forget();
+  }
+
+  void Resolve(typename PromiseType::ResolveValueType aResolveValue,
+               const char* aMethodName)
+  {
+    if (mMonitor) {
+      mMonitor->AssertCurrentThreadOwns();
+    }
+    MOZ_ASSERT(mPromise);
+    mPromise->Resolve(aResolveValue, aMethodName);
+    mPromise = nullptr;
+  }
+
+
+  void ResolveIfExists(typename PromiseType::ResolveValueType aResolveValue,
+                       const char* aMethodName)
+  {
+    if (!IsEmpty()) {
+      Resolve(aResolveValue, aMethodName);
+    }
+  }
+
+  void Reject(typename PromiseType::RejectValueType aRejectValue,
+              const char* aMethodName)
+  {
+    if (mMonitor) {
+      mMonitor->AssertCurrentThreadOwns();
+    }
+    MOZ_ASSERT(mPromise);
+    mPromise->Reject(aRejectValue, aMethodName);
+    mPromise = nullptr;
+  }
+
+
+  void RejectIfExists(typename PromiseType::RejectValueType aRejectValue,
+                      const char* aMethodName)
+  {
+    if (!IsEmpty()) {
+      Reject(aRejectValue, aMethodName);
+    }
+  }
+
+private:
+  Monitor* mMonitor;
+  RefPtr<typename PromiseType::Private> mPromise;
+};
+
+/*
+ * Class to encapsulate a MozPromise::Request reference. Use this as the member
+ * variable for a class waiting on a MozPromise.
+ */
+template<typename PromiseType>
+class MozPromiseRequestHolder
+{
+public:
+  MozPromiseRequestHolder() {}
+  ~MozPromiseRequestHolder() { MOZ_ASSERT(!mRequest); }
+
+  void Begin(RefPtr<typename PromiseType::Request>&& aRequest)
+  {
+    MOZ_DIAGNOSTIC_ASSERT(!Exists());
+    mRequest = Move(aRequest);
+  }
+
+  void Begin(typename PromiseType::Request* aRequest)
+  {
+    MOZ_DIAGNOSTIC_ASSERT(!Exists());
+    mRequest = aRequest;
+  }
+
+  void Complete()
+  {
+    MOZ_DIAGNOSTIC_ASSERT(Exists());
+    mRequest = nullptr;
+  }
+
+  // Disconnects and forgets an outstanding promise. The resolve/reject methods
+  // will never be called.
+  void Disconnect() {
+    MOZ_ASSERT(Exists());
+    mRequest->Disconnect();
+    mRequest = nullptr;
+  }
+
+  void DisconnectIfExists() {
+    if (Exists()) {
+      Disconnect();
+    }
+  }
+
+  bool Exists() const { return !!mRequest; }
+
+private:
+  RefPtr<typename PromiseType::Request> mRequest;
+};
+
+// Asynchronous Potentially-Cross-Thread Method Calls.
+//
+// This machinery allows callers to schedule a promise-returning method to be
+// invoked asynchronously on a given thread, while at the same time receiving
+// a promise upon which to invoke Then() immediately. InvokeAsync dispatches
+// a task to invoke the method on the proper thread and also chain the resulting
+// promise to the one that the caller received, so that resolve/reject values
+// are forwarded through.
+
+namespace detail {
+
+template<typename ReturnType, typename ThisType, typename... ArgTypes, size_t... Indices>
+ReturnType
+MethodCallInvokeHelper(ReturnType(ThisType::*aMethod)(ArgTypes...), ThisType* aThisVal,
+                       Tuple<ArgTypes...>& aArgs, IndexSequence<Indices...>)
+{
+  return ((*aThisVal).*aMethod)(Get<Indices>(aArgs)...);
+}
+
+// Non-templated base class to allow us to use MOZ_COUNT_{C,D}TOR, which cause
+// assertions when used on templated types.
+class MethodCallBase
+{
+public:
+  MethodCallBase() { MOZ_COUNT_CTOR(MethodCallBase); }
+  virtual ~MethodCallBase() { MOZ_COUNT_DTOR(MethodCallBase); }
+};
+
+template<typename PromiseType, typename ThisType, typename... ArgTypes>
+class MethodCall : public MethodCallBase
+{
+public:
+  typedef RefPtr<PromiseType>(ThisType::*MethodType)(ArgTypes...);
+  MethodCall(MethodType aMethod, ThisType* aThisVal, ArgTypes... aArgs)
+    : mMethod(aMethod)
+    , mThisVal(aThisVal)
+    , mArgs(Forward<ArgTypes>(aArgs)...)
+  {}
+
+  RefPtr<PromiseType> Invoke()
+  {
+    return MethodCallInvokeHelper(mMethod, mThisVal.get(), mArgs, typename IndexSequenceFor<ArgTypes...>::Type());
+  }
+
+private:
+  MethodType mMethod;
+  RefPtr<ThisType> mThisVal;
+  Tuple<ArgTypes...> mArgs;
+};
+
+template<typename PromiseType, typename ThisType, typename ...ArgTypes>
+class ProxyRunnable : public Runnable
+{
+public:
+  ProxyRunnable(typename PromiseType::Private* aProxyPromise, MethodCall<PromiseType, ThisType, ArgTypes...>* aMethodCall)
+    : mProxyPromise(aProxyPromise), mMethodCall(aMethodCall) {}
+
+  NS_IMETHOD Run() override
+  {
+    RefPtr<PromiseType> p = mMethodCall->Invoke();
+    mMethodCall = nullptr;
+    p->ChainTo(mProxyPromise.forget(), "<Proxy Promise>");
+    return NS_OK;
+  }
+
+private:
+  RefPtr<typename PromiseType::Private> mProxyPromise;
+  nsAutoPtr<MethodCall<PromiseType, ThisType, ArgTypes...>> mMethodCall;
+};
+
+constexpr bool Any()
+{
+  return false;
+}
+
+template <typename T1>
+constexpr bool Any(T1 a)
+{
+  return static_cast<bool>(a);
+}
+
+template <typename T1, typename... Ts>
+constexpr bool Any(T1 a, Ts... aOthers)
+{
+  return a || Any(aOthers...);
+}
+
+} // namespace detail
+
+template<typename PromiseType, typename ThisType, typename ...ArgTypes, typename ...ActualArgTypes>
+static RefPtr<PromiseType>
+InvokeAsync(AbstractThread* aTarget, ThisType* aThisVal, const char* aCallerName,
+            RefPtr<PromiseType>(ThisType::*aMethod)(ArgTypes...), ActualArgTypes&&... aArgs)
+{
+  static_assert(!detail::Any(IsReference<ArgTypes>::value...),
+                "Cannot pass reference types through InvokeAsync, see bug 1313497 if you require it");
+  typedef detail::MethodCall<PromiseType, ThisType, ArgTypes...> MethodCallType;
+  typedef detail::ProxyRunnable<PromiseType, ThisType, ArgTypes...> ProxyRunnableType;
+
+  MethodCallType* methodCall = new MethodCallType(aMethod, aThisVal, Forward<ActualArgTypes>(aArgs)...);
+  RefPtr<typename PromiseType::Private> p = new (typename PromiseType::Private)(aCallerName);
+  RefPtr<ProxyRunnableType> r = new ProxyRunnableType(p, methodCall);
+  MOZ_ASSERT(aTarget->IsDispatchReliable());
+  aTarget->Dispatch(r.forget());
+  return p.forget();
+}
+
+#undef PROMISE_LOG
+#undef PROMISE_ASSERT
+#undef PROMISE_DEBUG
+
+} // namespace mozilla
+
+#endif
diff --git a/xpcom/threads/SharedThreadPool.cpp b/xpcom/threads/SharedThreadPool.cpp
new file mode 100644
index 000000000..9adf6449e
--- /dev/null
+++ b/xpcom/threads/SharedThreadPool.cpp
@@ -0,0 +1,224 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "mozilla/SharedThreadPool.h"
+#include "mozilla/Monitor.h"
+#include "mozilla/ReentrantMonitor.h"
+#include "mozilla/Services.h"
+#include "mozilla/StaticPtr.h"
+#include "nsDataHashtable.h"
+#include "nsXPCOMCIDInternal.h"
+#include "nsComponentManagerUtils.h"
+#include "nsIObserver.h"
+#include "nsIObserverService.h"
+#ifdef XP_WIN
+#include "ThreadPoolCOMListener.h"
+#endif
+
+namespace mozilla {
+
+// Created and destroyed on the main thread.
+static StaticAutoPtr<ReentrantMonitor> sMonitor;
+
+// Hashtable, maps thread pool name to SharedThreadPool instance.
+// Modified only on the main thread.
+static StaticAutoPtr<nsDataHashtable<nsCStringHashKey, SharedThreadPool*>> sPools;
+
+static already_AddRefed<nsIThreadPool>
+CreateThreadPool(const nsCString& aName);
+
+class SharedThreadPoolShutdownObserver : public nsIObserver
+{
+public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIOBSERVER
+protected:
+  virtual ~SharedThreadPoolShutdownObserver() {}
+};
+
+NS_IMPL_ISUPPORTS(SharedThreadPoolShutdownObserver, nsIObserver, nsISupports)
+
+NS_IMETHODIMP
+SharedThreadPoolShutdownObserver::Observe(nsISupports* aSubject, const char *aTopic,
+                                          const char16_t *aData)
+{
+  MOZ_RELEASE_ASSERT(!strcmp(aTopic, "xpcom-shutdown-threads"));
+  SharedThreadPool::SpinUntilEmpty();
+  sMonitor = nullptr;
+  sPools = nullptr;
+  return NS_OK;
+}
+
+void
+SharedThreadPool::InitStatics()
+{
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(!sMonitor && !sPools);
+  sMonitor = new ReentrantMonitor("SharedThreadPool");
+  sPools = new nsDataHashtable<nsCStringHashKey, SharedThreadPool*>();
+  nsCOMPtr<nsIObserverService> obsService = mozilla::services::GetObserverService();
+  nsCOMPtr<nsIObserver> obs = new SharedThreadPoolShutdownObserver();
+  obsService->AddObserver(obs, "xpcom-shutdown-threads", false);
+}
+
+/* static */
+bool
+SharedThreadPool::IsEmpty()
+{
+  ReentrantMonitorAutoEnter mon(*sMonitor);
+  return !sPools->Count();
+}
+
+/* static */
+void
+SharedThreadPool::SpinUntilEmpty()
+{
+  MOZ_ASSERT(NS_IsMainThread());
+  while (!IsEmpty()) {
+    sMonitor->AssertNotCurrentThreadIn();
+    NS_ProcessNextEvent(NS_GetCurrentThread(), true);
+  }
+}
+
+already_AddRefed<SharedThreadPool>
+SharedThreadPool::Get(const nsCString& aName, uint32_t aThreadLimit)
+{
+  MOZ_ASSERT(sMonitor && sPools);
+  ReentrantMonitorAutoEnter mon(*sMonitor);
+  SharedThreadPool* pool = nullptr;
+  nsresult rv;
+  if (!sPools->Get(aName, &pool)) {
+    nsCOMPtr<nsIThreadPool> threadPool(CreateThreadPool(aName));
+    NS_ENSURE_TRUE(threadPool, nullptr);
+    pool = new SharedThreadPool(aName, threadPool);
+
+    // Set the thread and idle limits. Note that we don't rely on the
+    // EnsureThreadLimitIsAtLeast() call below, as the default thread limit
+    // is 4, and if aThreadLimit is less than 4 we'll end up with a pool
+    // with 4 threads rather than what we expected; so we'll have unexpected
+    // behaviour.
+    rv = pool->SetThreadLimit(aThreadLimit);
+    NS_ENSURE_SUCCESS(rv, nullptr);
+
+    rv = pool->SetIdleThreadLimit(aThreadLimit);
+    NS_ENSURE_SUCCESS(rv, nullptr);
+
+    sPools->Put(aName, pool);
+  } else if (NS_FAILED(pool->EnsureThreadLimitIsAtLeast(aThreadLimit))) {
+    NS_WARNING("Failed to set limits on thread pool");
+  }
+
+  MOZ_ASSERT(pool);
+  RefPtr<SharedThreadPool> instance(pool);
+  return instance.forget();
+}
+
+NS_IMETHODIMP_(MozExternalRefCountType) SharedThreadPool::AddRef(void)
+{
+  MOZ_ASSERT(sMonitor);
+  ReentrantMonitorAutoEnter mon(*sMonitor);
+  MOZ_ASSERT(int32_t(mRefCnt) >= 0, "illegal refcnt");
+  nsrefcnt count = ++mRefCnt;
+  NS_LOG_ADDREF(this, count, "SharedThreadPool", sizeof(*this));
+  return count;
+}
+
+NS_IMETHODIMP_(MozExternalRefCountType) SharedThreadPool::Release(void)
+{
+  MOZ_ASSERT(sMonitor);
+  ReentrantMonitorAutoEnter mon(*sMonitor);
+  nsrefcnt count = --mRefCnt;
+  NS_LOG_RELEASE(this, count, "SharedThreadPool");
+  if (count) {
+    return count;
+  }
+
+  // Remove SharedThreadPool from table of pools.
+  sPools->Remove(mName);
+  MOZ_ASSERT(!sPools->Get(mName));
+
+  // Dispatch an event to the main thread to call Shutdown() on
+  // the nsIThreadPool. The Runnable here will add a refcount to the pool,
+  // and when the Runnable releases the nsIThreadPool it will be deleted.
+  NS_DispatchToMainThread(NewRunnableMethod(mPool, &nsIThreadPool::Shutdown));
+
+  // Stabilize refcount, so that if something in the dtor QIs, it won't explode.
+  mRefCnt = 1;
+  delete this;
+  return 0;
+}
+
+NS_IMPL_QUERY_INTERFACE(SharedThreadPool, nsIThreadPool, nsIEventTarget)
+
+SharedThreadPool::SharedThreadPool(const nsCString& aName,
+                                   nsIThreadPool* aPool)
+  : mName(aName)
+  , mPool(aPool)
+  , mRefCnt(0)
+{
+  MOZ_COUNT_CTOR(SharedThreadPool);
+  mEventTarget = do_QueryInterface(aPool);
+}
+
+SharedThreadPool::~SharedThreadPool()
+{
+  MOZ_COUNT_DTOR(SharedThreadPool);
+}
+
+nsresult
+SharedThreadPool::EnsureThreadLimitIsAtLeast(uint32_t aLimit)
+{
+  // We limit the number of threads that we use. Note that we
+  // set the thread limit to the same as the idle limit so that we're not
+  // constantly creating and destroying threads (see Bug 881954). When the
+  // thread pool threads shutdown they dispatch an event to the main thread
+  // to call nsIThread::Shutdown(), and if we're very busy that can take a
+  // while to run, and we end up with dozens of extra threads. Note that
+  // threads that are idle for 60 seconds are shutdown naturally.
+  uint32_t existingLimit = 0;
+  nsresult rv;
+
+  rv = mPool->GetThreadLimit(&existingLimit);
+  NS_ENSURE_SUCCESS(rv, rv);
+  if (aLimit > existingLimit) {
+    rv = mPool->SetThreadLimit(aLimit);
+    NS_ENSURE_SUCCESS(rv, rv);
+  }
+
+  rv = mPool->GetIdleThreadLimit(&existingLimit);
+  NS_ENSURE_SUCCESS(rv, rv);
+  if (aLimit > existingLimit) {
+    rv = mPool->SetIdleThreadLimit(aLimit);
+    NS_ENSURE_SUCCESS(rv, rv);
+  }
+
+  return NS_OK;
+}
+
+static already_AddRefed<nsIThreadPool>
+CreateThreadPool(const nsCString& aName)
+{
+  nsresult rv;
+  nsCOMPtr<nsIThreadPool> pool = do_CreateInstance(NS_THREADPOOL_CONTRACTID, &rv);
+  NS_ENSURE_SUCCESS(rv, nullptr);
+
+  rv = pool->SetName(aName);
+  NS_ENSURE_SUCCESS(rv, nullptr);
+
+  rv = pool->SetThreadStackSize(SharedThreadPool::kStackSize);
+  NS_ENSURE_SUCCESS(rv, nullptr);
+
+#ifdef XP_WIN
+  // Ensure MSCOM is initialized on the thread pools threads.
+  nsCOMPtr<nsIThreadPoolListener> listener = new MSCOMInitThreadPoolListener();
+  rv = pool->SetListener(listener);
+  NS_ENSURE_SUCCESS(rv, nullptr);
+#endif
+
+  return pool.forget();
+}
+
+} // namespace mozilla
diff --git a/xpcom/threads/SharedThreadPool.h b/xpcom/threads/SharedThreadPool.h
new file mode 100644
index 000000000..185b9e76f
--- /dev/null
+++ b/xpcom/threads/SharedThreadPool.h
@@ -0,0 +1,129 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 SharedThreadPool_h_
+#define SharedThreadPool_h_
+
+#include <queue>
+#include "mozilla/RefPtr.h"
+#include "nsThreadUtils.h"
+#include "nsIThreadManager.h"
+#include "nsIThreadPool.h"
+#include "nsISupports.h"
+#include "nsISupportsImpl.h"
+#include "nsCOMPtr.h"
+
+namespace mozilla {
+
+// Wrapper that makes an nsIThreadPool a singleton, and provides a
+// consistent threadsafe interface to get instances. Callers simply get a
+// SharedThreadPool by the name of its nsIThreadPool. All get requests of
+// the same name get the same SharedThreadPool. Users must store a reference
+// to the pool, and when the last reference to a SharedThreadPool is dropped
+// the pool is shutdown and deleted. Users aren't required to manually
+// shutdown the pool, and can release references on any thread. This can make
+// it significantly easier to use thread pools, because the caller doesn't need
+// to worry about joining and tearing it down.
+//
+// On Windows all threads in the pool have MSCOM initialized with
+// COINIT_MULTITHREADED. Note that not all users of MSCOM use this mode see [1],
+// and mixing MSCOM objects between the two is terrible for performance, and can
+// cause some functions to fail. So be careful when using Win32 APIs on a
+// SharedThreadPool, and avoid sharing objects if at all possible.
+//
+// [1] https://dxr.mozilla.org/mozilla-central/search?q=coinitialize&redirect=false
+class SharedThreadPool : public nsIThreadPool
+{
+public:
+
+  // Gets (possibly creating) the shared thread pool singleton instance with
+  // thread pool named aName.
+  static already_AddRefed<SharedThreadPool> Get(const nsCString& aName,
+                                            uint32_t aThreadLimit = 4);
+
+  // We implement custom threadsafe AddRef/Release pair, that destroys the
+  // the shared pool singleton when the refcount drops to 0. The addref/release
+  // are implemented using locking, so it's not recommended that you use them
+  // in a tight loop.
+  NS_IMETHOD QueryInterface(REFNSIID aIID, void** aInstancePtr) override;
+  NS_IMETHOD_(MozExternalRefCountType) AddRef(void) override;
+  NS_IMETHOD_(MozExternalRefCountType) Release(void) override;
+
+  // Forward behaviour to wrapped thread pool implementation.
+  NS_FORWARD_SAFE_NSITHREADPOOL(mPool);
+
+  // Call this when dispatching from an event on the same
+  // threadpool that is about to complete. We should not create a new thread
+  // in that case since a thread is about to become idle.
+  nsresult DispatchFromEndOfTaskInThisPool(nsIRunnable *event)
+  {
+    return Dispatch(event, NS_DISPATCH_AT_END);
+  }
+
+  NS_IMETHOD DispatchFromScript(nsIRunnable *event, uint32_t flags) override {
+      return Dispatch(event, flags);
+  }
+
+  NS_IMETHOD Dispatch(already_AddRefed<nsIRunnable> event, uint32_t flags) override
+    { return !mEventTarget ? NS_ERROR_NULL_POINTER : mEventTarget->Dispatch(Move(event), flags); }
+
+  NS_IMETHOD DelayedDispatch(already_AddRefed<nsIRunnable>, uint32_t) override
+    { return NS_ERROR_NOT_IMPLEMENTED; }
+
+  using nsIEventTarget::Dispatch;
+
+  NS_IMETHOD IsOnCurrentThread(bool *_retval) override { return !mEventTarget ? NS_ERROR_NULL_POINTER : mEventTarget->IsOnCurrentThread(_retval); }
+
+  // Creates necessary statics. Called once at startup.
+  static void InitStatics();
+
+  // Spins the event loop until all thread pools are shutdown.
+  // *Must* be called on the main thread.
+  static void SpinUntilEmpty();
+
+#if defined(MOZ_ASAN)
+  // Use the system default in ASAN builds, because the default is assumed to be
+  // larger than the size we want to use and is hopefully sufficient for ASAN.
+  static const uint32_t kStackSize = nsIThreadManager::DEFAULT_STACK_SIZE;
+#elif defined(XP_WIN) || defined(XP_MACOSX) || defined(LINUX)
+  static const uint32_t kStackSize = (256 * 1024);
+#else
+  // All other platforms use their system defaults.
+  static const uint32_t kStackSize = nsIThreadManager::DEFAULT_STACK_SIZE;
+#endif
+
+private:
+
+  // Returns whether there are no pools in existence at the moment.
+  static bool IsEmpty();
+
+  // Creates a singleton SharedThreadPool wrapper around aPool.
+  // aName is the name of the aPool, and is used to lookup the
+  // SharedThreadPool in the hash table of all created pools.
+  SharedThreadPool(const nsCString& aName,
+                   nsIThreadPool* aPool);
+  virtual ~SharedThreadPool();
+
+  nsresult EnsureThreadLimitIsAtLeast(uint32_t aThreadLimit);
+
+  // Name of mPool.
+  const nsCString mName;
+
+  // Thread pool being wrapped.
+  nsCOMPtr<nsIThreadPool> mPool;
+
+  // Refcount. We implement custom ref counting so that the thread pool is
+  // shutdown in a threadsafe manner and singletonness is preserved.
+  nsrefcnt mRefCnt;
+
+  // mPool QI'd to nsIEventTarget. We cache this, so that we can use
+  // NS_FORWARD_SAFE_NSIEVENTTARGET above.
+  nsCOMPtr<nsIEventTarget> mEventTarget;
+};
+
+} // namespace mozilla
+
+#endif // SharedThreadPool_h_
diff --git a/xpcom/threads/StateMirroring.h b/xpcom/threads/StateMirroring.h
new file mode 100644
index 000000000..87d94ba74
--- /dev/null
+++ b/xpcom/threads/StateMirroring.h
@@ -0,0 +1,378 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 !defined(StateMirroring_h_)
+#define StateMirroring_h_
+
+#include "mozilla/Maybe.h"
+#include "mozilla/MozPromise.h"
+#include "mozilla/StateWatching.h"
+#include "mozilla/TaskDispatcher.h"
+#include "mozilla/UniquePtr.h"
+#include "mozilla/Unused.h"
+
+#include "mozilla/Logging.h"
+#include "nsISupportsImpl.h"
+
+/*
+ * The state-mirroring machinery allows pieces of interesting state to be
+ * observed on multiple thread without locking. The basic strategy is to track
+ * changes in a canonical value and post updates to other threads that hold
+ * mirrors for that value.
+ *
+ * One problem with the naive implementation of such a system is that some pieces
+ * of state need to be updated atomically, and certain other operations need to
+ * wait for these atomic updates to complete before executing. The state-mirroring
+ * machinery solves this problem by requiring that its owner thread uses tail
+ * dispatch, and posting state update events (which should always be run first by
+ * TaskDispatcher implementations) to that tail dispatcher. This ensures that
+ * state changes are always atomic from the perspective of observing threads.
+ *
+ * Given that state-mirroring is an automatic background process, we try to avoid
+ * burdening the caller with worrying too much about teardown. To that end, we
+ * don't assert dispatch success for any of the notifications, and assume that
+ * any canonical or mirror owned by a thread for whom dispatch fails will soon
+ * be disconnected by its holder anyway.
+ *
+ * Given that semantics may change and comments tend to go out of date, we
+ * deliberately don't provide usage examples here. Grep around to find them.
+ */
+
+namespace mozilla {
+
+// Mirror<T> and Canonical<T> inherit WatchTarget, so we piggy-back on the
+// logging that WatchTarget already does. Given that, it makes sense to share
+// the same log module.
+#define MIRROR_LOG(x, ...) \
+  MOZ_ASSERT(gStateWatchingLog); \
+  MOZ_LOG(gStateWatchingLog, LogLevel::Debug, (x, ##__VA_ARGS__))
+
+template<typename T> class AbstractMirror;
+
+/*
+ * AbstractCanonical is a superclass from which all Canonical values must
+ * inherit. It serves as the interface of operations which may be performed (via
+ * asynchronous dispatch) by other threads, in particular by the corresponding
+ * Mirror value.
+ */
+template<typename T>
+class AbstractCanonical
+{
+public:
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(AbstractCanonical)
+  AbstractCanonical(AbstractThread* aThread) : mOwnerThread(aThread) {}
+  virtual void AddMirror(AbstractMirror<T>* aMirror) = 0;
+  virtual void RemoveMirror(AbstractMirror<T>* aMirror) = 0;
+
+  AbstractThread* OwnerThread() const { return mOwnerThread; }
+protected:
+  virtual ~AbstractCanonical() {}
+  RefPtr<AbstractThread> mOwnerThread;
+};
+
+/*
+ * AbstractMirror is a superclass from which all Mirror values must
+ * inherit. It serves as the interface of operations which may be performed (via
+ * asynchronous dispatch) by other threads, in particular by the corresponding
+ * Canonical value.
+ */
+template<typename T>
+class AbstractMirror
+{
+public:
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(AbstractMirror)
+  AbstractMirror(AbstractThread* aThread) : mOwnerThread(aThread) {}
+  virtual void UpdateValue(const T& aNewValue) = 0;
+  virtual void NotifyDisconnected() = 0;
+
+  AbstractThread* OwnerThread() const { return mOwnerThread; }
+protected:
+  virtual ~AbstractMirror() {}
+  RefPtr<AbstractThread> mOwnerThread;
+};
+
+/*
+ * Canonical<T> is a wrapper class that allows a given value to be mirrored by other
+ * threads. It maintains a list of active mirrors, and queues updates for them
+ * when the internal value changes. When changing the value, the caller needs to
+ * pass a TaskDispatcher object, which fires the updates at the appropriate time.
+ * Canonical<T> is also a WatchTarget, and may be set up to trigger other routines
+ * (on the same thread) when the canonical value changes.
+ *
+ * Canonical<T> is intended to be used as a member variable, so it doesn't actually
+ * inherit AbstractCanonical<T> (a refcounted type). Rather, it contains an inner
+ * class called |Impl| that implements most of the interesting logic.
+ */
+template<typename T>
+class Canonical
+{
+public:
+  Canonical(AbstractThread* aThread, const T& aInitialValue, const char* aName)
+  {
+    mImpl = new Impl(aThread, aInitialValue, aName);
+  }
+
+
+  ~Canonical() {}
+
+private:
+  class Impl : public AbstractCanonical<T>, public WatchTarget
+  {
+  public:
+    using AbstractCanonical<T>::OwnerThread;
+
+    Impl(AbstractThread* aThread, const T& aInitialValue, const char* aName)
+      : AbstractCanonical<T>(aThread), WatchTarget(aName), mValue(aInitialValue)
+    {
+      MIRROR_LOG("%s [%p] initialized", mName, this);
+      MOZ_ASSERT(aThread->SupportsTailDispatch(), "Can't get coherency without tail dispatch");
+    }
+
+    void AddMirror(AbstractMirror<T>* aMirror) override
+    {
+      MIRROR_LOG("%s [%p] adding mirror %p", mName, this, aMirror);
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      MOZ_ASSERT(!mMirrors.Contains(aMirror));
+      mMirrors.AppendElement(aMirror);
+      aMirror->OwnerThread()->DispatchStateChange(MakeNotifier(aMirror));
+    }
+
+    void RemoveMirror(AbstractMirror<T>* aMirror) override
+    {
+      MIRROR_LOG("%s [%p] removing mirror %p", mName, this, aMirror);
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      MOZ_ASSERT(mMirrors.Contains(aMirror));
+      mMirrors.RemoveElement(aMirror);
+    }
+
+    void DisconnectAll()
+    {
+      MIRROR_LOG("%s [%p] Disconnecting all mirrors", mName, this);
+      for (size_t i = 0; i < mMirrors.Length(); ++i) {
+        mMirrors[i]->OwnerThread()->Dispatch(NewRunnableMethod(mMirrors[i],
+                                                               &AbstractMirror<T>::NotifyDisconnected),
+                                             AbstractThread::DontAssertDispatchSuccess);
+      }
+      mMirrors.Clear();
+    }
+
+    operator const T&()
+    {
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      return mValue;
+    }
+
+    void Set(const T& aNewValue)
+    {
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+
+      if (aNewValue == mValue) {
+        return;
+      }
+
+      // Notify same-thread watchers. The state watching machinery will make sure
+      // that notifications run at the right time.
+      NotifyWatchers();
+
+      // Check if we've already got a pending update. If so we won't schedule another
+      // one.
+      bool alreadyNotifying = mInitialValue.isSome();
+
+      // Stash the initial value if needed, then update to the new value.
+      if (mInitialValue.isNothing()) {
+        mInitialValue.emplace(mValue);
+      }
+      mValue = aNewValue;
+
+      // We wait until things have stablized before sending state updates so that
+      // we can avoid sending multiple updates, and possibly avoid sending any
+      // updates at all if the value ends up where it started.
+      if (!alreadyNotifying) {
+        AbstractThread::DispatchDirectTask(NewRunnableMethod(this, &Impl::DoNotify));
+      }
+    }
+
+    Impl& operator=(const T& aNewValue) { Set(aNewValue); return *this; }
+    Impl& operator=(const Impl& aOther) { Set(aOther); return *this; }
+    Impl(const Impl& aOther) = delete;
+
+  protected:
+    ~Impl() { MOZ_DIAGNOSTIC_ASSERT(mMirrors.IsEmpty()); }
+
+  private:
+    void DoNotify()
+    {
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      MOZ_ASSERT(mInitialValue.isSome());
+      bool same = mInitialValue.ref() == mValue;
+      mInitialValue.reset();
+
+      if (same) {
+        MIRROR_LOG("%s [%p] unchanged - not sending update", mName, this);
+        return;
+      }
+
+      for (size_t i = 0; i < mMirrors.Length(); ++i) {
+        mMirrors[i]->OwnerThread()->DispatchStateChange(MakeNotifier(mMirrors[i]));
+      }
+    }
+
+    already_AddRefed<nsIRunnable> MakeNotifier(AbstractMirror<T>* aMirror)
+    {
+      return NewRunnableMethod<T>(aMirror, &AbstractMirror<T>::UpdateValue, mValue);;
+    }
+
+    T mValue;
+    Maybe<T> mInitialValue;
+    nsTArray<RefPtr<AbstractMirror<T>>> mMirrors;
+  };
+public:
+
+  // NB: Because mirror-initiated disconnection can race with canonical-
+  // initiated disconnection, a canonical should never be reinitialized.
+  // Forward control operations to the Impl.
+  void DisconnectAll() { return mImpl->DisconnectAll(); }
+
+  // Access to the Impl.
+  operator Impl&() { return *mImpl; }
+  Impl* operator&() { return mImpl; }
+
+  // Access to the T.
+  const T& Ref() const { return *mImpl; }
+  operator const T&() const { return Ref(); }
+  void Set(const T& aNewValue) { mImpl->Set(aNewValue); }
+  Canonical& operator=(const T& aNewValue) { Set(aNewValue); return *this; }
+  Canonical& operator=(const Canonical& aOther) { Set(aOther); return *this; }
+  Canonical(const Canonical& aOther) = delete;
+
+private:
+  RefPtr<Impl> mImpl;
+};
+
+/*
+ * Mirror<T> is a wrapper class that allows a given value to mirror that of a
+ * Canonical<T> owned by another thread. It registers itself with a Canonical<T>,
+ * and is periodically updated with new values. Mirror<T> is also a WatchTarget,
+ * and may be set up to trigger other routines (on the same thread) when the
+ * mirrored value changes.
+ *
+ * Mirror<T> is intended to be used as a member variable, so it doesn't actually
+ * inherit AbstractMirror<T> (a refcounted type). Rather, it contains an inner
+ * class called |Impl| that implements most of the interesting logic.
+ */
+template<typename T>
+class Mirror
+{
+public:
+  Mirror(AbstractThread* aThread, const T& aInitialValue, const char* aName)
+  {
+    mImpl = new Impl(aThread, aInitialValue, aName);
+  }
+
+  ~Mirror()
+  {
+    // As a member of complex objects, a Mirror<T> may be destroyed on a
+    // different thread than its owner, or late in shutdown during CC. Given
+    // that, we require manual disconnection so that callers can put things in
+    // the right place.
+    MOZ_DIAGNOSTIC_ASSERT(!mImpl->IsConnected());
+  }
+
+private:
+  class Impl : public AbstractMirror<T>, public WatchTarget
+  {
+  public:
+    using AbstractMirror<T>::OwnerThread;
+
+    Impl(AbstractThread* aThread, const T& aInitialValue, const char* aName)
+      : AbstractMirror<T>(aThread), WatchTarget(aName), mValue(aInitialValue)
+    {
+      MIRROR_LOG("%s [%p] initialized", mName, this);
+      MOZ_ASSERT(aThread->SupportsTailDispatch(), "Can't get coherency without tail dispatch");
+    }
+
+    operator const T&()
+    {
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      return mValue;
+    }
+
+    virtual void UpdateValue(const T& aNewValue) override
+    {
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      if (mValue != aNewValue) {
+        mValue = aNewValue;
+        WatchTarget::NotifyWatchers();
+      }
+    }
+
+    virtual void NotifyDisconnected() override
+    {
+      MIRROR_LOG("%s [%p] Notifed of disconnection from %p", mName, this, mCanonical.get());
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      mCanonical = nullptr;
+    }
+
+    bool IsConnected() const { return !!mCanonical; }
+
+    void Connect(AbstractCanonical<T>* aCanonical)
+    {
+      MIRROR_LOG("%s [%p] Connecting to %p", mName, this, aCanonical);
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      MOZ_ASSERT(!IsConnected());
+      MOZ_ASSERT(OwnerThread()->RequiresTailDispatch(aCanonical->OwnerThread()), "Can't get coherency without tail dispatch");
+
+      nsCOMPtr<nsIRunnable> r = NewRunnableMethod<StorensRefPtrPassByPtr<AbstractMirror<T>>>
+                                  (aCanonical, &AbstractCanonical<T>::AddMirror, this);
+      aCanonical->OwnerThread()->Dispatch(r.forget(), AbstractThread::DontAssertDispatchSuccess);
+      mCanonical = aCanonical;
+    }
+  public:
+
+    void DisconnectIfConnected()
+    {
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      if (!IsConnected()) {
+        return;
+      }
+
+      MIRROR_LOG("%s [%p] Disconnecting from %p", mName, this, mCanonical.get());
+      nsCOMPtr<nsIRunnable> r = NewRunnableMethod<StorensRefPtrPassByPtr<AbstractMirror<T>>>
+                                  (mCanonical, &AbstractCanonical<T>::RemoveMirror, this);
+      mCanonical->OwnerThread()->Dispatch(r.forget(), AbstractThread::DontAssertDispatchSuccess);
+      mCanonical = nullptr;
+    }
+
+  protected:
+    ~Impl() { MOZ_DIAGNOSTIC_ASSERT(!IsConnected()); }
+
+  private:
+    T mValue;
+    RefPtr<AbstractCanonical<T>> mCanonical;
+  };
+public:
+
+  // Forward control operations to the Impl<T>.
+  void Connect(AbstractCanonical<T>* aCanonical) { mImpl->Connect(aCanonical); }
+  void DisconnectIfConnected() { mImpl->DisconnectIfConnected(); }
+
+  // Access to the Impl<T>.
+  operator Impl&() { return *mImpl; }
+  Impl* operator&() { return mImpl; }
+
+  // Access to the T.
+  const T& Ref() const { return *mImpl; }
+  operator const T&() const { return Ref(); }
+
+private:
+  RefPtr<Impl> mImpl;
+};
+
+#undef MIRROR_LOG
+
+} // namespace mozilla
+
+#endif
diff --git a/xpcom/threads/StateWatching.h b/xpcom/threads/StateWatching.h
new file mode 100644
index 000000000..99d521603
--- /dev/null
+++ b/xpcom/threads/StateWatching.h
@@ -0,0 +1,317 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 !defined(StateWatching_h_)
+#define StateWatching_h_
+
+#include "mozilla/AbstractThread.h"
+#include "mozilla/Logging.h"
+#include "mozilla/TaskDispatcher.h"
+#include "mozilla/UniquePtr.h"
+#include "mozilla/Unused.h"
+
+#include "nsISupportsImpl.h"
+
+/*
+ * The state-watching machinery automates the process of responding to changes
+ * in various pieces of state.
+ *
+ * A standard programming pattern is as follows:
+ *
+ * mFoo = ...;
+ * NotifyStuffChanged();
+ * ...
+ * mBar = ...;
+ * NotifyStuffChanged();
+ *
+ * This pattern is error-prone and difficult to audit because it requires the
+ * programmer to manually trigger the update routine. This can be especially
+ * problematic when the update routine depends on numerous pieces of state, and
+ * when that state is modified across a variety of helper methods. In these
+ * cases the responsibility for invoking the routine is often unclear, causing
+ * developers to scatter calls to it like pixie dust. This can result in
+ * duplicate invocations (which is wasteful) and missing invocations in corner-
+ * cases (which is a source of bugs).
+ *
+ * This file provides a set of primitives that automatically handle updates and
+ * allow the programmers to explicitly construct a graph of state dependencies.
+ * When used correctly, it eliminates the guess-work and wasted cycles described
+ * above.
+ *
+ * There are two basic pieces:
+ *   (1) Objects that can be watched for updates. These inherit WatchTarget.
+ *   (2) Objects that receive objects and trigger processing. These inherit
+ *       AbstractWatcher. In the current machinery, these exist only internally
+ *       within the WatchManager, though that could change.
+ *
+ * Note that none of this machinery is thread-safe - it must all happen on the
+ * same owning thread. To solve multi-threaded use-cases, use state mirroring
+ * and watch the mirrored value.
+ *
+ * Given that semantics may change and comments tend to go out of date, we
+ * deliberately don't provide usage examples here. Grep around to find them.
+ */
+
+namespace mozilla {
+
+extern LazyLogModule gStateWatchingLog;
+
+#define WATCH_LOG(x, ...) \
+  MOZ_LOG(gStateWatchingLog, LogLevel::Debug, (x, ##__VA_ARGS__))
+
+/*
+ * AbstractWatcher is a superclass from which all watchers must inherit.
+ */
+class AbstractWatcher
+{
+public:
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(AbstractWatcher)
+  AbstractWatcher() : mDestroyed(false) {}
+  bool IsDestroyed() { return mDestroyed; }
+  virtual void Notify() = 0;
+
+protected:
+  virtual ~AbstractWatcher() { MOZ_ASSERT(mDestroyed); }
+  bool mDestroyed;
+};
+
+/*
+ * WatchTarget is a superclass from which all watchable things must inherit.
+ * Unlike AbstractWatcher, it is a fully-implemented Mix-in, and the subclass
+ * needs only to invoke NotifyWatchers when something changes.
+ *
+ * The functionality that this class provides is not threadsafe, and should only
+ * be used on the thread that owns that WatchTarget.
+ */
+class WatchTarget
+{
+public:
+  explicit WatchTarget(const char* aName) : mName(aName) {}
+
+  void AddWatcher(AbstractWatcher* aWatcher)
+  {
+    MOZ_ASSERT(!mWatchers.Contains(aWatcher));
+    mWatchers.AppendElement(aWatcher);
+  }
+
+  void RemoveWatcher(AbstractWatcher* aWatcher)
+  {
+    MOZ_ASSERT(mWatchers.Contains(aWatcher));
+    mWatchers.RemoveElement(aWatcher);
+  }
+
+protected:
+  void NotifyWatchers()
+  {
+    WATCH_LOG("%s[%p] notifying watchers\n", mName, this);
+    PruneWatchers();
+    for (size_t i = 0; i < mWatchers.Length(); ++i) {
+      mWatchers[i]->Notify();
+    }
+  }
+
+private:
+  // We don't have Watchers explicitly unregister themselves when they die,
+  // because then they'd need back-references to all the WatchTargets they're
+  // subscribed to, and WatchTargets aren't reference-counted. So instead we
+  // just prune dead ones at appropriate times, which works just fine.
+  void PruneWatchers()
+  {
+    for (int i = mWatchers.Length() - 1; i >= 0; --i) {
+      if (mWatchers[i]->IsDestroyed()) {
+        mWatchers.RemoveElementAt(i);
+      }
+    }
+  }
+
+  nsTArray<RefPtr<AbstractWatcher>> mWatchers;
+
+protected:
+  const char* mName;
+};
+
+/*
+ * Watchable is a wrapper class that turns any primitive into a WatchTarget.
+ */
+template<typename T>
+class Watchable : public WatchTarget
+{
+public:
+  Watchable(const T& aInitialValue, const char* aName)
+    : WatchTarget(aName), mValue(aInitialValue) {}
+
+  const T& Ref() const { return mValue; }
+  operator const T&() const { return Ref(); }
+  Watchable& operator=(const T& aNewValue)
+  {
+    if (aNewValue != mValue) {
+      mValue = aNewValue;
+      NotifyWatchers();
+    }
+
+    return *this;
+  }
+
+private:
+  Watchable(const Watchable& aOther); // Not implemented
+  Watchable& operator=(const Watchable& aOther); // Not implemented
+
+  T mValue;
+};
+
+// Manager class for state-watching. Declare one of these in any class for which
+// you want to invoke method callbacks.
+//
+// Internally, WatchManager maintains one AbstractWatcher per callback method.
+// Consumers invoke Watch/Unwatch on a particular (WatchTarget, Callback) tuple.
+// This causes an AbstractWatcher for |Callback| to be instantiated if it doesn't
+// already exist, and registers it with |WatchTarget|.
+//
+// Using Direct Tasks on the TailDispatcher, WatchManager ensures that we fire
+// watch callbacks no more than once per task, once all other operations for that
+// task have been completed.
+//
+// WatchManager<OwnerType> is intended to be declared as a member of |OwnerType|
+// objects. Given that, it and its owned objects can't hold permanent strong refs to
+// the owner, since that would keep the owner alive indefinitely. Instead, it
+// _only_ holds strong refs while waiting for Direct Tasks to fire. This ensures
+// that everything is kept alive just long enough.
+template <typename OwnerType>
+class WatchManager
+{
+public:
+  typedef void(OwnerType::*CallbackMethod)();
+  explicit WatchManager(OwnerType* aOwner, AbstractThread* aOwnerThread)
+    : mOwner(aOwner), mOwnerThread(aOwnerThread) {}
+
+  ~WatchManager()
+  {
+    if (!IsShutdown()) {
+      Shutdown();
+    }
+  }
+
+  bool IsShutdown() const { return !mOwner; }
+
+  // Shutdown needs to happen on mOwnerThread. If the WatchManager will be
+  // destroyed on a different thread, Shutdown() must be called manually.
+  void Shutdown()
+  {
+    MOZ_ASSERT(mOwnerThread->IsCurrentThreadIn());
+    for (size_t i = 0; i < mWatchers.Length(); ++i) {
+      mWatchers[i]->Destroy();
+    }
+    mWatchers.Clear();
+    mOwner = nullptr;
+  }
+
+  void Watch(WatchTarget& aTarget, CallbackMethod aMethod)
+  {
+    MOZ_ASSERT(mOwnerThread->IsCurrentThreadIn());
+    aTarget.AddWatcher(&EnsureWatcher(aMethod));
+  }
+
+  void Unwatch(WatchTarget& aTarget, CallbackMethod aMethod)
+  {
+    MOZ_ASSERT(mOwnerThread->IsCurrentThreadIn());
+    PerCallbackWatcher* watcher = GetWatcher(aMethod);
+    MOZ_ASSERT(watcher);
+    aTarget.RemoveWatcher(watcher);
+  }
+
+  void ManualNotify(CallbackMethod aMethod)
+  {
+    MOZ_ASSERT(mOwnerThread->IsCurrentThreadIn());
+    PerCallbackWatcher* watcher = GetWatcher(aMethod);
+    MOZ_ASSERT(watcher);
+    watcher->Notify();
+  }
+
+private:
+  class PerCallbackWatcher : public AbstractWatcher
+  {
+  public:
+    PerCallbackWatcher(OwnerType* aOwner, AbstractThread* aOwnerThread, CallbackMethod aMethod)
+      : mOwner(aOwner), mOwnerThread(aOwnerThread), mCallbackMethod(aMethod) {}
+
+    void Destroy()
+    {
+      MOZ_ASSERT(mOwnerThread->IsCurrentThreadIn());
+      mDestroyed = true;
+      mOwner = nullptr;
+    }
+
+    void Notify() override
+    {
+      MOZ_ASSERT(mOwnerThread->IsCurrentThreadIn());
+      MOZ_DIAGNOSTIC_ASSERT(mOwner, "mOwner is only null after destruction, "
+                                    "at which point we shouldn't be notified");
+      if (mStrongRef) {
+        // We've already got a notification job in the pipe.
+        return;
+      }
+      mStrongRef = mOwner; // Hold the owner alive while notifying.
+
+      // Queue up our notification jobs to run in a stable state.
+      mOwnerThread->TailDispatcher().AddDirectTask(NewRunnableMethod(this, &PerCallbackWatcher::DoNotify));
+    }
+
+    bool CallbackMethodIs(CallbackMethod aMethod) const
+    {
+      return mCallbackMethod == aMethod;
+    }
+
+  private:
+    ~PerCallbackWatcher() {}
+
+    void DoNotify()
+    {
+      MOZ_ASSERT(mOwnerThread->IsCurrentThreadIn());
+      MOZ_ASSERT(mStrongRef);
+      RefPtr<OwnerType> ref = mStrongRef.forget();
+      if (!mDestroyed) {
+        ((*ref).*mCallbackMethod)();
+      }
+    }
+
+    OwnerType* mOwner; // Never null.
+    RefPtr<OwnerType> mStrongRef; // Only non-null when notifying.
+    RefPtr<AbstractThread> mOwnerThread;
+    CallbackMethod mCallbackMethod;
+  };
+
+  PerCallbackWatcher* GetWatcher(CallbackMethod aMethod)
+  {
+    MOZ_ASSERT(mOwnerThread->IsCurrentThreadIn());
+    for (size_t i = 0; i < mWatchers.Length(); ++i) {
+      if (mWatchers[i]->CallbackMethodIs(aMethod)) {
+        return mWatchers[i];
+      }
+    }
+    return nullptr;
+  }
+
+  PerCallbackWatcher& EnsureWatcher(CallbackMethod aMethod)
+  {
+    MOZ_ASSERT(mOwnerThread->IsCurrentThreadIn());
+    PerCallbackWatcher* watcher = GetWatcher(aMethod);
+    if (watcher) {
+      return *watcher;
+    }
+    watcher = mWatchers.AppendElement(new PerCallbackWatcher(mOwner, mOwnerThread, aMethod))->get();
+    return *watcher;
+  }
+
+  nsTArray<RefPtr<PerCallbackWatcher>> mWatchers;
+  OwnerType* mOwner;
+  RefPtr<AbstractThread> mOwnerThread;
+};
+
+#undef WATCH_LOG
+
+} // namespace mozilla
+
+#endif
diff --git a/xpcom/threads/SyncRunnable.h b/xpcom/threads/SyncRunnable.h
new file mode 100644
index 000000000..d96bac7ba
--- /dev/null
+++ b/xpcom/threads/SyncRunnable.h
@@ -0,0 +1,129 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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_SyncRunnable_h
+#define mozilla_SyncRunnable_h
+
+#include "nsThreadUtils.h"
+#include "mozilla/AbstractThread.h"
+#include "mozilla/Monitor.h"
+#include "mozilla/Move.h"
+
+namespace mozilla {
+
+/**
+ * This class will wrap a nsIRunnable and dispatch it to the main thread
+ * synchronously. This is different from nsIEventTarget.DISPATCH_SYNC:
+ * this class does not spin the event loop waiting for the event to be
+ * dispatched. This means that you don't risk reentrance from pending
+ * messages, but you must be sure that the target thread does not ever block
+ * on this thread, or else you will deadlock.
+ *
+ * Typical usage:
+ * RefPtr<SyncRunnable> sr = new SyncRunnable(new myrunnable...());
+ * sr->DispatchToThread(t);
+ *
+ * We also provide a convenience wrapper:
+ * SyncRunnable::DispatchToThread(new myrunnable...());
+ *
+ */
+class SyncRunnable : public Runnable
+{
+public:
+  explicit SyncRunnable(nsIRunnable* aRunnable)
+    : mRunnable(aRunnable)
+    , mMonitor("SyncRunnable")
+    , mDone(false)
+  {
+  }
+
+  explicit SyncRunnable(already_AddRefed<nsIRunnable> aRunnable)
+    : mRunnable(Move(aRunnable))
+    , mMonitor("SyncRunnable")
+    , mDone(false)
+  {
+  }
+
+  void DispatchToThread(nsIEventTarget* aThread, bool aForceDispatch = false)
+  {
+    nsresult rv;
+    bool on;
+
+    if (!aForceDispatch) {
+      rv = aThread->IsOnCurrentThread(&on);
+      MOZ_ASSERT(NS_SUCCEEDED(rv));
+      if (NS_SUCCEEDED(rv) && on) {
+        mRunnable->Run();
+        return;
+      }
+    }
+
+    rv = aThread->Dispatch(this, NS_DISPATCH_NORMAL);
+    if (NS_SUCCEEDED(rv)) {
+      mozilla::MonitorAutoLock lock(mMonitor);
+      while (!mDone) {
+        lock.Wait();
+      }
+    }
+  }
+
+  void DispatchToThread(AbstractThread* aThread, bool aForceDispatch = false)
+  {
+    if (!aForceDispatch && aThread->IsCurrentThreadIn()) {
+      mRunnable->Run();
+      return;
+    }
+
+    // Check we don't have tail dispatching here. Otherwise we will deadlock
+    // ourself when spinning the loop below.
+    MOZ_ASSERT(!aThread->RequiresTailDispatchFromCurrentThread());
+
+    aThread->Dispatch(RefPtr<nsIRunnable>(this).forget());
+    mozilla::MonitorAutoLock lock(mMonitor);
+    while (!mDone) {
+      lock.Wait();
+    }
+  }
+
+  static void DispatchToThread(nsIEventTarget* aThread,
+                               nsIRunnable* aRunnable,
+                               bool aForceDispatch = false)
+  {
+    RefPtr<SyncRunnable> s(new SyncRunnable(aRunnable));
+    s->DispatchToThread(aThread, aForceDispatch);
+  }
+
+  static void DispatchToThread(AbstractThread* aThread,
+                               nsIRunnable* aRunnable,
+                               bool aForceDispatch = false)
+  {
+    RefPtr<SyncRunnable> s(new SyncRunnable(aRunnable));
+    s->DispatchToThread(aThread, aForceDispatch);
+  }
+
+protected:
+  NS_IMETHOD Run() override
+  {
+    mRunnable->Run();
+
+    mozilla::MonitorAutoLock lock(mMonitor);
+    MOZ_ASSERT(!mDone);
+
+    mDone = true;
+    mMonitor.Notify();
+
+    return NS_OK;
+  }
+
+private:
+  nsCOMPtr<nsIRunnable> mRunnable;
+  mozilla::Monitor mMonitor;
+  bool mDone;
+};
+
+} // namespace mozilla
+
+#endif // mozilla_SyncRunnable_h
diff --git a/xpcom/threads/TaskDispatcher.h b/xpcom/threads/TaskDispatcher.h
new file mode 100644
index 000000000..405c3acfe
--- /dev/null
+++ b/xpcom/threads/TaskDispatcher.h
@@ -0,0 +1,276 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 !defined(TaskDispatcher_h_)
+#define TaskDispatcher_h_
+
+#include "mozilla/AbstractThread.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/UniquePtr.h"
+#include "mozilla/Unused.h"
+
+#include "nsISupportsImpl.h"
+#include "nsTArray.h"
+#include "nsThreadUtils.h"
+
+#include <queue>
+
+namespace mozilla {
+
+/*
+ * A classic approach to cross-thread communication is to dispatch asynchronous
+ * runnables to perform updates on other threads. This generally works well, but
+ * there are sometimes reasons why we might want to delay the actual dispatch of
+ * these tasks until a specified moment. At present, this is primarily useful to
+ * ensure that mirrored state gets updated atomically - but there may be other
+ * applications as well.
+ *
+ * TaskDispatcher is a general abstract class that accepts tasks and dispatches
+ * them at some later point. These groups of tasks are per-target-thread, and
+ * contain separate queues for several kinds of tasks (see comments  below). - "state change tasks" (which
+ * run first, and are intended to be used to update the value held by mirrors),
+ * and regular tasks, which are other arbitrary operations that the are gated
+ * to run after all the state changes have completed.
+ */
+class TaskDispatcher
+{
+public:
+  TaskDispatcher() {}
+  virtual ~TaskDispatcher() {}
+
+  // Direct tasks are run directly (rather than dispatched asynchronously) when
+  // the tail dispatcher fires. A direct task may cause other tasks to be added
+  // to the tail dispatcher.
+  virtual void AddDirectTask(already_AddRefed<nsIRunnable> aRunnable) = 0;
+
+  // State change tasks are dispatched asynchronously always run before regular
+  // tasks. They are intended to be used to update the value held by mirrors
+  // before any other dispatched tasks are run on the target thread.
+  virtual void AddStateChangeTask(AbstractThread* aThread,
+                                  already_AddRefed<nsIRunnable> aRunnable) = 0;
+
+  // Regular tasks are dispatched asynchronously, and run after state change
+  // tasks.
+  virtual void AddTask(AbstractThread* aThread,
+                       already_AddRefed<nsIRunnable> aRunnable,
+                       AbstractThread::DispatchFailureHandling aFailureHandling = AbstractThread::AssertDispatchSuccess) = 0;
+
+  virtual void DispatchTasksFor(AbstractThread* aThread) = 0;
+  virtual bool HasTasksFor(AbstractThread* aThread) = 0;
+  virtual void DrainDirectTasks() = 0;
+};
+
+/*
+ * AutoTaskDispatcher is a stack-scoped TaskDispatcher implementation that fires
+ * its queued tasks when it is popped off the stack.
+ */
+class AutoTaskDispatcher : public TaskDispatcher
+{
+public:
+  explicit AutoTaskDispatcher(bool aIsTailDispatcher = false)
+    : mIsTailDispatcher(aIsTailDispatcher)
+  {}
+
+  ~AutoTaskDispatcher()
+  {
+    // Given that direct tasks may trigger other code that uses the tail
+    // dispatcher, it's better to avoid processing them in the tail dispatcher's
+    // destructor. So we require TailDispatchers to manually invoke
+    // DrainDirectTasks before the AutoTaskDispatcher gets destroyed. In truth,
+    // this is only necessary in the case where this AutoTaskDispatcher can be
+    // accessed by the direct tasks it dispatches (true for TailDispatchers, but
+    // potentially not true for other hypothetical AutoTaskDispatchers). Feel
+    // free to loosen this restriction to apply only to mIsTailDispatcher if a
+    // use-case requires it.
+    MOZ_ASSERT(!HaveDirectTasks());
+
+    for (size_t i = 0; i < mTaskGroups.Length(); ++i) {
+      DispatchTaskGroup(Move(mTaskGroups[i]));
+    }
+  }
+
+  bool HaveDirectTasks() const
+  {
+    return mDirectTasks.isSome() && !mDirectTasks->empty();
+  }
+
+  void DrainDirectTasks() override
+  {
+    while (HaveDirectTasks()) {
+      nsCOMPtr<nsIRunnable> r = mDirectTasks->front();
+      mDirectTasks->pop();
+      r->Run();
+    }
+  }
+
+  void AddDirectTask(already_AddRefed<nsIRunnable> aRunnable) override
+  {
+    if (mDirectTasks.isNothing()) {
+      mDirectTasks.emplace();
+    }
+    mDirectTasks->push(Move(aRunnable));
+  }
+
+  void AddStateChangeTask(AbstractThread* aThread,
+                          already_AddRefed<nsIRunnable> aRunnable) override
+  {
+    EnsureTaskGroup(aThread).mStateChangeTasks.AppendElement(aRunnable);
+  }
+
+  void AddTask(AbstractThread* aThread,
+               already_AddRefed<nsIRunnable> aRunnable,
+               AbstractThread::DispatchFailureHandling aFailureHandling) override
+  {
+    PerThreadTaskGroup& group = EnsureTaskGroup(aThread);
+    group.mRegularTasks.AppendElement(aRunnable);
+
+    // The task group needs to assert dispatch success if any of the runnables
+    // it's dispatching want to assert it.
+    if (aFailureHandling == AbstractThread::AssertDispatchSuccess) {
+      group.mFailureHandling = AbstractThread::AssertDispatchSuccess;
+    }
+  }
+
+  bool HasTasksFor(AbstractThread* aThread) override
+  {
+    return !!GetTaskGroup(aThread) ||
+           (aThread == AbstractThread::GetCurrent() && HaveDirectTasks());
+  }
+
+  void DispatchTasksFor(AbstractThread* aThread) override
+  {
+    for (size_t i = 0; i < mTaskGroups.Length(); ++i) {
+      if (mTaskGroups[i]->mThread == aThread) {
+        DispatchTaskGroup(Move(mTaskGroups[i]));
+        mTaskGroups.RemoveElementAt(i);
+        return;
+      }
+    }
+  }
+
+private:
+
+  struct PerThreadTaskGroup
+  {
+  public:
+    explicit PerThreadTaskGroup(AbstractThread* aThread)
+      : mThread(aThread), mFailureHandling(AbstractThread::DontAssertDispatchSuccess)
+    {
+      MOZ_COUNT_CTOR(PerThreadTaskGroup);
+    }
+
+    ~PerThreadTaskGroup() { MOZ_COUNT_DTOR(PerThreadTaskGroup); }
+
+    RefPtr<AbstractThread> mThread;
+    nsTArray<nsCOMPtr<nsIRunnable>> mStateChangeTasks;
+    nsTArray<nsCOMPtr<nsIRunnable>> mRegularTasks;
+    AbstractThread::DispatchFailureHandling mFailureHandling;
+  };
+
+  class TaskGroupRunnable : public Runnable
+  {
+    public:
+      explicit TaskGroupRunnable(UniquePtr<PerThreadTaskGroup>&& aTasks) : mTasks(Move(aTasks)) {}
+
+      NS_IMETHOD Run() override
+      {
+        // State change tasks get run all together before any code is run, so
+        // that all state changes are made in an atomic unit.
+        for (size_t i = 0; i < mTasks->mStateChangeTasks.Length(); ++i) {
+          mTasks->mStateChangeTasks[i]->Run();
+        }
+
+        // Once the state changes have completed, drain any direct tasks
+        // generated by those state changes (i.e. watcher notification tasks).
+        // This needs to be outside the loop because we don't want to run code
+        // that might observe intermediate states.
+        MaybeDrainDirectTasks();
+
+        for (size_t i = 0; i < mTasks->mRegularTasks.Length(); ++i) {
+          mTasks->mRegularTasks[i]->Run();
+
+          // Scope direct tasks tightly to the task that generated them.
+          MaybeDrainDirectTasks();
+        }
+
+        return NS_OK;
+      }
+
+    private:
+      void MaybeDrainDirectTasks()
+      {
+        AbstractThread* currentThread = AbstractThread::GetCurrent();
+        if (currentThread) {
+          currentThread->TailDispatcher().DrainDirectTasks();
+        }
+      }
+
+      UniquePtr<PerThreadTaskGroup> mTasks;
+  };
+
+  PerThreadTaskGroup& EnsureTaskGroup(AbstractThread* aThread)
+  {
+    PerThreadTaskGroup* existing = GetTaskGroup(aThread);
+    if (existing) {
+      return *existing;
+    }
+
+    mTaskGroups.AppendElement(new PerThreadTaskGroup(aThread));
+    return *mTaskGroups.LastElement();
+  }
+
+  PerThreadTaskGroup* GetTaskGroup(AbstractThread* aThread)
+  {
+    for (size_t i = 0; i < mTaskGroups.Length(); ++i) {
+      if (mTaskGroups[i]->mThread == aThread) {
+        return mTaskGroups[i].get();
+      }
+    }
+
+    // Not found.
+    return nullptr;
+  }
+
+  void DispatchTaskGroup(UniquePtr<PerThreadTaskGroup> aGroup)
+  {
+    RefPtr<AbstractThread> thread = aGroup->mThread;
+
+    AbstractThread::DispatchFailureHandling failureHandling = aGroup->mFailureHandling;
+    AbstractThread::DispatchReason reason = mIsTailDispatcher ? AbstractThread::TailDispatch
+                                                              : AbstractThread::NormalDispatch;
+    nsCOMPtr<nsIRunnable> r = new TaskGroupRunnable(Move(aGroup));
+    thread->Dispatch(r.forget(), failureHandling, reason);
+  }
+
+  // Direct tasks. We use a Maybe<> because (a) this class is hot, (b)
+  // mDirectTasks often doesn't get anything put into it, and (c) the
+  // std::queue implementation in GNU libstdc++ does two largish heap
+  // allocations when creating a new std::queue.
+  mozilla::Maybe<std::queue<nsCOMPtr<nsIRunnable>>> mDirectTasks;
+
+  // Task groups, organized by thread.
+  nsTArray<UniquePtr<PerThreadTaskGroup>> mTaskGroups;
+
+  // True if this TaskDispatcher represents the tail dispatcher for the thread
+  // upon which it runs.
+  const bool mIsTailDispatcher;
+};
+
+// Little utility class to allow declaring AutoTaskDispatcher as a default
+// parameter for methods that take a TaskDispatcher&.
+template<typename T>
+class PassByRef
+{
+public:
+  PassByRef() {}
+  operator T&() { return mVal; }
+private:
+  T mVal;
+};
+
+} // namespace mozilla
+
+#endif
diff --git a/xpcom/threads/TaskQueue.cpp b/xpcom/threads/TaskQueue.cpp
new file mode 100644
index 000000000..2e593a773
--- /dev/null
+++ b/xpcom/threads/TaskQueue.cpp
@@ -0,0 +1,271 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "mozilla/TaskQueue.h"
+
+#include "nsIEventTarget.h"
+#include "nsThreadUtils.h"
+
+namespace mozilla {
+
+class TaskQueue::EventTargetWrapper final : public nsIEventTarget
+{
+  RefPtr<TaskQueue> mTaskQueue;
+
+  ~EventTargetWrapper()
+  {
+  }
+
+public:
+  explicit EventTargetWrapper(TaskQueue* aTaskQueue)
+    : mTaskQueue(aTaskQueue)
+  {
+    MOZ_ASSERT(mTaskQueue);
+  }
+
+  NS_IMETHOD
+  DispatchFromScript(nsIRunnable* aEvent, uint32_t aFlags) override
+  {
+    nsCOMPtr<nsIRunnable> ref = aEvent;
+    return Dispatch(ref.forget(), aFlags);
+  }
+
+  NS_IMETHOD
+  Dispatch(already_AddRefed<nsIRunnable> aEvent, uint32_t aFlags) override
+  {
+    nsCOMPtr<nsIRunnable> runnable = aEvent;
+    MonitorAutoLock mon(mTaskQueue->mQueueMonitor);
+    return mTaskQueue->DispatchLocked(/* passed by ref */runnable,
+                                      DontAssertDispatchSuccess,
+                                      NormalDispatch);
+  }
+
+  NS_IMETHOD
+  DelayedDispatch(already_AddRefed<nsIRunnable>, uint32_t aFlags) override
+  {
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+
+  NS_IMETHOD
+  IsOnCurrentThread(bool* aResult) override
+  {
+    *aResult = mTaskQueue->IsCurrentThreadIn();
+    return NS_OK;
+  }
+
+  NS_DECL_THREADSAFE_ISUPPORTS
+};
+
+NS_IMPL_ISUPPORTS(TaskQueue::EventTargetWrapper, nsIEventTarget)
+
+TaskQueue::TaskQueue(already_AddRefed<nsIEventTarget> aTarget,
+                     bool aRequireTailDispatch)
+  : AbstractThread(aRequireTailDispatch)
+  , mTarget(aTarget)
+  , mQueueMonitor("TaskQueue::Queue")
+  , mTailDispatcher(nullptr)
+  , mIsRunning(false)
+  , mIsShutdown(false)
+{
+  MOZ_COUNT_CTOR(TaskQueue);
+}
+
+TaskQueue::~TaskQueue()
+{
+  MonitorAutoLock mon(mQueueMonitor);
+  MOZ_ASSERT(mIsShutdown);
+  MOZ_COUNT_DTOR(TaskQueue);
+}
+
+TaskDispatcher&
+TaskQueue::TailDispatcher()
+{
+  MOZ_ASSERT(IsCurrentThreadIn());
+  MOZ_ASSERT(mTailDispatcher);
+  return *mTailDispatcher;
+}
+
+// Note aRunnable is passed by ref to support conditional ownership transfer.
+// See Dispatch() in TaskQueue.h for more details.
+nsresult
+TaskQueue::DispatchLocked(nsCOMPtr<nsIRunnable>& aRunnable,
+                          DispatchFailureHandling aFailureHandling,
+                          DispatchReason aReason)
+{
+  mQueueMonitor.AssertCurrentThreadOwns();
+  if (mIsShutdown) {
+    return NS_ERROR_FAILURE;
+  }
+
+  AbstractThread* currentThread;
+  if (aReason != TailDispatch && (currentThread = GetCurrent()) && RequiresTailDispatch(currentThread)) {
+    currentThread->TailDispatcher().AddTask(this, aRunnable.forget(), aFailureHandling);
+    return NS_OK;
+  }
+
+  mTasks.push(aRunnable.forget());
+  if (mIsRunning) {
+    return NS_OK;
+  }
+  RefPtr<nsIRunnable> runner(new Runner(this));
+  nsresult rv = mTarget->Dispatch(runner.forget(), NS_DISPATCH_NORMAL);
+  if (NS_FAILED(rv)) {
+    NS_WARNING("Failed to dispatch runnable to run TaskQueue");
+    return rv;
+  }
+  mIsRunning = true;
+
+  return NS_OK;
+}
+
+void
+TaskQueue::AwaitIdle()
+{
+  MonitorAutoLock mon(mQueueMonitor);
+  AwaitIdleLocked();
+}
+
+void
+TaskQueue::AwaitIdleLocked()
+{
+  // Make sure there are no tasks for this queue waiting in the caller's tail
+  // dispatcher.
+  MOZ_ASSERT_IF(AbstractThread::GetCurrent(),
+                !AbstractThread::GetCurrent()->HasTailTasksFor(this));
+
+  mQueueMonitor.AssertCurrentThreadOwns();
+  MOZ_ASSERT(mIsRunning || mTasks.empty());
+  while (mIsRunning) {
+    mQueueMonitor.Wait();
+  }
+}
+
+void
+TaskQueue::AwaitShutdownAndIdle()
+{
+  MOZ_ASSERT(!IsCurrentThreadIn());
+  // Make sure there are no tasks for this queue waiting in the caller's tail
+  // dispatcher.
+  MOZ_ASSERT_IF(AbstractThread::GetCurrent(),
+                !AbstractThread::GetCurrent()->HasTailTasksFor(this));
+
+  MonitorAutoLock mon(mQueueMonitor);
+  while (!mIsShutdown) {
+    mQueueMonitor.Wait();
+  }
+  AwaitIdleLocked();
+}
+
+RefPtr<ShutdownPromise>
+TaskQueue::BeginShutdown()
+{
+  // Dispatch any tasks for this queue waiting in the caller's tail dispatcher,
+  // since this is the last opportunity to do so.
+  if (AbstractThread* currentThread = AbstractThread::GetCurrent()) {
+    currentThread->TailDispatchTasksFor(this);
+  }
+
+  MonitorAutoLock mon(mQueueMonitor);
+  mIsShutdown = true;
+  RefPtr<ShutdownPromise> p = mShutdownPromise.Ensure(__func__);
+  MaybeResolveShutdown();
+  mon.NotifyAll();
+  return p;
+}
+
+bool
+TaskQueue::IsEmpty()
+{
+  MonitorAutoLock mon(mQueueMonitor);
+  return mTasks.empty();
+}
+
+uint32_t
+TaskQueue::ImpreciseLengthForHeuristics()
+{
+  MonitorAutoLock mon(mQueueMonitor);
+  return mTasks.size();
+}
+
+bool
+TaskQueue::IsCurrentThreadIn()
+{
+  bool in = NS_GetCurrentThread() == mRunningThread;
+  return in;
+}
+
+already_AddRefed<nsIEventTarget>
+TaskQueue::WrapAsEventTarget()
+{
+  nsCOMPtr<nsIEventTarget> ref = new EventTargetWrapper(this);
+  return ref.forget();
+}
+
+nsresult
+TaskQueue::Runner::Run()
+{
+  RefPtr<nsIRunnable> event;
+  {
+    MonitorAutoLock mon(mQueue->mQueueMonitor);
+    MOZ_ASSERT(mQueue->mIsRunning);
+    if (mQueue->mTasks.size() == 0) {
+      mQueue->mIsRunning = false;
+      mQueue->MaybeResolveShutdown();
+      mon.NotifyAll();
+      return NS_OK;
+    }
+    event = mQueue->mTasks.front().forget();
+    mQueue->mTasks.pop();
+  }
+  MOZ_ASSERT(event);
+
+  // Note that dropping the queue monitor before running the task, and
+  // taking the monitor again after the task has run ensures we have memory
+  // fences enforced. This means that if the object we're calling wasn't
+  // designed to be threadsafe, it will be, provided we're only calling it
+  // in this task queue.
+  {
+    AutoTaskGuard g(mQueue);
+    event->Run();
+  }
+
+  // Drop the reference to event. The event will hold a reference to the
+  // object it's calling, and we don't want to keep it alive, it may be
+  // making assumptions what holds references to it. This is especially
+  // the case if the object is waiting for us to shutdown, so that it
+  // can shutdown (like in the MediaDecoderStateMachine's SHUTDOWN case).
+  event = nullptr;
+
+  {
+    MonitorAutoLock mon(mQueue->mQueueMonitor);
+    if (mQueue->mTasks.size() == 0) {
+      // No more events to run. Exit the task runner.
+      mQueue->mIsRunning = false;
+      mQueue->MaybeResolveShutdown();
+      mon.NotifyAll();
+      return NS_OK;
+    }
+  }
+
+  // There's at least one more event that we can run. Dispatch this Runner
+  // to the target again to ensure it runs again. Note that we don't just
+  // run in a loop here so that we don't hog the target. This means we may
+  // run on another thread next time, but we rely on the memory fences from
+  // mQueueMonitor for thread safety of non-threadsafe tasks.
+  nsresult rv = mQueue->mTarget->Dispatch(this, NS_DISPATCH_AT_END);
+  if (NS_FAILED(rv)) {
+    // Failed to dispatch, shutdown!
+    MonitorAutoLock mon(mQueue->mQueueMonitor);
+    mQueue->mIsRunning = false;
+    mQueue->mIsShutdown = true;
+    mQueue->MaybeResolveShutdown();
+    mon.NotifyAll();
+  }
+
+  return NS_OK;
+}
+
+} // namespace mozilla
diff --git a/xpcom/threads/TaskQueue.h b/xpcom/threads/TaskQueue.h
new file mode 100644
index 000000000..aafd206a7
--- /dev/null
+++ b/xpcom/threads/TaskQueue.h
@@ -0,0 +1,203 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 TaskQueue_h_
+#define TaskQueue_h_
+
+#include "mozilla/Monitor.h"
+#include "mozilla/MozPromise.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/TaskDispatcher.h"
+#include "mozilla/Unused.h"
+
+#include <queue>
+
+#include "nsThreadUtils.h"
+
+class nsIEventTarget;
+class nsIRunnable;
+
+namespace mozilla {
+
+typedef MozPromise<bool, bool, false> ShutdownPromise;
+
+// Abstracts executing runnables in order on an arbitrary event target. The
+// runnables dispatched to the TaskQueue will be executed in the order in which
+// they're received, and are guaranteed to not be executed concurrently.
+// They may be executed on different threads, and a memory barrier is used
+// to make this threadsafe for objects that aren't already threadsafe.
+//
+// Note, since a TaskQueue can also be converted to an nsIEventTarget using
+// WrapAsEventTarget() its possible to construct a hierarchy of TaskQueues.
+// Consider these three TaskQueues:
+//
+//  TQ1 dispatches to the main thread
+//  TQ2 dispatches to TQ1
+//  TQ3 dispatches to TQ1
+//
+// This ensures there is only ever a single runnable from the entire chain on
+// the main thread.  It also ensures that TQ2 and TQ3 only have a single runnable
+// in TQ1 at any time.
+//
+// This arrangement lets you prioritize work by dispatching runnables directly
+// to TQ1.  You can issue many runnables for important work.  Meanwhile the TQ2
+// and TQ3 work will always execute at most one runnable and then yield.
+class TaskQueue : public AbstractThread
+{
+  class EventTargetWrapper;
+
+public:
+  explicit TaskQueue(already_AddRefed<nsIEventTarget> aTarget,
+                     bool aSupportsTailDispatch = false);
+
+  TaskDispatcher& TailDispatcher() override;
+
+  TaskQueue* AsTaskQueue() override { return this; }
+
+  void Dispatch(already_AddRefed<nsIRunnable> aRunnable,
+                DispatchFailureHandling aFailureHandling = AssertDispatchSuccess,
+                DispatchReason aReason = NormalDispatch) override
+  {
+    nsCOMPtr<nsIRunnable> r = aRunnable;
+    {
+      MonitorAutoLock mon(mQueueMonitor);
+      nsresult rv = DispatchLocked(/* passed by ref */r, aFailureHandling, aReason);
+      MOZ_DIAGNOSTIC_ASSERT(aFailureHandling == DontAssertDispatchSuccess || NS_SUCCEEDED(rv));
+      Unused << rv;
+    }
+    // If the ownership of |r| is not transferred in DispatchLocked() due to
+    // dispatch failure, it will be deleted here outside the lock. We do so
+    // since the destructor of the runnable might access TaskQueue and result
+    // in deadlocks.
+  }
+
+  // Puts the queue in a shutdown state and returns immediately. The queue will
+  // remain alive at least until all the events are drained, because the Runners
+  // hold a strong reference to the task queue, and one of them is always held
+  // by the target event queue when the task queue is non-empty.
+  //
+  // The returned promise is resolved when the queue goes empty.
+  RefPtr<ShutdownPromise> BeginShutdown();
+
+  // Blocks until all task finish executing.
+  void AwaitIdle();
+
+  // Blocks until the queue is flagged for shutdown and all tasks have finished
+  // executing.
+  void AwaitShutdownAndIdle();
+
+  bool IsEmpty();
+  uint32_t ImpreciseLengthForHeuristics();
+
+  // Returns true if the current thread is currently running a Runnable in
+  // the task queue.
+  bool IsCurrentThreadIn() override;
+
+  // Create a new nsIEventTarget wrapper object that dispatches to this
+  // TaskQueue.
+  already_AddRefed<nsIEventTarget> WrapAsEventTarget();
+
+protected:
+  virtual ~TaskQueue();
+
+
+  // Blocks until all task finish executing. Called internally by methods
+  // that need to wait until the task queue is idle.
+  // mQueueMonitor must be held.
+  void AwaitIdleLocked();
+
+  nsresult DispatchLocked(nsCOMPtr<nsIRunnable>& aRunnable,
+                          DispatchFailureHandling aFailureHandling,
+                          DispatchReason aReason = NormalDispatch);
+
+  void MaybeResolveShutdown()
+  {
+    mQueueMonitor.AssertCurrentThreadOwns();
+    if (mIsShutdown && !mIsRunning) {
+      mShutdownPromise.ResolveIfExists(true, __func__);
+      mTarget = nullptr;
+    }
+  }
+
+  nsCOMPtr<nsIEventTarget> mTarget;
+
+  // Monitor that protects the queue and mIsRunning;
+  Monitor mQueueMonitor;
+
+  // Queue of tasks to run.
+  std::queue<nsCOMPtr<nsIRunnable>> mTasks;
+
+  // The thread currently running the task queue. We store a reference
+  // to this so that IsCurrentThreadIn() can tell if the current thread
+  // is the thread currently running in the task queue.
+  //
+  // This may be read on any thread, but may only be written on mRunningThread.
+  // The thread can't die while we're running in it, and we only use it for
+  // pointer-comparison with the current thread anyway - so we make it atomic
+  // and don't refcount it.
+  Atomic<nsIThread*> mRunningThread;
+
+  // RAII class that gets instantiated for each dispatched task.
+  class AutoTaskGuard : public AutoTaskDispatcher
+  {
+  public:
+    explicit AutoTaskGuard(TaskQueue* aQueue)
+      : AutoTaskDispatcher(/* aIsTailDispatcher = */ true), mQueue(aQueue)
+      , mLastCurrentThread(nullptr)
+    {
+      // NB: We don't hold the lock to aQueue here. Don't do anything that
+      // might require it.
+      MOZ_ASSERT(!mQueue->mTailDispatcher);
+      mQueue->mTailDispatcher = this;
+
+      mLastCurrentThread = sCurrentThreadTLS.get();
+      sCurrentThreadTLS.set(aQueue);
+
+      MOZ_ASSERT(mQueue->mRunningThread == nullptr);
+      mQueue->mRunningThread = NS_GetCurrentThread();
+    }
+
+    ~AutoTaskGuard()
+    {
+      DrainDirectTasks();
+
+      MOZ_ASSERT(mQueue->mRunningThread == NS_GetCurrentThread());
+      mQueue->mRunningThread = nullptr;
+
+      sCurrentThreadTLS.set(mLastCurrentThread);
+      mQueue->mTailDispatcher = nullptr;
+    }
+
+  private:
+  TaskQueue* mQueue;
+  AbstractThread* mLastCurrentThread;
+  };
+
+  TaskDispatcher* mTailDispatcher;
+
+  // True if we've dispatched an event to the target to execute events from
+  // the queue.
+  bool mIsRunning;
+
+  // True if we've started our shutdown process.
+  bool mIsShutdown;
+  MozPromiseHolder<ShutdownPromise> mShutdownPromise;
+
+  class Runner : public Runnable {
+  public:
+    explicit Runner(TaskQueue* aQueue)
+      : mQueue(aQueue)
+    {
+    }
+    NS_IMETHOD Run() override;
+  private:
+    RefPtr<TaskQueue> mQueue;
+  };
+};
+
+} // namespace mozilla
+
+#endif // TaskQueue_h_
diff --git a/xpcom/threads/ThreadStackHelper.cpp b/xpcom/threads/ThreadStackHelper.cpp
new file mode 100644
index 000000000..d31bf6359
--- /dev/null
+++ b/xpcom/threads/ThreadStackHelper.cpp
@@ -0,0 +1,726 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "ThreadStackHelper.h"
+#include "MainThreadUtils.h"
+#include "nsJSPrincipals.h"
+#include "nsScriptSecurityManager.h"
+#include "jsfriendapi.h"
+#ifdef MOZ_THREADSTACKHELPER_NATIVE
+#include "shared-libraries.h"
+#endif
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/IntegerPrintfMacros.h"
+#include "mozilla/Move.h"
+#include "mozilla/Scoped.h"
+#include "mozilla/UniquePtr.h"
+#include "mozilla/MemoryChecking.h"
+#include "mozilla/Sprintf.h"
+
+#ifdef __GNUC__
+# pragma GCC diagnostic push
+# pragma GCC diagnostic ignored "-Wshadow"
+#endif
+
+#if defined(MOZ_VALGRIND)
+# include <valgrind/valgrind.h>
+#endif
+
+#include <string.h>
+#include <vector>
+#include <cstdlib>
+
+#ifdef XP_LINUX
+#include <ucontext.h>
+#include <unistd.h>
+#include <sys/syscall.h>
+#endif
+
+#ifdef __GNUC__
+# pragma GCC diagnostic pop // -Wshadow
+#endif
+
+#if defined(XP_LINUX) || defined(XP_MACOSX)
+#include <pthread.h>
+#endif
+
+#ifdef ANDROID
+#ifndef SYS_gettid
+#define SYS_gettid __NR_gettid
+#endif
+#if defined(__arm__) && !defined(__NR_rt_tgsigqueueinfo)
+// Some NDKs don't define this constant even though the kernel supports it.
+#define __NR_rt_tgsigqueueinfo (__NR_SYSCALL_BASE+363)
+#endif
+#ifndef SYS_rt_tgsigqueueinfo
+#define SYS_rt_tgsigqueueinfo __NR_rt_tgsigqueueinfo
+#endif
+#endif
+
+#ifdef MOZ_THREADSTACKHELPER_NATIVE
+#if defined(MOZ_THREADSTACKHELPER_X86) || \
+    defined(MOZ_THREADSTACKHELPER_X64) || \
+    defined(MOZ_THREADSTACKHELPER_ARM)
+// On these architectures, the stack grows downwards (toward lower addresses).
+#define MOZ_THREADSTACKHELPER_STACK_GROWS_DOWN
+#else
+#error "Unsupported architecture"
+#endif
+#endif // MOZ_THREADSTACKHELPER_NATIVE
+
+namespace mozilla {
+
+void
+ThreadStackHelper::Startup()
+{
+#if defined(XP_LINUX)
+  MOZ_ASSERT(NS_IsMainThread());
+  if (!sInitialized) {
+    // TODO: centralize signal number allocation
+    sFillStackSignum = SIGRTMIN + 4;
+    if (sFillStackSignum > SIGRTMAX) {
+      // Leave uninitialized
+      MOZ_ASSERT(false);
+      return;
+    }
+    struct sigaction sigact = {};
+    sigact.sa_sigaction = FillStackHandler;
+    sigemptyset(&sigact.sa_mask);
+    sigact.sa_flags = SA_SIGINFO | SA_RESTART;
+    MOZ_ALWAYS_TRUE(!::sigaction(sFillStackSignum, &sigact, nullptr));
+  }
+  sInitialized++;
+#endif
+}
+
+void
+ThreadStackHelper::Shutdown()
+{
+#if defined(XP_LINUX)
+  MOZ_ASSERT(NS_IsMainThread());
+  if (sInitialized == 1) {
+    struct sigaction sigact = {};
+    sigact.sa_handler = SIG_DFL;
+    MOZ_ALWAYS_TRUE(!::sigaction(sFillStackSignum, &sigact, nullptr));
+  }
+  sInitialized--;
+#endif
+}
+
+ThreadStackHelper::ThreadStackHelper()
+  : mStackToFill(nullptr)
+#ifdef MOZ_THREADSTACKHELPER_PSEUDO
+  , mPseudoStack(mozilla_get_pseudo_stack())
+#ifdef MOZ_THREADSTACKHELPER_NATIVE
+  , mContextToFill(nullptr)
+#endif
+  , mMaxStackSize(Stack::sMaxInlineStorage)
+  , mMaxBufferSize(512)
+#endif
+{
+#if defined(XP_LINUX)
+  MOZ_ALWAYS_TRUE(!::sem_init(&mSem, 0, 0));
+  mThreadID = ::syscall(SYS_gettid);
+#elif defined(XP_WIN)
+  mInitialized = !!::DuplicateHandle(
+    ::GetCurrentProcess(), ::GetCurrentThread(),
+    ::GetCurrentProcess(), &mThreadID,
+    THREAD_SUSPEND_RESUME
+#ifdef MOZ_THREADSTACKHELPER_NATIVE
+    | THREAD_GET_CONTEXT | THREAD_QUERY_INFORMATION
+#endif
+    , FALSE, 0);
+  MOZ_ASSERT(mInitialized);
+#elif defined(XP_MACOSX)
+  mThreadID = mach_thread_self();
+#endif
+
+#ifdef MOZ_THREADSTACKHELPER_NATIVE
+  GetThreadStackBase();
+#endif
+}
+
+ThreadStackHelper::~ThreadStackHelper()
+{
+#if defined(XP_LINUX)
+  MOZ_ALWAYS_TRUE(!::sem_destroy(&mSem));
+#elif defined(XP_WIN)
+  if (mInitialized) {
+    MOZ_ALWAYS_TRUE(!!::CloseHandle(mThreadID));
+  }
+#endif
+}
+
+#ifdef MOZ_THREADSTACKHELPER_NATIVE
+void ThreadStackHelper::GetThreadStackBase()
+{
+  mThreadStackBase = 0;
+
+#if defined(XP_LINUX)
+  void* stackAddr;
+  size_t stackSize;
+  ::pthread_t pthr = ::pthread_self();
+  ::pthread_attr_t pthr_attr;
+  NS_ENSURE_TRUE_VOID(!::pthread_getattr_np(pthr, &pthr_attr));
+  if (!::pthread_attr_getstack(&pthr_attr, &stackAddr, &stackSize)) {
+#ifdef MOZ_THREADSTACKHELPER_STACK_GROWS_DOWN
+    mThreadStackBase = intptr_t(stackAddr) + stackSize;
+#else
+    mThreadStackBase = intptr_t(stackAddr);
+#endif
+  }
+  MOZ_ALWAYS_TRUE(!::pthread_attr_destroy(&pthr_attr));
+
+#elif defined(XP_WIN)
+  ::MEMORY_BASIC_INFORMATION meminfo = {};
+  NS_ENSURE_TRUE_VOID(::VirtualQuery(&meminfo, &meminfo, sizeof(meminfo)));
+#ifdef MOZ_THREADSTACKHELPER_STACK_GROWS_DOWN
+  mThreadStackBase = intptr_t(meminfo.BaseAddress) + meminfo.RegionSize;
+#else
+  mThreadStackBase = intptr_t(meminfo.AllocationBase);
+#endif
+
+#elif defined(XP_MACOSX)
+  ::pthread_t pthr = ::pthread_self();
+  mThreadStackBase = intptr_t(::pthread_get_stackaddr_np(pthr));
+
+#else
+  #error "Unsupported platform"
+#endif // platform
+}
+#endif // MOZ_THREADSTACKHELPER_NATIVE
+
+namespace {
+template<typename T>
+class ScopedSetPtr
+{
+private:
+  T*& mPtr;
+public:
+  ScopedSetPtr(T*& p, T* val) : mPtr(p) { mPtr = val; }
+  ~ScopedSetPtr() { mPtr = nullptr; }
+};
+} // namespace
+
+void
+ThreadStackHelper::GetStack(Stack& aStack)
+{
+  // Always run PrepareStackBuffer first to clear aStack
+  if (!PrepareStackBuffer(aStack)) {
+    // Skip and return empty aStack
+    return;
+  }
+
+  ScopedSetPtr<Stack> stackPtr(mStackToFill, &aStack);
+
+#if defined(XP_LINUX)
+  if (!sInitialized) {
+    MOZ_ASSERT(false);
+    return;
+  }
+  siginfo_t uinfo = {};
+  uinfo.si_signo = sFillStackSignum;
+  uinfo.si_code = SI_QUEUE;
+  uinfo.si_pid = getpid();
+  uinfo.si_uid = getuid();
+  uinfo.si_value.sival_ptr = this;
+  if (::syscall(SYS_rt_tgsigqueueinfo, uinfo.si_pid,
+                mThreadID, sFillStackSignum, &uinfo)) {
+    // rt_tgsigqueueinfo was added in Linux 2.6.31.
+    // Could have failed because the syscall did not exist.
+    return;
+  }
+  MOZ_ALWAYS_TRUE(!::sem_wait(&mSem));
+
+#elif defined(XP_WIN)
+  if (!mInitialized) {
+    MOZ_ASSERT(false);
+    return;
+  }
+  if (::SuspendThread(mThreadID) == DWORD(-1)) {
+    MOZ_ASSERT(false);
+    return;
+  }
+
+  // SuspendThread is asynchronous, so the thread may still be running. Use
+  // GetThreadContext to ensure it's really suspended.
+  // See https://blogs.msdn.microsoft.com/oldnewthing/20150205-00/?p=44743.
+  CONTEXT context;
+  context.ContextFlags = CONTEXT_CONTROL;
+  if (::GetThreadContext(mThreadID, &context)) {
+    FillStackBuffer();
+    FillThreadContext();
+  }
+
+  MOZ_ALWAYS_TRUE(::ResumeThread(mThreadID) != DWORD(-1));
+
+#elif defined(XP_MACOSX)
+# if defined(MOZ_VALGRIND) && defined(RUNNING_ON_VALGRIND)
+  if (RUNNING_ON_VALGRIND) {
+    /* thread_suspend and thread_resume sometimes hang runs on Valgrind,
+       for unknown reasons.  So, just avoid them.  See bug 1100911. */
+    return;
+  }
+# endif
+
+  if (::thread_suspend(mThreadID) != KERN_SUCCESS) {
+    MOZ_ASSERT(false);
+    return;
+  }
+
+  FillStackBuffer();
+  FillThreadContext();
+
+  MOZ_ALWAYS_TRUE(::thread_resume(mThreadID) == KERN_SUCCESS);
+
+#endif
+}
+
+#ifdef MOZ_THREADSTACKHELPER_NATIVE
+class ThreadStackHelper::ThreadContext final
+{
+public:
+  // TODO: provide per-platform definition of Context.
+  typedef struct {} Context;
+
+  // Limit copied stack to 4kB
+  static const size_t kMaxStackSize = 0x1000;
+  // Limit unwound stack to 32 frames
+  static const unsigned int kMaxStackFrames = 32;
+  // Whether this structure contains valid data
+  bool mValid;
+  // Processor context
+  Context mContext;
+  // Stack area
+  UniquePtr<uint8_t[]> mStack;
+  // Start of stack area
+  uintptr_t mStackBase;
+  // Size of stack area
+  size_t mStackSize;
+  // End of stack area
+  const void* mStackEnd;
+
+  ThreadContext()
+    : mValid(false)
+    , mStackBase(0)
+    , mStackSize(0)
+    , mStackEnd(nullptr) {}
+};
+#endif // MOZ_THREADSTACKHELPER_NATIVE
+
+void
+ThreadStackHelper::GetNativeStack(Stack& aStack)
+{
+#ifdef MOZ_THREADSTACKHELPER_NATIVE
+  ThreadContext context;
+  context.mStack = MakeUnique<uint8_t[]>(ThreadContext::kMaxStackSize);
+
+  ScopedSetPtr<ThreadContext> contextPtr(mContextToFill, &context);
+
+  // Get pseudostack first and fill the thread context.
+  GetStack(aStack);
+  NS_ENSURE_TRUE_VOID(context.mValid);
+
+  // TODO: walk the saved stack frames.
+#endif // MOZ_THREADSTACKHELPER_NATIVE
+}
+
+#ifdef XP_LINUX
+
+int ThreadStackHelper::sInitialized;
+int ThreadStackHelper::sFillStackSignum;
+
+void
+ThreadStackHelper::FillStackHandler(int aSignal, siginfo_t* aInfo,
+                                    void* aContext)
+{
+  ThreadStackHelper* const helper =
+    reinterpret_cast<ThreadStackHelper*>(aInfo->si_value.sival_ptr);
+  helper->FillStackBuffer();
+  helper->FillThreadContext(aContext);
+  ::sem_post(&helper->mSem);
+}
+
+#endif // XP_LINUX
+
+bool
+ThreadStackHelper::PrepareStackBuffer(Stack& aStack)
+{
+  // Return false to skip getting the stack and return an empty stack
+  aStack.clear();
+#ifdef MOZ_THREADSTACKHELPER_PSEUDO
+  /* Normally, provided the profiler is enabled, it would be an error if we
+     don't have a pseudostack here (the thread probably forgot to call
+     profiler_register_thread). However, on B2G, profiling secondary threads
+     may be disabled despite profiler being enabled. This is by-design and
+     is not an error. */
+#ifdef MOZ_WIDGET_GONK
+  if (!mPseudoStack) {
+    return false;
+  }
+#endif
+  MOZ_ASSERT(mPseudoStack);
+  if (!aStack.reserve(mMaxStackSize) ||
+      !aStack.reserve(aStack.capacity()) || // reserve up to the capacity
+      !aStack.EnsureBufferCapacity(mMaxBufferSize)) {
+    return false;
+  }
+  return true;
+#else
+  return false;
+#endif
+}
+
+#ifdef MOZ_THREADSTACKHELPER_PSEUDO
+
+namespace {
+
+bool
+IsChromeJSScript(JSScript* aScript)
+{
+  // May be called from another thread or inside a signal handler.
+  // We assume querying the script is safe but we must not manipulate it.
+
+  nsIScriptSecurityManager* const secman =
+    nsScriptSecurityManager::GetScriptSecurityManager();
+  NS_ENSURE_TRUE(secman, false);
+
+  JSPrincipals* const principals = JS_GetScriptPrincipals(aScript);
+  return secman->IsSystemPrincipal(nsJSPrincipals::get(principals));
+}
+
+// Get the full path after the URI scheme, if the URI matches the scheme.
+// For example, GetFullPathForScheme("a://b/c/d/e", "a://") returns "b/c/d/e".
+template <size_t LEN>
+const char*
+GetFullPathForScheme(const char* filename, const char (&scheme)[LEN]) {
+  // Account for the null terminator included in LEN.
+  if (!strncmp(filename, scheme, LEN - 1)) {
+    return filename + LEN - 1;
+  }
+  return nullptr;
+}
+
+// Get the full path after a URI component, if the URI contains the component.
+// For example, GetPathAfterComponent("a://b/c/d/e", "/c/") returns "d/e".
+template <size_t LEN>
+const char*
+GetPathAfterComponent(const char* filename, const char (&component)[LEN]) {
+  const char* found = nullptr;
+  const char* next = strstr(filename, component);
+  while (next) {
+    // Move 'found' to end of the component, after the separator '/'.
+    // 'LEN - 1' accounts for the null terminator included in LEN,
+    found = next + LEN - 1;
+    // Resume searching before the separator '/'.
+    next = strstr(found - 1, component);
+  }
+  return found;
+}
+
+} // namespace
+
+const char*
+ThreadStackHelper::AppendJSEntry(const volatile StackEntry* aEntry,
+                                 intptr_t& aAvailableBufferSize,
+                                 const char* aPrevLabel)
+{
+  // May be called from another thread or inside a signal handler.
+  // We assume querying the script is safe but we must not manupulate it.
+  // Also we must not allocate any memory from heap.
+  MOZ_ASSERT(aEntry->isJs());
+
+  const char* label;
+  JSScript* script = aEntry->script();
+  if (!script) {
+    label = "(profiling suppressed)";
+  } else if (IsChromeJSScript(aEntry->script())) {
+    const char* filename = JS_GetScriptFilename(aEntry->script());
+    const unsigned lineno = JS_PCToLineNumber(aEntry->script(), aEntry->pc());
+    MOZ_ASSERT(filename);
+
+    char buffer[128]; // Enough to fit longest js file name from the tree
+
+    // Some script names are in the form "foo -> bar -> baz".
+    // Here we find the origin of these redirected scripts.
+    const char* basename = GetPathAfterComponent(filename, " -> ");
+    if (basename) {
+      filename = basename;
+    }
+
+    basename = GetFullPathForScheme(filename, "chrome://");
+    if (!basename) {
+      basename = GetFullPathForScheme(filename, "resource://");
+    }
+    if (!basename) {
+      // If the (add-on) script is located under the {profile}/extensions
+      // directory, extract the path after the /extensions/ part.
+      basename = GetPathAfterComponent(filename, "/extensions/");
+    }
+    if (!basename) {
+      // Only keep the file base name for paths outside the above formats.
+      basename = strrchr(filename, '/');
+      basename = basename ? basename + 1 : filename;
+      // Look for Windows path separator as well.
+      filename = strrchr(basename, '\\');
+      if (filename) {
+        basename = filename + 1;
+      }
+    }
+
+    size_t len = SprintfLiteral(buffer, "%s:%u", basename, lineno);
+    if (len < sizeof(buffer)) {
+      if (mStackToFill->IsSameAsEntry(aPrevLabel, buffer)) {
+        return aPrevLabel;
+      }
+
+      // Keep track of the required buffer size
+      aAvailableBufferSize -= (len + 1);
+      if (aAvailableBufferSize >= 0) {
+        // Buffer is big enough.
+        return mStackToFill->InfallibleAppendViaBuffer(buffer, len);
+      }
+      // Buffer is not big enough; fall through to using static label below.
+    }
+    // snprintf failed or buffer is not big enough.
+    label = "(chrome script)";
+  } else {
+    label = "(content script)";
+  }
+
+  if (mStackToFill->IsSameAsEntry(aPrevLabel, label)) {
+    return aPrevLabel;
+  }
+  mStackToFill->infallibleAppend(label);
+  return label;
+}
+
+#endif // MOZ_THREADSTACKHELPER_PSEUDO
+
+void
+ThreadStackHelper::FillStackBuffer()
+{
+  MOZ_ASSERT(mStackToFill->empty());
+
+#ifdef MOZ_THREADSTACKHELPER_PSEUDO
+  size_t reservedSize = mStackToFill->capacity();
+  size_t reservedBufferSize = mStackToFill->AvailableBufferSize();
+  intptr_t availableBufferSize = intptr_t(reservedBufferSize);
+
+  // Go from front to back
+  const volatile StackEntry* entry = mPseudoStack->mStack;
+  const volatile StackEntry* end = entry + mPseudoStack->stackSize();
+  // Deduplicate identical, consecutive frames
+  const char* prevLabel = nullptr;
+  for (; reservedSize-- && entry != end; entry++) {
+    /* We only accept non-copy labels, including js::RunScript,
+       because we only want static labels in the hang stack. */
+    if (entry->isCopyLabel()) {
+      continue;
+    }
+    if (entry->isJs()) {
+      prevLabel = AppendJSEntry(entry, availableBufferSize, prevLabel);
+      continue;
+    }
+#ifdef MOZ_THREADSTACKHELPER_NATIVE
+    if (mContextToFill) {
+      mContextToFill->mStackEnd = entry->stackAddress();
+    }
+#endif
+    const char* const label = entry->label();
+    if (mStackToFill->IsSameAsEntry(prevLabel, label)) {
+      // Avoid duplicate labels to save space in the stack.
+      continue;
+    }
+    mStackToFill->infallibleAppend(label);
+    prevLabel = label;
+  }
+
+  // end != entry if we exited early due to not enough reserved frames.
+  // Expand the number of reserved frames for next time.
+  mMaxStackSize = mStackToFill->capacity() + (end - entry);
+
+  // availableBufferSize < 0 if we needed a larger buffer than we reserved.
+  // Calculate a new reserve size for next time.
+  if (availableBufferSize < 0) {
+    mMaxBufferSize = reservedBufferSize - availableBufferSize;
+  }
+#endif
+}
+
+MOZ_ASAN_BLACKLIST void
+ThreadStackHelper::FillThreadContext(void* aContext)
+{
+#ifdef MOZ_THREADSTACKHELPER_NATIVE
+  if (!mContextToFill) {
+    return;
+  }
+
+#if 0 // TODO: remove dependency on Breakpad structs.
+#if defined(XP_LINUX)
+  const ucontext_t& context = *reinterpret_cast<ucontext_t*>(aContext);
+#if defined(MOZ_THREADSTACKHELPER_X86)
+  mContextToFill->mContext.context_flags = MD_CONTEXT_X86_FULL;
+  mContextToFill->mContext.edi = context.uc_mcontext.gregs[REG_EDI];
+  mContextToFill->mContext.esi = context.uc_mcontext.gregs[REG_ESI];
+  mContextToFill->mContext.ebx = context.uc_mcontext.gregs[REG_EBX];
+  mContextToFill->mContext.edx = context.uc_mcontext.gregs[REG_EDX];
+  mContextToFill->mContext.ecx = context.uc_mcontext.gregs[REG_ECX];
+  mContextToFill->mContext.eax = context.uc_mcontext.gregs[REG_EAX];
+  mContextToFill->mContext.ebp = context.uc_mcontext.gregs[REG_EBP];
+  mContextToFill->mContext.eip = context.uc_mcontext.gregs[REG_EIP];
+  mContextToFill->mContext.eflags = context.uc_mcontext.gregs[REG_EFL];
+  mContextToFill->mContext.esp = context.uc_mcontext.gregs[REG_ESP];
+#elif defined(MOZ_THREADSTACKHELPER_X64)
+  mContextToFill->mContext.context_flags = MD_CONTEXT_AMD64_FULL;
+  mContextToFill->mContext.eflags = uint32_t(context.uc_mcontext.gregs[REG_EFL]);
+  mContextToFill->mContext.rax = context.uc_mcontext.gregs[REG_RAX];
+  mContextToFill->mContext.rcx = context.uc_mcontext.gregs[REG_RCX];
+  mContextToFill->mContext.rdx = context.uc_mcontext.gregs[REG_RDX];
+  mContextToFill->mContext.rbx = context.uc_mcontext.gregs[REG_RBX];
+  mContextToFill->mContext.rsp = context.uc_mcontext.gregs[REG_RSP];
+  mContextToFill->mContext.rbp = context.uc_mcontext.gregs[REG_RBP];
+  mContextToFill->mContext.rsi = context.uc_mcontext.gregs[REG_RSI];
+  mContextToFill->mContext.rdi = context.uc_mcontext.gregs[REG_RDI];
+  memcpy(&mContextToFill->mContext.r8,
+         &context.uc_mcontext.gregs[REG_R8], 8 * sizeof(int64_t));
+  mContextToFill->mContext.rip = context.uc_mcontext.gregs[REG_RIP];
+#elif defined(MOZ_THREADSTACKHELPER_ARM)
+  mContextToFill->mContext.context_flags = MD_CONTEXT_ARM_FULL;
+  memcpy(&mContextToFill->mContext.iregs[0],
+         &context.uc_mcontext.arm_r0, 17 * sizeof(int32_t));
+#else
+  #error "Unsupported architecture"
+#endif // architecture
+
+#elif defined(XP_WIN)
+  // Breakpad context struct is based off of the Windows CONTEXT struct,
+  // so we assume they are the same; do some sanity checks to make sure.
+  static_assert(sizeof(ThreadContext::Context) == sizeof(::CONTEXT),
+                "Context struct mismatch");
+  static_assert(offsetof(ThreadContext::Context, context_flags) ==
+                offsetof(::CONTEXT, ContextFlags),
+                "Context struct mismatch");
+  mContextToFill->mContext.context_flags = CONTEXT_FULL;
+  NS_ENSURE_TRUE_VOID(::GetThreadContext(mThreadID,
+      reinterpret_cast<::CONTEXT*>(&mContextToFill->mContext)));
+
+#elif defined(XP_MACOSX)
+#if defined(MOZ_THREADSTACKHELPER_X86)
+  const thread_state_flavor_t flavor = x86_THREAD_STATE32;
+  x86_thread_state32_t state = {};
+  mach_msg_type_number_t count = x86_THREAD_STATE32_COUNT;
+#elif defined(MOZ_THREADSTACKHELPER_X64)
+  const thread_state_flavor_t flavor = x86_THREAD_STATE64;
+  x86_thread_state64_t state = {};
+  mach_msg_type_number_t count = x86_THREAD_STATE64_COUNT;
+#elif defined(MOZ_THREADSTACKHELPER_ARM)
+  const thread_state_flavor_t flavor = ARM_THREAD_STATE;
+  arm_thread_state_t state = {};
+  mach_msg_type_number_t count = ARM_THREAD_STATE_COUNT;
+#endif
+  NS_ENSURE_TRUE_VOID(KERN_SUCCESS == ::thread_get_state(
+    mThreadID, flavor, reinterpret_cast<thread_state_t>(&state), &count));
+#if __DARWIN_UNIX03
+#define GET_REGISTER(s, r) ((s).__##r)
+#else
+#define GET_REGISTER(s, r) ((s).r)
+#endif
+#if defined(MOZ_THREADSTACKHELPER_X86)
+  mContextToFill->mContext.context_flags = MD_CONTEXT_X86_FULL;
+  mContextToFill->mContext.edi = GET_REGISTER(state, edi);
+  mContextToFill->mContext.esi = GET_REGISTER(state, esi);
+  mContextToFill->mContext.ebx = GET_REGISTER(state, ebx);
+  mContextToFill->mContext.edx = GET_REGISTER(state, edx);
+  mContextToFill->mContext.ecx = GET_REGISTER(state, ecx);
+  mContextToFill->mContext.eax = GET_REGISTER(state, eax);
+  mContextToFill->mContext.ebp = GET_REGISTER(state, ebp);
+  mContextToFill->mContext.eip = GET_REGISTER(state, eip);
+  mContextToFill->mContext.eflags = GET_REGISTER(state, eflags);
+  mContextToFill->mContext.esp = GET_REGISTER(state, esp);
+#elif defined(MOZ_THREADSTACKHELPER_X64)
+  mContextToFill->mContext.context_flags = MD_CONTEXT_AMD64_FULL;
+  mContextToFill->mContext.eflags = uint32_t(GET_REGISTER(state, rflags));
+  mContextToFill->mContext.rax = GET_REGISTER(state, rax);
+  mContextToFill->mContext.rcx = GET_REGISTER(state, rcx);
+  mContextToFill->mContext.rdx = GET_REGISTER(state, rdx);
+  mContextToFill->mContext.rbx = GET_REGISTER(state, rbx);
+  mContextToFill->mContext.rsp = GET_REGISTER(state, rsp);
+  mContextToFill->mContext.rbp = GET_REGISTER(state, rbp);
+  mContextToFill->mContext.rsi = GET_REGISTER(state, rsi);
+  mContextToFill->mContext.rdi = GET_REGISTER(state, rdi);
+  memcpy(&mContextToFill->mContext.r8,
+         &GET_REGISTER(state, r8), 8 * sizeof(int64_t));
+  mContextToFill->mContext.rip = GET_REGISTER(state, rip);
+#elif defined(MOZ_THREADSTACKHELPER_ARM)
+  mContextToFill->mContext.context_flags = MD_CONTEXT_ARM_FULL;
+  memcpy(mContextToFill->mContext.iregs,
+         GET_REGISTER(state, r), 17 * sizeof(int32_t));
+#else
+  #error "Unsupported architecture"
+#endif // architecture
+#undef GET_REGISTER
+
+#else
+  #error "Unsupported platform"
+#endif // platform
+
+  intptr_t sp = 0;
+#if defined(MOZ_THREADSTACKHELPER_X86)
+  sp = mContextToFill->mContext.esp;
+#elif defined(MOZ_THREADSTACKHELPER_X64)
+  sp = mContextToFill->mContext.rsp;
+#elif defined(MOZ_THREADSTACKHELPER_ARM)
+  sp = mContextToFill->mContext.iregs[13];
+#else
+  #error "Unsupported architecture"
+#endif // architecture
+  NS_ENSURE_TRUE_VOID(sp);
+  NS_ENSURE_TRUE_VOID(mThreadStackBase);
+
+  size_t stackSize = std::min(intptr_t(ThreadContext::kMaxStackSize),
+                              std::abs(sp - mThreadStackBase));
+
+  if (mContextToFill->mStackEnd) {
+    // Limit the start of stack to a certain location if specified.
+    stackSize = std::min(intptr_t(stackSize),
+      std::abs(sp - intptr_t(mContextToFill->mStackEnd)));
+  }
+
+#ifndef MOZ_THREADSTACKHELPER_STACK_GROWS_DOWN
+  // If if the stack grows upwards, and we need to recalculate our
+  // stack copy's base address. Subtract sizeof(void*) so that the
+  // location pointed to by sp is included.
+  sp -= stackSize - sizeof(void*);
+#endif
+
+#ifndef MOZ_ASAN
+  memcpy(mContextToFill->mStack.get(), reinterpret_cast<void*>(sp), stackSize);
+  // Valgrind doesn't care about the access outside the stack frame, but
+  // the presence of uninitialised values on the stack does cause it to
+  // later report a lot of false errors when Breakpad comes to unwind it.
+  // So mark the extracted data as defined.
+  MOZ_MAKE_MEM_DEFINED(mContextToFill->mStack.get(), stackSize);
+#else
+  // ASan will flag memcpy for access outside of stack frames,
+  // so roll our own memcpy here.
+  intptr_t* dst = reinterpret_cast<intptr_t*>(&mContextToFill->mStack[0]);
+  const intptr_t* src = reinterpret_cast<intptr_t*>(sp);
+  for (intptr_t len = stackSize; len > 0; len -= sizeof(*src)) {
+    *(dst++) = *(src++);
+  }
+#endif
+
+  mContextToFill->mStackBase = uintptr_t(sp);
+  mContextToFill->mStackSize = stackSize;
+  mContextToFill->mValid = true;
+#endif
+#endif // MOZ_THREADSTACKHELPER_NATIVE
+}
+
+} // namespace mozilla
diff --git a/xpcom/threads/ThreadStackHelper.h b/xpcom/threads/ThreadStackHelper.h
new file mode 100644
index 000000000..9c40ad5e2
--- /dev/null
+++ b/xpcom/threads/ThreadStackHelper.h
@@ -0,0 +1,147 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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_ThreadStackHelper_h
+#define mozilla_ThreadStackHelper_h
+
+#include "mozilla/ThreadHangStats.h"
+
+#include "GeckoProfiler.h"
+
+#include <stddef.h>
+
+#if defined(XP_LINUX)
+#include <signal.h>
+#include <semaphore.h>
+#include <sys/types.h>
+#elif defined(XP_WIN)
+#include <windows.h>
+#elif defined(XP_MACOSX)
+#include <mach/mach.h>
+#endif
+
+// Support pseudostack on these platforms.
+#if defined(XP_LINUX) || defined(XP_WIN) || defined(XP_MACOSX)
+#  ifdef MOZ_ENABLE_PROFILER_SPS
+#    define MOZ_THREADSTACKHELPER_PSEUDO
+#  endif
+#endif
+
+#ifdef MOZ_THREADSTACKHELPER_PSEUDO
+#  define MOZ_THREADSTACKHELPER_NATIVE
+#  if defined(__i386__) || defined(_M_IX86)
+#    define MOZ_THREADSTACKHELPER_X86
+#  elif defined(__x86_64__) || defined(_M_X64)
+#    define MOZ_THREADSTACKHELPER_X64
+#  elif defined(__arm__) || defined(_M_ARM)
+#    define MOZ_THREADSTACKHELPER_ARM
+#  else
+     // Unsupported architecture
+#    undef MOZ_THREADSTACKHELPER_NATIVE
+#  endif
+#endif
+
+namespace mozilla {
+
+/**
+ * ThreadStackHelper is used to retrieve the profiler pseudo-stack of a
+ * thread, as an alternative of using the profiler to take a profile.
+ * The target thread first declares an ThreadStackHelper instance;
+ * then another thread can call ThreadStackHelper::GetStack to retrieve
+ * the pseudo-stack of the target thread at that instant.
+ *
+ * Only non-copying labels are included in the stack, which means labels
+ * with custom text and markers are not included.
+ */
+class ThreadStackHelper
+{
+public:
+  typedef Telemetry::HangStack Stack;
+
+private:
+  Stack* mStackToFill;
+#ifdef MOZ_THREADSTACKHELPER_PSEUDO
+  const PseudoStack* const mPseudoStack;
+#ifdef MOZ_THREADSTACKHELPER_NATIVE
+  class ThreadContext;
+  // Set to non-null if GetStack should get the thread context.
+  ThreadContext* mContextToFill;
+  intptr_t mThreadStackBase;
+#endif
+  size_t mMaxStackSize;
+  size_t mMaxBufferSize;
+#endif
+
+  bool PrepareStackBuffer(Stack& aStack);
+  void FillStackBuffer();
+  void FillThreadContext(void* aContext = nullptr);
+#ifdef MOZ_THREADSTACKHELPER_PSEUDO
+  const char* AppendJSEntry(const volatile StackEntry* aEntry,
+                            intptr_t& aAvailableBufferSize,
+                            const char* aPrevLabel);
+#endif
+#ifdef MOZ_THREADSTACKHELPER_NATIVE
+  void GetThreadStackBase();
+#endif
+
+public:
+  /**
+   * Initialize ThreadStackHelper. Must be called from main thread.
+   */
+  static void Startup();
+  /**
+   * Uninitialize ThreadStackHelper. Must be called from main thread.
+   */
+  static void Shutdown();
+
+  /**
+   * Create a ThreadStackHelper instance targeting the current thread.
+   */
+  ThreadStackHelper();
+
+  ~ThreadStackHelper();
+
+  /**
+   * Retrieve the current pseudostack of the thread associated
+   * with this ThreadStackHelper.
+   *
+   * @param aStack Stack instance to be filled.
+   */
+  void GetStack(Stack& aStack);
+
+  /**
+   * Retrieve the current native stack of the thread associated
+   * with this ThreadStackHelper.
+   *
+   * @param aNativeStack Stack instance to be filled.
+   */
+  void GetNativeStack(Stack& aStack);
+
+#if defined(XP_LINUX)
+private:
+  static int sInitialized;
+  static int sFillStackSignum;
+
+  static void FillStackHandler(int aSignal, siginfo_t* aInfo, void* aContext);
+
+  sem_t mSem;
+  pid_t mThreadID;
+
+#elif defined(XP_WIN)
+private:
+  bool mInitialized;
+  HANDLE mThreadID;
+
+#elif defined(XP_MACOSX)
+private:
+  thread_act_t mThreadID;
+
+#endif
+};
+
+} // namespace mozilla
+
+#endif // mozilla_ThreadStackHelper_h
diff --git a/xpcom/threads/ThrottledEventQueue.cpp b/xpcom/threads/ThrottledEventQueue.cpp
new file mode 100644
index 000000000..941566ef2
--- /dev/null
+++ b/xpcom/threads/ThrottledEventQueue.cpp
@@ -0,0 +1,446 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "ThrottledEventQueue.h"
+
+#include "mozilla/Atomics.h"
+#include "mozilla/ClearOnShutdown.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/Unused.h"
+#include "nsEventQueue.h"
+
+namespace mozilla {
+
+using mozilla::services::GetObserverService;
+
+namespace {
+
+static const char kShutdownTopic[] = "xpcom-shutdown";
+
+} // anonymous namespace
+
+// The ThrottledEventQueue is designed with inner and outer objects:
+//
+//       XPCOM code    nsObserverService
+//            |               |
+//            |               |
+//            v               |
+//        +-------+           |
+//        | Outer |           |
+//        +-------+           |
+//            |               |
+//            |   +-------+   |
+//            +-->| Inner |<--+
+//                +-------+
+//
+// Client code references the outer nsIEventTarget which in turn references
+// an inner object.  The inner object is also held alive by the observer
+// service.
+//
+// If the outer object is dereferenced and destroyed, it will trigger a
+// shutdown operation on the inner object.  Similarly if the observer
+// service notifies that the browser is shutting down, then the inner
+// object also starts shutting down.
+//
+// Once the queue has drained we unregister from the observer service.  If
+// the outer object is already gone, then the inner object is free'd at this
+// point.  If the outer object still exists then calls fall back to the
+// ThrottledEventQueue's base target.  We just don't queue things
+// any more.  The inner is then released once the outer object is released.
+//
+// Note, we must keep the inner object alive and attached to the observer
+// service until the TaskQueue is fully shutdown and idle.  We must delay
+// xpcom shutdown if the TaskQueue is in the middle of draining.
+class ThrottledEventQueue::Inner final : public nsIObserver
+{
+  // The runnable which is dispatched to the underlying base target.  Since
+  // we only execute one event at a time we just re-use a single instance
+  // of this class while there are events left in the queue.
+  class Executor final : public Runnable
+  {
+    RefPtr<Inner> mInner;
+
+  public:
+    explicit Executor(Inner* aInner)
+      : mInner(aInner)
+    { }
+
+    NS_IMETHODIMP
+    Run()
+    {
+      mInner->ExecuteRunnable();
+      return NS_OK;
+    }
+  };
+
+  mutable Mutex mMutex;
+  mutable CondVar mIdleCondVar;
+
+  mozilla::CondVar mEventsAvailable;
+
+  // any thread, protected by mutex
+  nsEventQueue mEventQueue;
+
+  // written on main thread, read on any thread
+  nsCOMPtr<nsIEventTarget> mBaseTarget;
+
+  // any thread, protected by mutex
+  nsCOMPtr<nsIRunnable> mExecutor;
+
+  // any thread, atomic
+  Atomic<uint32_t> mExecutionDepth;
+
+  // any thread, protected by mutex
+  bool mShutdownStarted;
+
+  explicit Inner(nsIEventTarget* aBaseTarget)
+    : mMutex("ThrottledEventQueue")
+    , mIdleCondVar(mMutex, "ThrottledEventQueue:Idle")
+    , mEventsAvailable(mMutex, "[ThrottledEventQueue::Inner.mEventsAvailable]")
+    , mEventQueue(mEventsAvailable, nsEventQueue::eNormalQueue)
+    , mBaseTarget(aBaseTarget)
+    , mExecutionDepth(0)
+    , mShutdownStarted(false)
+  {
+  }
+
+  ~Inner()
+  {
+    MOZ_ASSERT(!mExecutor);
+    MOZ_ASSERT(mShutdownStarted);
+  }
+
+  void
+  ExecuteRunnable()
+  {
+    // Any thread
+    nsCOMPtr<nsIRunnable> event;
+    bool shouldShutdown = false;
+
+#ifdef DEBUG
+    bool currentThread = false;
+    mBaseTarget->IsOnCurrentThread(&currentThread);
+    MOZ_ASSERT(currentThread);
+#endif
+
+    {
+      MutexAutoLock lock(mMutex);
+
+      // We only dispatch an executor runnable when we know there is something
+      // in the queue, so this should never fail.
+      MOZ_ALWAYS_TRUE(mEventQueue.GetPendingEvent(getter_AddRefs(event), lock));
+
+      // If there are more events in the queue, then dispatch the next
+      // executor.  We do this now, before running the event, because
+      // the event might spin the event loop and we don't want to stall
+      // the queue.
+      if (mEventQueue.HasPendingEvent(lock)) {
+        // Dispatch the next base target runnable to attempt to execute
+        // the next throttled event.  We must do this before executing
+        // the event in case the event spins the event loop.
+        MOZ_ALWAYS_SUCCEEDS(
+          mBaseTarget->Dispatch(mExecutor, NS_DISPATCH_NORMAL));
+      }
+
+      // Otherwise the queue is empty and we can stop dispatching the
+      // executor.  We might also need to shutdown after running the
+      // last event.
+      else {
+        shouldShutdown = mShutdownStarted;
+        // Note, this breaks a ref cycle.
+        mExecutor = nullptr;
+        mIdleCondVar.NotifyAll();
+      }
+    }
+
+    // Execute the event now that we have unlocked.
+    ++mExecutionDepth;
+    Unused << event->Run();
+    --mExecutionDepth;
+
+    // If shutdown was started and the queue is now empty we can now
+    // finalize the shutdown.  This is performed separately at the end
+    // of the method in order to wait for the event to finish running.
+    if (shouldShutdown) {
+      MOZ_ASSERT(IsEmpty());
+      NS_DispatchToMainThread(NewRunnableMethod(this, &Inner::ShutdownComplete));
+    }
+  }
+
+  void
+  ShutdownComplete()
+  {
+    MOZ_ASSERT(NS_IsMainThread());
+    MOZ_ASSERT(IsEmpty());
+    nsCOMPtr<nsIObserverService> obs = GetObserverService();
+    obs->RemoveObserver(this, kShutdownTopic);
+  }
+
+public:
+  static already_AddRefed<Inner>
+  Create(nsIEventTarget* aBaseTarget)
+  {
+    MOZ_ASSERT(NS_IsMainThread());
+
+    if (ClearOnShutdown_Internal::sCurrentShutdownPhase != ShutdownPhase::NotInShutdown) {
+      return nullptr;
+    }
+
+    nsCOMPtr<nsIObserverService> obs = GetObserverService();
+    if (NS_WARN_IF(!obs)) {
+      return nullptr;
+    }
+
+    RefPtr<Inner> ref = new Inner(aBaseTarget);
+
+    nsresult rv = obs->AddObserver(ref, kShutdownTopic,
+                                   false /* means OS will hold a strong ref */);
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      ref->MaybeStartShutdown();
+      MOZ_ASSERT(ref->IsEmpty());
+      return nullptr;
+    }
+
+    return ref.forget();
+  }
+
+  NS_IMETHOD
+  Observe(nsISupports*, const char* aTopic, const char16_t*) override
+  {
+    MOZ_ASSERT(NS_IsMainThread());
+    MOZ_ASSERT(!strcmp(aTopic, kShutdownTopic));
+
+    MaybeStartShutdown();
+
+    // Once shutdown begins we set the Atomic<bool> mShutdownStarted flag.
+    // This prevents any new runnables from being dispatched into the
+    // TaskQueue.  Therefore this loop should be finite.
+    while (!IsEmpty()) {
+      MOZ_ALWAYS_TRUE(NS_ProcessNextEvent());
+    }
+
+    return NS_OK;
+  }
+
+  void
+  MaybeStartShutdown()
+  {
+    // Any thread
+    MutexAutoLock lock(mMutex);
+
+    if (mShutdownStarted) {
+      return;
+    }
+    mShutdownStarted = true;
+
+    // We are marked for shutdown now, but we are still processing runnables.
+    // Return for now.  The shutdown will be completed once the queue is
+    // drained.
+    if (mExecutor) {
+      return;
+    }
+
+    // The queue is empty, so we can complete immediately.
+    NS_DispatchToMainThread(NewRunnableMethod(this, &Inner::ShutdownComplete));
+  }
+
+  bool
+  IsEmpty() const
+  {
+    // Any thread
+    return Length() == 0;
+  }
+
+  uint32_t
+  Length() const
+  {
+    // Any thread
+    MutexAutoLock lock(mMutex);
+    return mEventQueue.Count(lock);
+  }
+
+  void
+  AwaitIdle() const
+  {
+    // Any thread, except the main thread or our base target.  Blocking the
+    // main thread is forbidden.  Blocking the base target is guaranteed to
+    // produce a deadlock.
+    MOZ_ASSERT(!NS_IsMainThread());
+#ifdef DEBUG
+    bool onBaseTarget = false;
+    Unused << mBaseTarget->IsOnCurrentThread(&onBaseTarget);
+    MOZ_ASSERT(!onBaseTarget);
+#endif
+
+    MutexAutoLock lock(mMutex);
+    while (mExecutor) {
+      mIdleCondVar.Wait();
+    }
+  }
+
+  nsresult
+  DispatchFromScript(nsIRunnable* aEvent, uint32_t aFlags)
+  {
+    // Any thread
+    nsCOMPtr<nsIRunnable> r = aEvent;
+    return Dispatch(r.forget(), aFlags);
+  }
+
+  nsresult
+  Dispatch(already_AddRefed<nsIRunnable> aEvent, uint32_t aFlags)
+  {
+    MOZ_ASSERT(aFlags == NS_DISPATCH_NORMAL ||
+               aFlags == NS_DISPATCH_AT_END);
+
+    // Any thread
+    MutexAutoLock lock(mMutex);
+
+    // If we are shutting down, just fall back to our base target
+    // directly.
+    if (mShutdownStarted) {
+      return mBaseTarget->Dispatch(Move(aEvent), aFlags);
+    }
+
+    // We are not currently processing events, so we must start
+    // operating on our base target.  This is fallible, so do
+    // it first.  Our lock will prevent the executor from accessing
+    // the event queue before we add the event below.
+    if (!mExecutor) {
+      // Note, this creates a ref cycle keeping the inner alive
+      // until the queue is drained.
+      mExecutor = new Executor(this);
+      nsresult rv = mBaseTarget->Dispatch(mExecutor, NS_DISPATCH_NORMAL);
+      if (NS_WARN_IF(NS_FAILED(rv))) {
+        mExecutor = nullptr;
+        return rv;
+      }
+    }
+
+    // Only add the event to the underlying queue if are able to
+    // dispatch to our base target.
+    mEventQueue.PutEvent(Move(aEvent), lock);
+    return NS_OK;
+  }
+
+  nsresult
+  DelayedDispatch(already_AddRefed<nsIRunnable> aEvent, uint32_t aDelay)
+  {
+    // The base target may implement this, but we don't.  Always fail
+    // to provide consistent behavior.
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+
+  nsresult
+  IsOnCurrentThread(bool* aResult)
+  {
+    // Any thread
+
+    bool shutdownAndIdle = false;
+    {
+      MutexAutoLock lock(mMutex);
+      shutdownAndIdle = mShutdownStarted && mEventQueue.Count(lock) == 0;
+    }
+
+    bool onBaseTarget = false;
+    nsresult rv = mBaseTarget->IsOnCurrentThread(&onBaseTarget);
+    if (NS_FAILED(rv)) {
+      return rv;
+    }
+
+    // We consider the current stack on this event target if are on
+    // the base target and one of the following is true
+    //  1) We are currently running an event OR
+    //  2) We are both shutting down and the queue is idle
+    *aResult = onBaseTarget && (mExecutionDepth || shutdownAndIdle);
+
+    return NS_OK;
+  }
+
+  NS_DECL_THREADSAFE_ISUPPORTS
+};
+
+NS_IMPL_ISUPPORTS(ThrottledEventQueue::Inner, nsIObserver);
+
+NS_IMPL_ISUPPORTS(ThrottledEventQueue, nsIEventTarget);
+
+ThrottledEventQueue::ThrottledEventQueue(already_AddRefed<Inner> aInner)
+  : mInner(aInner)
+{
+  MOZ_ASSERT(mInner);
+}
+
+ThrottledEventQueue::~ThrottledEventQueue()
+{
+  mInner->MaybeStartShutdown();
+}
+
+void
+ThrottledEventQueue::MaybeStartShutdown()
+{
+  return mInner->MaybeStartShutdown();
+}
+
+already_AddRefed<ThrottledEventQueue>
+ThrottledEventQueue::Create(nsIEventTarget* aBaseTarget)
+{
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(aBaseTarget);
+
+  RefPtr<Inner> inner = Inner::Create(aBaseTarget);
+  if (NS_WARN_IF(!inner)) {
+    return nullptr;
+  }
+
+  RefPtr<ThrottledEventQueue> ref =
+    new ThrottledEventQueue(inner.forget());
+  return ref.forget();
+}
+
+bool
+ThrottledEventQueue::IsEmpty() const
+{
+  return mInner->IsEmpty();
+}
+
+uint32_t
+ThrottledEventQueue::Length() const
+{
+  return mInner->Length();
+}
+
+void
+ThrottledEventQueue::AwaitIdle() const
+{
+  return mInner->AwaitIdle();
+}
+
+NS_IMETHODIMP
+ThrottledEventQueue::DispatchFromScript(nsIRunnable* aEvent, uint32_t aFlags)
+{
+  return mInner->DispatchFromScript(aEvent, aFlags);
+}
+
+NS_IMETHODIMP
+ThrottledEventQueue::Dispatch(already_AddRefed<nsIRunnable> aEvent,
+                                     uint32_t aFlags)
+{
+  return mInner->Dispatch(Move(aEvent), aFlags);
+}
+
+NS_IMETHODIMP
+ThrottledEventQueue::DelayedDispatch(already_AddRefed<nsIRunnable> aEvent,
+                                            uint32_t aFlags)
+{
+  return mInner->DelayedDispatch(Move(aEvent), aFlags);
+}
+
+NS_IMETHODIMP
+ThrottledEventQueue::IsOnCurrentThread(bool* aResult)
+{
+  return mInner->IsOnCurrentThread(aResult);
+}
+
+} // namespace mozilla
diff --git a/xpcom/threads/ThrottledEventQueue.h b/xpcom/threads/ThrottledEventQueue.h
new file mode 100644
index 000000000..e0762bcce
--- /dev/null
+++ b/xpcom/threads/ThrottledEventQueue.h
@@ -0,0 +1,94 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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/. */
+
+// nsIEventTarget wrapper for throttling event dispatch.
+
+#ifndef mozilla_ThrottledEventQueue_h
+#define mozilla_ThrottledEventQueue_h
+
+#include "nsIEventTarget.h"
+
+namespace mozilla {
+
+// A ThrottledEventQueue is an event target that can be used to throttle
+// events being dispatched to another base target.  It maintains its
+// own queue of events and only dispatches one at a time to the wrapped
+// target.  This can be used to avoid flooding the base target.
+//
+// Flooding is avoided via a very simply principal.  Runnables dispatched
+// to the ThrottledEventQueue are only dispatched to the base target
+// one at a time.  Only once that runnable has executed will we dispatch
+// the next runnable to the base target.  This in effect makes all
+// runnables passing through the ThrottledEventQueue yield to other work
+// on the base target.
+//
+// ThrottledEventQueue keeps runnables waiting to be dispatched to the
+// base in its own internal queue.  Code can query the length of this
+// queue using IsEmpty() and Length().  Further, code implement back
+// pressure by checking the depth of the queue and deciding to stop
+// issuing runnables if they see the ThrottledEventQueue is backed up.
+// Code running on other threads could even use AwaitIdle() to block
+// all operation until the ThrottledEventQueue drains.
+//
+// Note, this class is similar to TaskQueue, but also differs in a few
+// ways.  First, it is a very simple nsIEventTarget implementation.  It
+// does not use the AbstractThread API.
+//
+// In addition, ThrottledEventQueue currently dispatches its next
+// runnable to the base target *before* running the current event.  This
+// allows the event code to spin the event loop without stalling the
+// ThrottledEventQueue.  In contrast, TaskQueue only dispatches its next
+// runnable after running the current event.  That approach is necessary
+// for TaskQueue in order to work with thread pool targets.
+//
+// So, if you are targeting a thread pool you probably want a TaskQueue.
+// If you are targeting a single thread or other non-concurrent event
+// target, you probably want a ThrottledEventQueue.
+//
+// ThrottledEventQueue also implements an automatic shutdown mechanism.
+// De-referencing the queue or browser shutdown will automatically begin
+// shutdown.
+//
+// Once shutdown begins all events will bypass the queue and be dispatched
+// straight to the underlying base target.
+class ThrottledEventQueue final : public nsIEventTarget
+{
+  class Inner;
+  RefPtr<Inner> mInner;
+
+  explicit ThrottledEventQueue(already_AddRefed<Inner> aInner);
+  ~ThrottledEventQueue();
+
+  // Begin shutdown of the event queue.  This has no effect if shutdown
+  // is already in process.  After this is called nsIEventTarget methods
+  // will bypass the queue and operate directly on the base target.
+  // Note, this could be made public if code needs to explicitly shutdown
+  // for some reason.
+  void MaybeStartShutdown();
+
+public:
+  // Attempt to create a ThrottledEventQueue for the given target.  This
+  // may return nullptr if the browser is already shutting down.
+  static already_AddRefed<ThrottledEventQueue>
+  Create(nsIEventTarget* aBaseTarget);
+
+  // Determine if there are any events pending in the queue.
+  bool IsEmpty() const;
+
+  // Determine how many events are pending in the queue.
+  uint32_t Length() const;
+
+  // Block the current thread until the queue is empty.  This may not
+  // be called on the main thread or the base target.
+  void AwaitIdle() const;
+
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIEVENTTARGET
+};
+
+} // namespace mozilla
+
+#endif // mozilla_ThrottledEventQueue_h
diff --git a/xpcom/threads/TimerThread.cpp b/xpcom/threads/TimerThread.cpp
new file mode 100644
index 000000000..0127e2dd1
--- /dev/null
+++ b/xpcom/threads/TimerThread.cpp
@@ -0,0 +1,752 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "nsTimerImpl.h"
+#include "TimerThread.h"
+
+#include "nsThreadUtils.h"
+#include "plarena.h"
+#include "pratom.h"
+
+#include "nsIObserverService.h"
+#include "nsIServiceManager.h"
+#include "mozilla/Services.h"
+#include "mozilla/ChaosMode.h"
+#include "mozilla/ArrayUtils.h"
+#include "mozilla/BinarySearch.h"
+
+#include <math.h>
+
+using namespace mozilla;
+#ifdef MOZ_TASK_TRACER
+#include "GeckoTaskTracerImpl.h"
+using namespace mozilla::tasktracer;
+#endif
+
+NS_IMPL_ISUPPORTS(TimerThread, nsIRunnable, nsIObserver)
+
+TimerThread::TimerThread() :
+  mInitInProgress(false),
+  mInitialized(false),
+  mMonitor("TimerThread.mMonitor"),
+  mShutdown(false),
+  mWaiting(false),
+  mNotified(false),
+  mSleeping(false)
+{
+}
+
+TimerThread::~TimerThread()
+{
+  mThread = nullptr;
+
+  NS_ASSERTION(mTimers.IsEmpty(), "Timers remain in TimerThread::~TimerThread");
+}
+
+nsresult
+TimerThread::InitLocks()
+{
+  return NS_OK;
+}
+
+namespace {
+
+class TimerObserverRunnable : public Runnable
+{
+public:
+  explicit TimerObserverRunnable(nsIObserver* aObserver)
+    : mObserver(aObserver)
+  {
+  }
+
+  NS_DECL_NSIRUNNABLE
+
+private:
+  nsCOMPtr<nsIObserver> mObserver;
+};
+
+NS_IMETHODIMP
+TimerObserverRunnable::Run()
+{
+  nsCOMPtr<nsIObserverService> observerService =
+    mozilla::services::GetObserverService();
+  if (observerService) {
+    observerService->AddObserver(mObserver, "sleep_notification", false);
+    observerService->AddObserver(mObserver, "wake_notification", false);
+    observerService->AddObserver(mObserver, "suspend_process_notification", false);
+    observerService->AddObserver(mObserver, "resume_process_notification", false);
+  }
+  return NS_OK;
+}
+
+} // namespace
+
+namespace {
+
+// TimerEventAllocator is a thread-safe allocator used only for nsTimerEvents.
+// It's needed to avoid contention over the default allocator lock when
+// firing timer events (see bug 733277).  The thread-safety is required because
+// nsTimerEvent objects are allocated on the timer thread, and freed on another
+// thread.  Because TimerEventAllocator has its own lock, contention over that
+// lock is limited to the allocation and deallocation of nsTimerEvent objects.
+//
+// Because this allocator is layered over PLArenaPool, it never shrinks -- even
+// "freed" nsTimerEvents aren't truly freed, they're just put onto a free-list
+// for later recycling.  So the amount of memory consumed will always be equal
+// to the high-water mark consumption.  But nsTimerEvents are small and it's
+// unusual to have more than a few hundred of them, so this shouldn't be a
+// problem in practice.
+
+class TimerEventAllocator
+{
+private:
+  struct FreeEntry
+  {
+    FreeEntry* mNext;
+  };
+
+  PLArenaPool mPool;
+  FreeEntry* mFirstFree;
+  mozilla::Monitor mMonitor;
+
+public:
+  TimerEventAllocator()
+    : mFirstFree(nullptr)
+    , mMonitor("TimerEventAllocator")
+  {
+    PL_InitArenaPool(&mPool, "TimerEventPool", 4096, /* align = */ 0);
+  }
+
+  ~TimerEventAllocator()
+  {
+    PL_FinishArenaPool(&mPool);
+  }
+
+  void* Alloc(size_t aSize);
+  void Free(void* aPtr);
+};
+
+} // namespace
+
+// This is a nsICancelableRunnable because we can dispatch it to Workers and
+// those can be shut down at any time, and in these cases, Cancel() is called
+// instead of Run().
+class nsTimerEvent final : public CancelableRunnable
+{
+public:
+  NS_IMETHOD Run() override;
+
+  nsresult Cancel() override
+  {
+    // Since nsTimerImpl is not thread-safe, we should release |mTimer|
+    // here in the target thread to avoid race condition. Otherwise,
+    // ~nsTimerEvent() which calls nsTimerImpl::Release() could run in the
+    // timer thread and result in race condition.
+    mTimer = nullptr;
+    return NS_OK;
+  }
+
+  nsTimerEvent()
+    : mTimer()
+    , mGeneration(0)
+  {
+    // Note: We override operator new for this class, and the override is
+    // fallible!
+    sAllocatorUsers++;
+  }
+
+  TimeStamp mInitTime;
+
+  static void Init();
+  static void Shutdown();
+  static void DeleteAllocatorIfNeeded();
+
+  static void* operator new(size_t aSize) CPP_THROW_NEW
+  {
+    return sAllocator->Alloc(aSize);
+  }
+  void operator delete(void* aPtr)
+  {
+    sAllocator->Free(aPtr);
+    DeleteAllocatorIfNeeded();
+  }
+
+  already_AddRefed<nsTimerImpl> ForgetTimer()
+  {
+    return mTimer.forget();
+  }
+
+  void SetTimer(already_AddRefed<nsTimerImpl> aTimer)
+  {
+    mTimer = aTimer;
+    mGeneration = mTimer->GetGeneration();
+  }
+
+private:
+  nsTimerEvent(const nsTimerEvent&) = delete;
+  nsTimerEvent& operator=(const nsTimerEvent&) = delete;
+  nsTimerEvent& operator=(const nsTimerEvent&&) = delete;
+
+  ~nsTimerEvent()
+  {
+    MOZ_ASSERT(!sCanDeleteAllocator || sAllocatorUsers > 0,
+               "This will result in us attempting to deallocate the nsTimerEvent allocator twice");
+    sAllocatorUsers--;
+  }
+
+  RefPtr<nsTimerImpl> mTimer;
+  int32_t      mGeneration;
+
+  static TimerEventAllocator* sAllocator;
+  static Atomic<int32_t> sAllocatorUsers;
+  static bool sCanDeleteAllocator;
+};
+
+TimerEventAllocator* nsTimerEvent::sAllocator = nullptr;
+Atomic<int32_t> nsTimerEvent::sAllocatorUsers;
+bool nsTimerEvent::sCanDeleteAllocator = false;
+
+namespace {
+
+void*
+TimerEventAllocator::Alloc(size_t aSize)
+{
+  MOZ_ASSERT(aSize == sizeof(nsTimerEvent));
+
+  mozilla::MonitorAutoLock lock(mMonitor);
+
+  void* p;
+  if (mFirstFree) {
+    p = mFirstFree;
+    mFirstFree = mFirstFree->mNext;
+  } else {
+    PL_ARENA_ALLOCATE(p, &mPool, aSize);
+    if (!p) {
+      return nullptr;
+    }
+  }
+
+  return p;
+}
+
+void
+TimerEventAllocator::Free(void* aPtr)
+{
+  mozilla::MonitorAutoLock lock(mMonitor);
+
+  FreeEntry* entry = reinterpret_cast<FreeEntry*>(aPtr);
+
+  entry->mNext = mFirstFree;
+  mFirstFree = entry;
+}
+
+} // namespace
+
+void
+nsTimerEvent::Init()
+{
+  sAllocator = new TimerEventAllocator();
+}
+
+void
+nsTimerEvent::Shutdown()
+{
+  sCanDeleteAllocator = true;
+  DeleteAllocatorIfNeeded();
+}
+
+void
+nsTimerEvent::DeleteAllocatorIfNeeded()
+{
+  if (sCanDeleteAllocator && sAllocatorUsers == 0) {
+    delete sAllocator;
+    sAllocator = nullptr;
+  }
+}
+
+NS_IMETHODIMP
+nsTimerEvent::Run()
+{
+  if (!mTimer) {
+    MOZ_ASSERT(false);
+    return NS_OK;
+  }
+
+  if (MOZ_LOG_TEST(GetTimerLog(), LogLevel::Debug)) {
+    TimeStamp now = TimeStamp::Now();
+    MOZ_LOG(GetTimerLog(), LogLevel::Debug,
+           ("[this=%p] time between PostTimerEvent() and Fire(): %fms\n",
+            this, (now - mInitTime).ToMilliseconds()));
+  }
+
+  mTimer->Fire(mGeneration);
+
+  // We call Cancel() to correctly release mTimer.
+  // Read more in the Cancel() implementation.
+  return Cancel();
+}
+
+nsresult
+TimerThread::Init()
+{
+  MOZ_LOG(GetTimerLog(), LogLevel::Debug,
+         ("TimerThread::Init [%d]\n", mInitialized));
+
+  if (mInitialized) {
+    if (!mThread) {
+      return NS_ERROR_FAILURE;
+    }
+
+    return NS_OK;
+  }
+
+  nsTimerEvent::Init();
+
+  if (mInitInProgress.exchange(true) == false) {
+    // We hold on to mThread to keep the thread alive.
+    nsresult rv = NS_NewThread(getter_AddRefs(mThread), this);
+    if (NS_FAILED(rv)) {
+      mThread = nullptr;
+    } else {
+      RefPtr<TimerObserverRunnable> r = new TimerObserverRunnable(this);
+      if (NS_IsMainThread()) {
+        r->Run();
+      } else {
+        NS_DispatchToMainThread(r);
+      }
+    }
+
+    {
+      MonitorAutoLock lock(mMonitor);
+      mInitialized = true;
+      mMonitor.NotifyAll();
+    }
+  } else {
+    MonitorAutoLock lock(mMonitor);
+    while (!mInitialized) {
+      mMonitor.Wait();
+    }
+  }
+
+  if (!mThread) {
+    return NS_ERROR_FAILURE;
+  }
+
+  return NS_OK;
+}
+
+nsresult
+TimerThread::Shutdown()
+{
+  MOZ_LOG(GetTimerLog(), LogLevel::Debug, ("TimerThread::Shutdown begin\n"));
+
+  if (!mThread) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  nsTArray<nsTimerImpl*> timers;
+  {
+    // lock scope
+    MonitorAutoLock lock(mMonitor);
+
+    mShutdown = true;
+
+    // notify the cond var so that Run() can return
+    if (mWaiting) {
+      mNotified = true;
+      mMonitor.Notify();
+    }
+
+    // Need to copy content of mTimers array to a local array
+    // because call to timers' Cancel() (and release its self)
+    // must not be done under the lock. Destructor of a callback
+    // might potentially call some code reentering the same lock
+    // that leads to unexpected behavior or deadlock.
+    // See bug 422472.
+    timers.AppendElements(mTimers);
+    mTimers.Clear();
+  }
+
+  uint32_t timersCount = timers.Length();
+  for (uint32_t i = 0; i < timersCount; i++) {
+    nsTimerImpl* timer = timers[i];
+    timer->Cancel();
+    ReleaseTimerInternal(timer);
+  }
+
+  mThread->Shutdown();    // wait for the thread to die
+
+  nsTimerEvent::Shutdown();
+
+  MOZ_LOG(GetTimerLog(), LogLevel::Debug, ("TimerThread::Shutdown end\n"));
+  return NS_OK;
+}
+
+namespace {
+
+struct MicrosecondsToInterval
+{
+  PRIntervalTime operator[](size_t aMs) const {
+    return PR_MicrosecondsToInterval(aMs);
+  }
+};
+
+struct IntervalComparator
+{
+  int operator()(PRIntervalTime aInterval) const {
+    return (0 < aInterval) ? -1 : 1;
+  }
+};
+
+} // namespace
+
+NS_IMETHODIMP
+TimerThread::Run()
+{
+  PR_SetCurrentThreadName("Timer");
+
+  MonitorAutoLock lock(mMonitor);
+
+  // We need to know how many microseconds give a positive PRIntervalTime. This
+  // is platform-dependent and we calculate it at runtime, finding a value |v|
+  // such that |PR_MicrosecondsToInterval(v) > 0| and then binary-searching in
+  // the range [0, v) to find the ms-to-interval scale.
+  uint32_t usForPosInterval = 1;
+  while (PR_MicrosecondsToInterval(usForPosInterval) == 0) {
+    usForPosInterval <<= 1;
+  }
+
+  size_t usIntervalResolution;
+  BinarySearchIf(MicrosecondsToInterval(), 0, usForPosInterval, IntervalComparator(), &usIntervalResolution);
+  MOZ_ASSERT(PR_MicrosecondsToInterval(usIntervalResolution - 1) == 0);
+  MOZ_ASSERT(PR_MicrosecondsToInterval(usIntervalResolution) == 1);
+
+  // Half of the amount of microseconds needed to get positive PRIntervalTime.
+  // We use this to decide how to round our wait times later
+  int32_t halfMicrosecondsIntervalResolution = usIntervalResolution / 2;
+  bool forceRunNextTimer = false;
+
+  while (!mShutdown) {
+    // Have to use PRIntervalTime here, since PR_WaitCondVar takes it
+    PRIntervalTime waitFor;
+    bool forceRunThisTimer = forceRunNextTimer;
+    forceRunNextTimer = false;
+
+    if (mSleeping) {
+      // Sleep for 0.1 seconds while not firing timers.
+      uint32_t milliseconds = 100;
+      if (ChaosMode::isActive(ChaosFeature::TimerScheduling)) {
+        milliseconds = ChaosMode::randomUint32LessThan(200);
+      }
+      waitFor = PR_MillisecondsToInterval(milliseconds);
+    } else {
+      waitFor = PR_INTERVAL_NO_TIMEOUT;
+      TimeStamp now = TimeStamp::Now();
+      nsTimerImpl* timer = nullptr;
+
+      if (!mTimers.IsEmpty()) {
+        timer = mTimers[0];
+
+        if (now >= timer->mTimeout || forceRunThisTimer) {
+    next:
+          // NB: AddRef before the Release under RemoveTimerInternal to avoid
+          // mRefCnt passing through zero, in case all other refs than the one
+          // from mTimers have gone away (the last non-mTimers[i]-ref's Release
+          // must be racing with us, blocked in gThread->RemoveTimer waiting
+          // for TimerThread::mMonitor, under nsTimerImpl::Release.
+
+          RefPtr<nsTimerImpl> timerRef(timer);
+          RemoveTimerInternal(timer);
+          timer = nullptr;
+
+          MOZ_LOG(GetTimerLog(), LogLevel::Debug,
+                 ("Timer thread woke up %fms from when it was supposed to\n",
+                  fabs((now - timerRef->mTimeout).ToMilliseconds())));
+
+          // We are going to let the call to PostTimerEvent here handle the
+          // release of the timer so that we don't end up releasing the timer
+          // on the TimerThread instead of on the thread it targets.
+          timerRef = PostTimerEvent(timerRef.forget());
+
+          if (timerRef) {
+            // We got our reference back due to an error.
+            // Unhook the nsRefPtr, and release manually so we can get the
+            // refcount.
+            nsrefcnt rc = timerRef.forget().take()->Release();
+            (void)rc;
+
+            // The nsITimer interface requires that its users keep a reference
+            // to the timers they use while those timers are initialized but
+            // have not yet fired.  If this ever happens, it is a bug in the
+            // code that created and used the timer.
+            //
+            // Further, note that this should never happen even with a
+            // misbehaving user, because nsTimerImpl::Release checks for a
+            // refcount of 1 with an armed timer (a timer whose only reference
+            // is from the timer thread) and when it hits this will remove the
+            // timer from the timer thread and thus destroy the last reference,
+            // preventing this situation from occurring.
+            MOZ_ASSERT(rc != 0, "destroyed timer off its target thread!");
+          }
+
+          if (mShutdown) {
+            break;
+          }
+
+          // Update now, as PostTimerEvent plus the locking may have taken a
+          // tick or two, and we may goto next below.
+          now = TimeStamp::Now();
+        }
+      }
+
+      if (!mTimers.IsEmpty()) {
+        timer = mTimers[0];
+
+        TimeStamp timeout = timer->mTimeout;
+
+        // Don't wait at all (even for PR_INTERVAL_NO_WAIT) if the next timer
+        // is due now or overdue.
+        //
+        // Note that we can only sleep for integer values of a certain
+        // resolution. We use halfMicrosecondsIntervalResolution, calculated
+        // before, to do the optimal rounding (i.e., of how to decide what
+        // interval is so small we should not wait at all).
+        double microseconds = (timeout - now).ToMilliseconds() * 1000;
+
+        if (ChaosMode::isActive(ChaosFeature::TimerScheduling)) {
+          // The mean value of sFractions must be 1 to ensure that
+          // the average of a long sequence of timeouts converges to the
+          // actual sum of their times.
+          static const float sFractions[] = {
+            0.0f, 0.25f, 0.5f, 0.75f, 1.0f, 1.75f, 2.75f
+          };
+          microseconds *=
+            sFractions[ChaosMode::randomUint32LessThan(ArrayLength(sFractions))];
+          forceRunNextTimer = true;
+        }
+
+        if (microseconds < halfMicrosecondsIntervalResolution) {
+          forceRunNextTimer = false;
+          goto next; // round down; execute event now
+        }
+        waitFor = PR_MicrosecondsToInterval(
+          static_cast<uint32_t>(microseconds)); // Floor is accurate enough.
+        if (waitFor == 0) {
+          waitFor = 1;  // round up, wait the minimum time we can wait
+        }
+      }
+
+      if (MOZ_LOG_TEST(GetTimerLog(), LogLevel::Debug)) {
+        if (waitFor == PR_INTERVAL_NO_TIMEOUT)
+          MOZ_LOG(GetTimerLog(), LogLevel::Debug,
+                 ("waiting for PR_INTERVAL_NO_TIMEOUT\n"));
+        else
+          MOZ_LOG(GetTimerLog(), LogLevel::Debug,
+                 ("waiting for %u\n", PR_IntervalToMilliseconds(waitFor)));
+      }
+    }
+
+    mWaiting = true;
+    mNotified = false;
+    mMonitor.Wait(waitFor);
+    if (mNotified) {
+      forceRunNextTimer = false;
+    }
+    mWaiting = false;
+  }
+
+  return NS_OK;
+}
+
+nsresult
+TimerThread::AddTimer(nsTimerImpl* aTimer)
+{
+  MonitorAutoLock lock(mMonitor);
+
+  if (!aTimer->mEventTarget) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  // Add the timer to our list.
+  int32_t i = AddTimerInternal(aTimer);
+  if (i < 0) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  // Awaken the timer thread.
+  if (mWaiting && i == 0) {
+    mNotified = true;
+    mMonitor.Notify();
+  }
+
+  return NS_OK;
+}
+
+nsresult
+TimerThread::RemoveTimer(nsTimerImpl* aTimer)
+{
+  MonitorAutoLock lock(mMonitor);
+
+  // Remove the timer from our array.  Tell callers that aTimer was not found
+  // by returning NS_ERROR_NOT_AVAILABLE.
+
+  if (!RemoveTimerInternal(aTimer)) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+
+  // Awaken the timer thread.
+  if (mWaiting) {
+    mNotified = true;
+    mMonitor.Notify();
+  }
+
+  return NS_OK;
+}
+
+// This function must be called from within a lock
+int32_t
+TimerThread::AddTimerInternal(nsTimerImpl* aTimer)
+{
+  mMonitor.AssertCurrentThreadOwns();
+  if (mShutdown) {
+    return -1;
+  }
+
+  TimeStamp now = TimeStamp::Now();
+
+  TimerAdditionComparator c(now, aTimer);
+  nsTimerImpl** insertSlot = mTimers.InsertElementSorted(aTimer, c);
+
+  if (!insertSlot) {
+    return -1;
+  }
+
+  NS_ADDREF(aTimer);
+
+#ifdef MOZ_TASK_TRACER
+  // Caller of AddTimer is the parent task of its timer event, so we store the
+  // TraceInfo here for later used.
+  aTimer->GetTLSTraceInfo();
+#endif
+
+  return insertSlot - mTimers.Elements();
+}
+
+bool
+TimerThread::RemoveTimerInternal(nsTimerImpl* aTimer)
+{
+  mMonitor.AssertCurrentThreadOwns();
+  if (!mTimers.RemoveElement(aTimer)) {
+    return false;
+  }
+
+  ReleaseTimerInternal(aTimer);
+  return true;
+}
+
+void
+TimerThread::ReleaseTimerInternal(nsTimerImpl* aTimer)
+{
+  if (!mShutdown) {
+    // copied to a local array before releasing in shutdown
+    mMonitor.AssertCurrentThreadOwns();
+  }
+  NS_RELEASE(aTimer);
+}
+
+already_AddRefed<nsTimerImpl>
+TimerThread::PostTimerEvent(already_AddRefed<nsTimerImpl> aTimerRef)
+{
+  mMonitor.AssertCurrentThreadOwns();
+
+  RefPtr<nsTimerImpl> timer(aTimerRef);
+  if (!timer->mEventTarget) {
+    NS_ERROR("Attempt to post timer event to NULL event target");
+    return timer.forget();
+  }
+
+  // XXX we may want to reuse this nsTimerEvent in the case of repeating timers.
+
+  // Since we already addref'd 'timer', we don't need to addref here.
+  // We will release either in ~nsTimerEvent(), or pass the reference back to
+  // the caller. We need to copy the generation number from this timer into the
+  // event, so we can avoid firing a timer that was re-initialized after being
+  // canceled.
+
+  RefPtr<nsTimerEvent> event = new nsTimerEvent;
+  if (!event) {
+    return timer.forget();
+  }
+
+  if (MOZ_LOG_TEST(GetTimerLog(), LogLevel::Debug)) {
+    event->mInitTime = TimeStamp::Now();
+  }
+
+#ifdef MOZ_TASK_TRACER
+  // During the dispatch of TimerEvent, we overwrite the current TraceInfo
+  // partially with the info saved in timer earlier, and restore it back by
+  // AutoSaveCurTraceInfo.
+  AutoSaveCurTraceInfo saveCurTraceInfo;
+  (timer->GetTracedTask()).SetTLSTraceInfo();
+#endif
+
+  nsCOMPtr<nsIEventTarget> target = timer->mEventTarget;
+  event->SetTimer(timer.forget());
+
+  nsresult rv;
+  {
+    // We release mMonitor around the Dispatch because if this timer is targeted
+    // at the TimerThread we'll deadlock.
+    MonitorAutoUnlock unlock(mMonitor);
+    rv = target->Dispatch(event, NS_DISPATCH_NORMAL);
+  }
+
+  if (NS_FAILED(rv)) {
+    timer = event->ForgetTimer();
+    RemoveTimerInternal(timer);
+    return timer.forget();
+  }
+
+  return nullptr;
+}
+
+void
+TimerThread::DoBeforeSleep()
+{
+  // Mainthread
+  MonitorAutoLock lock(mMonitor);
+  mSleeping = true;
+}
+
+// Note: wake may be notified without preceding sleep notification
+void
+TimerThread::DoAfterSleep()
+{
+  // Mainthread
+  MonitorAutoLock lock(mMonitor);
+  mSleeping = false;
+
+  // Wake up the timer thread to re-process the array to ensure the sleep delay is correct,
+  // and fire any expired timers (perhaps quite a few)
+  mNotified = true;
+  mMonitor.Notify();
+}
+
+
+NS_IMETHODIMP
+TimerThread::Observe(nsISupports* /* aSubject */, const char* aTopic,
+                     const char16_t* /* aData */)
+{
+  if (strcmp(aTopic, "sleep_notification") == 0 ||
+      strcmp(aTopic, "suspend_process_notification") == 0) {
+    DoBeforeSleep();
+  } else if (strcmp(aTopic, "wake_notification") == 0 ||
+             strcmp(aTopic, "resume_process_notification") == 0) {
+    DoAfterSleep();
+  }
+
+  return NS_OK;
+}
diff --git a/xpcom/threads/TimerThread.h b/xpcom/threads/TimerThread.h
new file mode 100644
index 000000000..a7204810a
--- /dev/null
+++ b/xpcom/threads/TimerThread.h
@@ -0,0 +1,115 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 TimerThread_h___
+#define TimerThread_h___
+
+#include "nsIObserver.h"
+#include "nsIRunnable.h"
+#include "nsIThread.h"
+
+#include "nsTimerImpl.h"
+#include "nsThreadUtils.h"
+
+#include "nsTArray.h"
+
+#include "mozilla/Atomics.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/Monitor.h"
+
+namespace mozilla {
+class TimeStamp;
+} // namespace mozilla
+
+class TimerThread final
+  : public nsIRunnable
+  , public nsIObserver
+{
+public:
+  typedef mozilla::Monitor Monitor;
+  typedef mozilla::TimeStamp TimeStamp;
+  typedef mozilla::TimeDuration TimeDuration;
+
+  TimerThread();
+  nsresult InitLocks();
+
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIRUNNABLE
+  NS_DECL_NSIOBSERVER
+
+  nsresult Init();
+  nsresult Shutdown();
+
+  nsresult AddTimer(nsTimerImpl* aTimer);
+  nsresult RemoveTimer(nsTimerImpl* aTimer);
+
+  void DoBeforeSleep();
+  void DoAfterSleep();
+
+  bool IsOnTimerThread() const
+  {
+    return mThread == NS_GetCurrentThread();
+  }
+
+private:
+  ~TimerThread();
+
+  mozilla::Atomic<bool> mInitInProgress;
+  bool    mInitialized;
+
+  // These two internal helper methods must be called while mMonitor is held.
+  // AddTimerInternal returns the position where the timer was added in the
+  // list, or -1 if it failed.
+  int32_t AddTimerInternal(nsTimerImpl* aTimer);
+  bool    RemoveTimerInternal(nsTimerImpl* aTimer);
+  void    ReleaseTimerInternal(nsTimerImpl* aTimer);
+
+  already_AddRefed<nsTimerImpl> PostTimerEvent(already_AddRefed<nsTimerImpl> aTimerRef);
+
+  nsCOMPtr<nsIThread> mThread;
+  Monitor mMonitor;
+
+  bool mShutdown;
+  bool mWaiting;
+  bool mNotified;
+  bool mSleeping;
+
+  nsTArray<nsTimerImpl*> mTimers;
+};
+
+struct TimerAdditionComparator
+{
+  TimerAdditionComparator(const mozilla::TimeStamp& aNow,
+                          nsTimerImpl* aTimerToInsert) :
+    now(aNow)
+#ifdef DEBUG
+    , timerToInsert(aTimerToInsert)
+#endif
+  {
+  }
+
+  bool LessThan(nsTimerImpl* aFromArray, nsTimerImpl* aNewTimer) const
+  {
+    MOZ_ASSERT(aNewTimer == timerToInsert, "Unexpected timer ordering");
+
+    // Skip any overdue timers.
+    return aFromArray->mTimeout <= now ||
+           aFromArray->mTimeout <= aNewTimer->mTimeout;
+  }
+
+  bool Equals(nsTimerImpl* aFromArray, nsTimerImpl* aNewTimer) const
+  {
+    return false;
+  }
+
+private:
+  const mozilla::TimeStamp& now;
+#ifdef DEBUG
+  const nsTimerImpl* const timerToInsert;
+#endif
+};
+
+#endif /* TimerThread_h___ */
diff --git a/xpcom/threads/moz.build b/xpcom/threads/moz.build
new file mode 100644
index 000000000..5d54a4bf4
--- /dev/null
+++ b/xpcom/threads/moz.build
@@ -0,0 +1,89 @@
+# -*- 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/.
+
+XPIDL_SOURCES += [
+    'nsIEnvironment.idl',
+    'nsIEventTarget.idl',
+    'nsIIdlePeriod.idl',
+    'nsIProcess.idl',
+    'nsIRunnable.idl',
+    'nsISupportsPriority.idl',
+    'nsIThread.idl',
+    'nsIThreadInternal.idl',
+    'nsIThreadManager.idl',
+    'nsIThreadPool.idl',
+    'nsITimer.idl',
+]
+
+XPIDL_MODULE = 'xpcom_threads'
+
+EXPORTS += [
+    'nsEventQueue.h',
+    'nsICancelableRunnable.h',
+    'nsIIncrementalRunnable.h',
+    'nsMemoryPressure.h',
+    'nsProcess.h',
+    'nsThread.h',
+]
+
+EXPORTS.mozilla += [
+    'AbstractThread.h',
+    'BackgroundHangMonitor.h',
+    'HangAnnotations.h',
+    'HangMonitor.h',
+    'LazyIdleThread.h',
+    'MainThreadIdlePeriod.h',
+    'MozPromise.h',
+    'SharedThreadPool.h',
+    'StateMirroring.h',
+    'StateWatching.h',
+    'SyncRunnable.h',
+    'TaskDispatcher.h',
+    'TaskQueue.h',
+    'ThrottledEventQueue.h',
+]
+
+UNIFIED_SOURCES += [
+    'AbstractThread.cpp',
+    'BackgroundHangMonitor.cpp',
+    'HangAnnotations.cpp',
+    'HangMonitor.cpp',
+    'LazyIdleThread.cpp',
+    'MainThreadIdlePeriod.cpp',
+    'nsEnvironment.cpp',
+    'nsEventQueue.cpp',
+    'nsMemoryPressure.cpp',
+    'nsProcessCommon.cpp',
+    'nsThread.cpp',
+    'nsThreadManager.cpp',
+    'nsThreadPool.cpp',
+    'nsTimerImpl.cpp',
+    'SharedThreadPool.cpp',
+    'TaskQueue.cpp',
+    'ThreadStackHelper.cpp',
+    'ThrottledEventQueue.cpp',
+    'TimerThread.cpp',
+]
+
+LOCAL_INCLUDES += [
+    '../build',
+    '/caps',
+    '/tools/profiler',
+]
+
+# BHR disabled for Release builds because of bug 965392.
+# BHR disabled for debug builds because of bug 979069.
+# BHR disabled on gonk because of bug 1180533
+# BHR disabled for TSan builds because of bug 1121216.
+if CONFIG['MOZ_UPDATE_CHANNEL'] not in ('release') and \
+   not CONFIG['MOZ_DEBUG'] and \
+   not CONFIG['MOZ_WIDGET_TOOLKIT'] == 'gonk' and \
+   not CONFIG['MOZ_TSAN']:
+    DEFINES['MOZ_ENABLE_BACKGROUND_HANG_MONITOR'] = 1
+
+FINAL_LIBRARY = 'xul'
+
+include('/ipc/chromium/chromium-config.mozbuild')
diff --git a/xpcom/threads/nsEnvironment.cpp b/xpcom/threads/nsEnvironment.cpp
new file mode 100644
index 000000000..0de56675e
--- /dev/null
+++ b/xpcom/threads/nsEnvironment.cpp
@@ -0,0 +1,163 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "nsEnvironment.h"
+#include "prenv.h"
+#include "prprf.h"
+#include "nsBaseHashtable.h"
+#include "nsHashKeys.h"
+#include "nsPromiseFlatString.h"
+#include "nsDependentString.h"
+#include "nsNativeCharsetUtils.h"
+
+using namespace mozilla;
+
+NS_IMPL_ISUPPORTS(nsEnvironment, nsIEnvironment)
+
+nsresult
+nsEnvironment::Create(nsISupports* aOuter, REFNSIID aIID, void** aResult)
+{
+  nsresult rv;
+  *aResult = nullptr;
+
+  if (aOuter) {
+    return NS_ERROR_NO_AGGREGATION;
+  }
+
+  nsEnvironment* obj = new nsEnvironment();
+
+  rv = obj->QueryInterface(aIID, aResult);
+  if (NS_FAILED(rv)) {
+    delete obj;
+  }
+  return rv;
+}
+
+nsEnvironment::~nsEnvironment()
+{
+}
+
+NS_IMETHODIMP
+nsEnvironment::Exists(const nsAString& aName, bool* aOutValue)
+{
+  nsAutoCString nativeName;
+  nsresult rv = NS_CopyUnicodeToNative(aName, nativeName);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  nsAutoCString nativeVal;
+#if defined(XP_UNIX)
+  /* For Unix/Linux platforms we follow the Unix definition:
+   * An environment variable exists when |getenv()| returns a non-nullptr
+   * value. An environment variable does not exist when |getenv()| returns
+   * nullptr.
+   */
+  const char* value = PR_GetEnv(nativeName.get());
+  *aOutValue = value && *value;
+#else
+  /* For non-Unix/Linux platforms we have to fall back to a
+   * "portable" definition (which is incorrect for Unix/Linux!!!!)
+   * which simply checks whether the string returned by |Get()| is empty
+   * or not.
+   */
+  nsAutoString value;
+  Get(aName, value);
+  *aOutValue = !value.IsEmpty();
+#endif /* XP_UNIX */
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsEnvironment::Get(const nsAString& aName, nsAString& aOutValue)
+{
+  nsAutoCString nativeName;
+  nsresult rv = NS_CopyUnicodeToNative(aName, nativeName);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  nsAutoCString nativeVal;
+  const char* value = PR_GetEnv(nativeName.get());
+  if (value && *value) {
+    rv = NS_CopyNativeToUnicode(nsDependentCString(value), aOutValue);
+  } else {
+    aOutValue.Truncate();
+    rv = NS_OK;
+  }
+
+  return rv;
+}
+
+/* Environment strings must have static duration; We're gonna leak all of this
+ * at shutdown: this is by design, caused how Unix/Linux implement environment
+ * vars.
+ */
+
+typedef nsBaseHashtableET<nsCharPtrHashKey, char*> EnvEntryType;
+typedef nsTHashtable<EnvEntryType> EnvHashType;
+
+static EnvHashType* gEnvHash = nullptr;
+
+static bool
+EnsureEnvHash()
+{
+  if (gEnvHash) {
+    return true;
+  }
+
+  gEnvHash = new EnvHashType;
+  if (!gEnvHash) {
+    return false;
+  }
+
+  return true;
+}
+
+NS_IMETHODIMP
+nsEnvironment::Set(const nsAString& aName, const nsAString& aValue)
+{
+  nsAutoCString nativeName;
+  nsAutoCString nativeVal;
+
+  nsresult rv = NS_CopyUnicodeToNative(aName, nativeName);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  rv = NS_CopyUnicodeToNative(aValue, nativeVal);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  MutexAutoLock lock(mLock);
+
+  if (!EnsureEnvHash()) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  EnvEntryType* entry = gEnvHash->PutEntry(nativeName.get());
+  if (!entry) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  char* newData = PR_smprintf("%s=%s",
+                              nativeName.get(),
+                              nativeVal.get());
+  if (!newData) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  PR_SetEnv(newData);
+  if (entry->mData) {
+    PR_smprintf_free(entry->mData);
+  }
+  entry->mData = newData;
+  return NS_OK;
+}
+
+
diff --git a/xpcom/threads/nsEnvironment.h b/xpcom/threads/nsEnvironment.h
new file mode 100644
index 000000000..234055a07
--- /dev/null
+++ b/xpcom/threads/nsEnvironment.h
@@ -0,0 +1,36 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 nsEnvironment_h__
+#define nsEnvironment_h__
+
+#include "mozilla/Attributes.h"
+#include "mozilla/Mutex.h"
+#include "nsIEnvironment.h"
+
+#define NS_ENVIRONMENT_CID \
+  { 0X3D68F92UL, 0X9513, 0X4E25, \
+  { 0X9B, 0XE9, 0X7C, 0XB2, 0X39, 0X87, 0X41, 0X72 } }
+#define NS_ENVIRONMENT_CONTRACTID "@mozilla.org/process/environment;1"
+
+class nsEnvironment final : public nsIEnvironment
+{
+public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIENVIRONMENT
+
+  static nsresult Create(nsISupports* aOuter, REFNSIID aIID, void** aResult);
+
+private:
+  nsEnvironment() : mLock("nsEnvironment.mLock")
+  {
+  }
+  ~nsEnvironment();
+
+  mozilla::Mutex mLock;
+};
+
+#endif /* !nsEnvironment_h__ */
diff --git a/xpcom/threads/nsEventQueue.cpp b/xpcom/threads/nsEventQueue.cpp
new file mode 100644
index 000000000..4ca2f11ea
--- /dev/null
+++ b/xpcom/threads/nsEventQueue.cpp
@@ -0,0 +1,155 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "nsEventQueue.h"
+#include "nsAutoPtr.h"
+#include "mozilla/Logging.h"
+#include "nsThreadUtils.h"
+#include "prthread.h"
+#include "mozilla/ChaosMode.h"
+
+using namespace mozilla;
+
+static LazyLogModule sEventQueueLog("nsEventQueue");
+#ifdef LOG
+#undef LOG
+#endif
+#define LOG(args) MOZ_LOG(sEventQueueLog, mozilla::LogLevel::Debug, args)
+
+nsEventQueue::nsEventQueue(mozilla::CondVar& aCondVar, EventQueueType aType)
+  : mHead(nullptr)
+  , mTail(nullptr)
+  , mOffsetHead(0)
+  , mOffsetTail(0)
+  , mEventsAvailable(aCondVar)
+  , mType(aType)
+{
+}
+
+nsEventQueue::~nsEventQueue()
+{
+  // It'd be nice to be able to assert that no one else is holding the lock,
+  // but NSPR doesn't really expose APIs for it.
+  NS_ASSERTION(IsEmpty(),
+               "Non-empty event queue being destroyed; events being leaked.");
+
+  if (mHead) {
+    FreePage(mHead);
+  }
+}
+
+bool
+nsEventQueue::GetEvent(bool aMayWait, nsIRunnable** aResult,
+                       MutexAutoLock& aProofOfLock)
+{
+  if (aResult) {
+    *aResult = nullptr;
+  }
+
+  while (IsEmpty()) {
+    if (!aMayWait) {
+      return false;
+    }
+    LOG(("EVENTQ(%p): wait begin\n", this));
+    mEventsAvailable.Wait();
+    LOG(("EVENTQ(%p): wait end\n", this));
+
+    if (mType == eSharedCondVarQueue) {
+      if (IsEmpty()) {
+        return false;
+      }
+      break;
+    }
+  }
+
+  if (aResult) {
+    MOZ_ASSERT(mOffsetHead < EVENTS_PER_PAGE);
+    MOZ_ASSERT_IF(mHead == mTail, mOffsetHead <= mOffsetTail);
+    *aResult = mHead->mEvents[mOffsetHead++];
+
+    MOZ_ASSERT(*aResult);
+    MOZ_ASSERT(mOffsetHead <= EVENTS_PER_PAGE);
+
+    // Check if mHead points to empty Page
+    if (mOffsetHead == EVENTS_PER_PAGE) {
+      Page* dead = mHead;
+      mHead = mHead->mNext;
+      FreePage(dead);
+      mOffsetHead = 0;
+    }
+  }
+
+  return true;
+}
+
+void
+nsEventQueue::PutEvent(already_AddRefed<nsIRunnable>&& aRunnable,
+                       MutexAutoLock& aProofOfLock)
+{
+  if (!mHead) {
+    mHead = NewPage();
+    MOZ_ASSERT(mHead);
+
+    mTail = mHead;
+    mOffsetHead = 0;
+    mOffsetTail = 0;
+  } else if (mOffsetTail == EVENTS_PER_PAGE) {
+    Page* page = NewPage();
+    MOZ_ASSERT(page);
+
+    mTail->mNext = page;
+    mTail = page;
+    mOffsetTail = 0;
+  }
+
+  nsIRunnable*& queueLocation = mTail->mEvents[mOffsetTail];
+  MOZ_ASSERT(!queueLocation);
+  queueLocation = aRunnable.take();
+  ++mOffsetTail;
+  LOG(("EVENTQ(%p): notify\n", this));
+  mEventsAvailable.Notify();
+}
+
+void
+nsEventQueue::PutEvent(nsIRunnable* aRunnable, MutexAutoLock& aProofOfLock)
+{
+  nsCOMPtr<nsIRunnable> event(aRunnable);
+  PutEvent(event.forget(), aProofOfLock);
+}
+
+size_t
+nsEventQueue::Count(MutexAutoLock& aProofOfLock) const
+{
+  // It is obvious count is 0 when the queue is empty.
+  if (!mHead) {
+    return 0;
+  }
+
+  /* How we count the number of events in the queue:
+   * 1. Let pageCount(x, y) denote the number of pages excluding the tail page
+   *    where x is the index of head page and y is the index of the tail page.
+   * 2. Then we have pageCount(x, y) = y - x.
+   *
+   * Ex: pageCount(0, 0) = 0 where both head and tail pages point to page 0.
+   *     pageCount(0, 1) = 1 where head points to page 0 and tail points page 1.
+   *
+   * 3. number of events = (EVENTS_PER_PAGE * pageCount(x, y))
+   *      - (empty slots in head page) + (non-empty slots in tail page)
+   *      = (EVENTS_PER_PAGE * pageCount(x, y)) - mOffsetHead + mOffsetTail
+   */
+
+  int count = -mOffsetHead;
+
+  // Compute (EVENTS_PER_PAGE * pageCount(x, y))
+  for (Page* page = mHead; page != mTail; page = page->mNext) {
+    count += EVENTS_PER_PAGE;
+  }
+
+  count += mOffsetTail;
+  MOZ_ASSERT(count >= 0);
+
+  return count;
+}
diff --git a/xpcom/threads/nsEventQueue.h b/xpcom/threads/nsEventQueue.h
new file mode 100644
index 000000000..23b55e63d
--- /dev/null
+++ b/xpcom/threads/nsEventQueue.h
@@ -0,0 +1,123 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 nsEventQueue_h__
+#define nsEventQueue_h__
+
+#include <stdlib.h>
+#include "mozilla/CondVar.h"
+#include "mozilla/Mutex.h"
+#include "nsIRunnable.h"
+#include "nsCOMPtr.h"
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/UniquePtr.h"
+
+class nsThreadPool;
+
+// A threadsafe FIFO event queue...
+class nsEventQueue
+{
+public:
+  typedef mozilla::MutexAutoLock MutexAutoLock;
+
+  enum EventQueueType
+  {
+    eNormalQueue,
+    eSharedCondVarQueue
+  };
+
+  nsEventQueue(mozilla::CondVar& aCondVar, EventQueueType aType);
+  ~nsEventQueue();
+
+  // This method adds a new event to the pending event queue.  The queue holds
+  // a strong reference to the event after this method returns.  This method
+  // cannot fail.
+  void PutEvent(nsIRunnable* aEvent, MutexAutoLock& aProofOfLock);
+  void PutEvent(already_AddRefed<nsIRunnable>&& aEvent,
+                MutexAutoLock& aProofOfLock);
+
+  // This method gets an event from the event queue.  If mayWait is true, then
+  // the method will block the calling thread until an event is available.  If
+  // the event is null, then the method returns immediately indicating whether
+  // or not an event is pending.  When the resulting event is non-null, the
+  // caller is responsible for releasing the event object.  This method does
+  // not alter the reference count of the resulting event.
+  bool GetEvent(bool aMayWait, nsIRunnable** aEvent,
+                MutexAutoLock& aProofOfLock);
+
+  // This method returns true if there is a pending event.
+  bool HasPendingEvent(MutexAutoLock& aProofOfLock)
+  {
+    return GetEvent(false, nullptr, aProofOfLock);
+  }
+
+  // This method returns the next pending event or null.
+  bool GetPendingEvent(nsIRunnable** aRunnable, MutexAutoLock& aProofOfLock)
+  {
+    return GetEvent(false, aRunnable, aProofOfLock);
+  }
+
+  size_t Count(MutexAutoLock&) const;
+
+private:
+  bool IsEmpty()
+  {
+    return !mHead || (mHead == mTail && mOffsetHead == mOffsetTail);
+  }
+
+  enum
+  {
+    EVENTS_PER_PAGE = 255
+  };
+
+  // Page objects are linked together to form a simple deque.
+
+  struct Page
+  {
+    struct Page* mNext;
+    nsIRunnable* mEvents[EVENTS_PER_PAGE];
+  };
+
+  static_assert((sizeof(Page) & (sizeof(Page) - 1)) == 0,
+                "sizeof(Page) should be a power of two to avoid heap slop.");
+
+  static Page* NewPage()
+  {
+    return static_cast<Page*>(moz_xcalloc(1, sizeof(Page)));
+  }
+
+  static void FreePage(Page* aPage)
+  {
+    free(aPage);
+  }
+
+  Page* mHead;
+  Page* mTail;
+
+  uint16_t mOffsetHead;  // offset into mHead where next item is removed
+  uint16_t mOffsetTail;  // offset into mTail where next item is added
+  mozilla::CondVar& mEventsAvailable;
+
+  EventQueueType mType;
+
+  // These methods are made available to nsThreadPool as a hack, since
+  // nsThreadPool needs to have its threads sleep for fixed amounts of
+  // time as well as being able to wake up all threads when thread
+  // limits change.
+  friend class nsThreadPool;
+  void Wait(PRIntervalTime aInterval)
+  {
+    MOZ_ASSERT(mType == eNormalQueue);
+    mEventsAvailable.Wait(aInterval);
+  }
+  void NotifyAll()
+  {
+    MOZ_ASSERT(mType == eNormalQueue);
+    mEventsAvailable.NotifyAll();
+  }
+};
+
+#endif  // nsEventQueue_h__
diff --git a/xpcom/threads/nsICancelableRunnable.h b/xpcom/threads/nsICancelableRunnable.h
new file mode 100644
index 000000000..5ae9f5b14
--- /dev/null
+++ b/xpcom/threads/nsICancelableRunnable.h
@@ -0,0 +1,38 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 nsICancelableRunnable_h__
+#define nsICancelableRunnable_h__
+
+#include "nsISupports.h"
+
+#define NS_ICANCELABLERUNNABLE_IID \
+{ 0xde93dc4c, 0x5eea, 0x4eb7, \
+{ 0xb6, 0xd1, 0xdb, 0xf1, 0xe0, 0xce, 0xf6, 0x5c } }
+
+class nsICancelableRunnable : public nsISupports
+{
+public:
+  NS_DECLARE_STATIC_IID_ACCESSOR(NS_ICANCELABLERUNNABLE_IID)
+
+  /*
+   * Cancels a pending task.  If the task has already been executed this will
+   * be a no-op.  Calling this method twice is considered an error.
+   *
+   * @throws NS_ERROR_UNEXPECTED
+   *   Indicates that the runnable has already been canceled.
+   */
+  virtual nsresult Cancel() = 0;
+
+protected:
+  nsICancelableRunnable() { }
+  virtual ~nsICancelableRunnable() {}
+};
+
+NS_DEFINE_STATIC_IID_ACCESSOR(nsICancelableRunnable,
+                              NS_ICANCELABLERUNNABLE_IID)
+
+#endif // nsICancelableRunnable_h__
diff --git a/xpcom/threads/nsIEnvironment.idl b/xpcom/threads/nsIEnvironment.idl
new file mode 100644
index 000000000..afbc3eb7c
--- /dev/null
+++ b/xpcom/threads/nsIEnvironment.idl
@@ -0,0 +1,55 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* 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"
+
+/**
+ * Scriptable access to the current process environment.
+ *
+ */
+[scriptable, uuid(101d5941-d820-4e85-a266-9a3469940807)]
+interface nsIEnvironment : nsISupports
+{
+    /**
+     * Set the value of an environment variable.
+     *
+     * @param aName   the variable name to set.
+     * @param aValue  the value to set.
+     */
+    void set(in AString aName, in AString aValue);
+
+    /**
+     * Get the value of an environment variable.
+     *
+     * @param aName   the variable name to retrieve.
+     * @return        returns the value of the env variable. An empty string
+     *                will be returned when the env variable does not exist or
+     *                when the value itself is an empty string - please use
+     *                |exists()| to probe whether the env variable exists
+     *                or not.
+     */
+    AString get(in AString aName);
+
+    /**
+     * Check the existence of an environment variable.
+     * This method checks whether an environment variable is present in
+     * the environment or not.
+     *
+     * - For Unix/Linux platforms we follow the Unix definition:
+     * An environment variable exists when |getenv()| returns a non-NULL value.
+     * An environment variable does not exist when |getenv()| returns NULL.
+     * - For non-Unix/Linux platforms we have to fall back to a 
+     * "portable" definition (which is incorrect for Unix/Linux!!!!)
+     * which simply checks whether the string returned by |Get()| is empty
+     * or not.
+     *
+     * @param aName   the variable name to probe.
+     * @return        if the variable has been set, the value returned is
+     *                PR_TRUE. If the variable was not defined in the
+     *                environment PR_FALSE will be returned.
+     */
+    boolean exists(in AString aName);
+};
+
diff --git a/xpcom/threads/nsIEventTarget.idl b/xpcom/threads/nsIEventTarget.idl
new file mode 100644
index 000000000..a6f9068dc
--- /dev/null
+++ b/xpcom/threads/nsIEventTarget.idl
@@ -0,0 +1,127 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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"
+#include "nsIRunnable.idl"
+%{C++
+#include "nsCOMPtr.h"
+#include "mozilla/AlreadyAddRefed.h"
+%}
+
+native alreadyAddRefed_nsIRunnable(already_AddRefed<nsIRunnable>);
+
+[scriptable, uuid(88145945-3278-424e-9f37-d874cbdd9f6f)]
+interface nsIEventTarget : nsISupports
+{
+  /* until we can get rid of all uses, keep the non-alreadyAddRefed<> version */
+%{C++
+    nsresult Dispatch(nsIRunnable* aEvent, uint32_t aFlags) {
+      return Dispatch(nsCOMPtr<nsIRunnable>(aEvent).forget(), aFlags);
+    }
+%}
+
+  /**
+   * This flag specifies the default mode of event dispatch, whereby the event
+   * is simply queued for later processing.  When this flag is specified,
+   * dispatch returns immediately after the event is queued.
+   */
+  const unsigned long DISPATCH_NORMAL = 0;
+
+  /**
+   * This flag specifies the synchronous mode of event dispatch, in which the
+   * dispatch method does not return until the event has been processed.
+   *
+   * NOTE: passing this flag to dispatch may have the side-effect of causing
+   * other events on the current thread to be processed while waiting for the
+   * given event to be processed.
+   */
+  const unsigned long DISPATCH_SYNC = 1;
+
+  /**
+   * This flag specifies that the dispatch is occurring from a running event
+   * that was dispatched to the same event target, and that event is about to
+   * finish.
+   *
+   * A thread pool can use this as an optimization hint to not spin up
+   * another thread, since the current thread is about to become idle.
+   *
+   * These events are always async.
+   */
+  const unsigned long DISPATCH_AT_END = 2;
+
+  /**
+   * Check to see if this event target is associated with the current thread.
+   *
+   * @returns
+   *   A boolean value that if "true" indicates that events dispatched to this
+   *   event target will run on the current thread (i.e., the thread calling
+   *   this method).
+   */
+  boolean isOnCurrentThread();
+
+  /**
+   * Dispatch an event to this event target.  This function may be called from
+   * any thread, and it may be called re-entrantly.
+   *
+   * @param event
+   *   The alreadyAddRefed<> event to dispatch.
+   *   NOTE that the event will be leaked if it fails to dispatch.
+   * @param flags
+   *   The flags modifying event dispatch.  The flags are described in detail
+   *   below.
+   *
+   * @throws NS_ERROR_INVALID_ARG
+   *   Indicates that event is null.
+   * @throws NS_ERROR_UNEXPECTED
+   *   Indicates that the thread is shutting down and has finished processing
+   * events, so this event would never run and has not been dispatched.
+   */
+  [noscript, binaryname(Dispatch)] void dispatchFromC(in alreadyAddRefed_nsIRunnable event, in unsigned long flags);
+  /**
+   * Version of Dispatch to expose to JS, which doesn't require an alreadyAddRefed<>
+   * (it will be converted to that internally)
+   *
+   * @param event
+   *   The (raw) event to dispatch.
+   * @param flags
+   *   The flags modifying event dispatch.  The flags are described in detail
+   *   below.
+   *
+   * @throws NS_ERROR_INVALID_ARG
+   *   Indicates that event is null.
+   * @throws NS_ERROR_UNEXPECTED
+   *   Indicates that the thread is shutting down and has finished processing
+   * events, so this event would never run and has not been dispatched.
+   */
+  [binaryname(DispatchFromScript)] void dispatch(in nsIRunnable event, in unsigned long flags);
+  /**
+   * Dispatch an event to this event target, but do not run it before delay
+   * milliseconds have passed.  This function may be called from any thread.
+   *
+   * @param event
+   *   The alreadyAddrefed<> event to dispatch.
+   * @param delay
+   *   The delay (in ms) before running the event.  If event does not rise to
+   *   the top of the event queue before the delay has passed, it will be set
+   *   aside to execute once the delay has passed.  Otherwise, it will be
+   *   executed immediately.
+   *
+   * @throws NS_ERROR_INVALID_ARG
+   *   Indicates that event is null.
+   * @throws NS_ERROR_UNEXPECTED
+   *   Indicates that the thread is shutting down and has finished processing
+   * events, so this event would never run and has not been dispatched, or
+   * that delay is zero.
+   */
+  [noscript] void delayedDispatch(in alreadyAddRefed_nsIRunnable event, in unsigned long delay);
+};
+
+%{C++
+// convenient aliases:
+#define NS_DISPATCH_NORMAL nsIEventTarget::DISPATCH_NORMAL
+#define NS_DISPATCH_SYNC   nsIEventTarget::DISPATCH_SYNC
+#define NS_DISPATCH_AT_END nsIEventTarget::DISPATCH_AT_END
+%}
diff --git a/xpcom/threads/nsIIdlePeriod.idl b/xpcom/threads/nsIIdlePeriod.idl
new file mode 100644
index 000000000..aa72b2171
--- /dev/null
+++ b/xpcom/threads/nsIIdlePeriod.idl
@@ -0,0 +1,32 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* 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"
+
+%{C++
+namespace mozilla {
+class TimeStamp;
+}
+%}
+
+native TimeStamp(mozilla::TimeStamp);
+
+/**
+ * An instance implementing nsIIdlePeriod is used by an associated
+ * nsIThread to estimate when it is likely that it will receive an
+ * event.
+ */
+[builtinclass, uuid(21dd35a2-eae9-4bd8-b470-0dfa35a0e3b9)]
+interface nsIIdlePeriod : nsISupports
+{
+    /**
+     * Return an estimate of a point in time in the future when we
+     * think that the associated thread will become busy. Should
+     * return TimeStamp() (i.e. the null time) or a time less than
+     * TimeStamp::Now() if the thread is currently busy or will become
+     * busy very soon.
+     */
+    TimeStamp getIdlePeriodHint();
+};
diff --git a/xpcom/threads/nsIIncrementalRunnable.h b/xpcom/threads/nsIIncrementalRunnable.h
new file mode 100644
index 000000000..526bc165e
--- /dev/null
+++ b/xpcom/threads/nsIIncrementalRunnable.h
@@ -0,0 +1,41 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 nsIIncrementalRunnable_h__
+#define nsIIncrementalRunnable_h__
+
+#include "nsISupports.h"
+#include "mozilla/TimeStamp.h"
+
+#define NS_IINCREMENTALRUNNABLE_IID \
+{ 0x688be92e, 0x7ade, 0x4fdc, \
+{ 0x9d, 0x83, 0x74, 0xcb, 0xef, 0xf4, 0xa5, 0x2c } }
+
+
+/**
+ * A task interface for tasks that can schedule their work to happen
+ * in increments bounded by a deadline.
+ */
+class nsIIncrementalRunnable : public nsISupports
+{
+public:
+  NS_DECLARE_STATIC_IID_ACCESSOR(NS_IINCREMENTALRUNNABLE_IID)
+
+  /**
+   * Notify the task of a point in time in the future when the task
+   * should stop executing.
+   */
+  virtual void SetDeadline(mozilla::TimeStamp aDeadline) = 0;
+
+protected:
+  nsIIncrementalRunnable() { }
+  virtual ~nsIIncrementalRunnable() {}
+};
+
+NS_DEFINE_STATIC_IID_ACCESSOR(nsIIncrementalRunnable,
+                              NS_IINCREMENTALRUNNABLE_IID)
+
+#endif // nsIIncrementalRunnable_h__
diff --git a/xpcom/threads/nsIProcess.idl b/xpcom/threads/nsIProcess.idl
new file mode 100644
index 000000000..2c7dcd55e
--- /dev/null
+++ b/xpcom/threads/nsIProcess.idl
@@ -0,0 +1,99 @@
+/* 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"
+
+interface nsIFile;
+interface nsIObserver;
+
+[scriptable, uuid(609610de-9954-4a63-8a7c-346350a86403)]
+interface nsIProcess : nsISupports
+{
+  /**
+   * Initialises the process with an executable to be run. Call the run method
+   * to run the executable.
+   * @param executable The executable to run.
+   */
+  void init(in nsIFile executable);
+
+  /**
+   * Kills the running process. After exiting the process will either have
+   * been killed or a failure will have been returned.
+   */
+  void kill();
+
+  /**
+   * Executes the file this object was initialized with
+   * @param blocking   Whether to wait until the process terminates before
+                       returning or not.
+   * @param args       An array of arguments to pass to the process in the
+   *                   native character set.
+   * @param count      The length of the args array.
+   */
+  void run(in boolean blocking, [array, size_is(count)] in string args,
+           in unsigned long count);
+
+  /**
+   * Executes the file this object was initialized with optionally calling
+   * an observer after the process has finished running.
+   * @param args       An array of arguments to pass to the process in the
+   *                   native character set.
+   * @param count      The length of the args array.
+   * @param observer   An observer to notify when the process has completed. It
+   *                   will receive this process instance as the subject and
+   *                   "process-finished" or "process-failed" as the topic. The
+   *                   observer will be notified on the main thread.
+   * @param holdWeak   Whether to use a weak reference to hold the observer.
+   */
+  void runAsync([array, size_is(count)] in string args, in unsigned long count,
+                [optional] in nsIObserver observer, [optional] in boolean holdWeak);
+
+  /**
+   * Executes the file this object was initialized with
+   * @param blocking   Whether to wait until the process terminates before
+                       returning or not.
+   * @param args       An array of arguments to pass to the process in UTF-16
+   * @param count      The length of the args array.
+   */
+  void runw(in boolean blocking, [array, size_is(count)] in wstring args,
+	    in unsigned long count);
+
+  /**
+   * Executes the file this object was initialized with optionally calling
+   * an observer after the process has finished running.
+   * @param args       An array of arguments to pass to the process in UTF-16
+   * @param count      The length of the args array.
+   * @param observer   An observer to notify when the process has completed. It
+   *                   will receive this process instance as the subject and
+   *                   "process-finished" or "process-failed" as the topic. The
+   *                   observer will be notified on the main thread.
+   * @param holdWeak   Whether to use a weak reference to hold the observer.
+   */
+  void runwAsync([array, size_is(count)] in wstring args,
+		 in unsigned long count,
+		 [optional] in nsIObserver observer, [optional] in boolean holdWeak);
+
+  /**
+   * The process identifier of the currently running process. This will only
+   * be available after the process has started and may not be available on
+   * some platforms.
+   */
+  readonly attribute unsigned long pid;
+
+  /**
+   * The exit value of the process. This is only valid after the process has
+   * exited.
+   */
+  readonly attribute long exitValue;
+
+  /**
+   * Returns whether the process is currently running or not.
+   */
+  readonly attribute boolean isRunning;
+};
+
+%{C++
+
+#define NS_PROCESS_CONTRACTID "@mozilla.org/process/util;1"
+%}
diff --git a/xpcom/threads/nsIRunnable.idl b/xpcom/threads/nsIRunnable.idl
new file mode 100644
index 000000000..4d26f72d9
--- /dev/null
+++ b/xpcom/threads/nsIRunnable.idl
@@ -0,0 +1,27 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* 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"
+
+/**
+ * Represents a task which can be dispatched to a thread for execution.
+ */
+
+[scriptable, function, uuid(4a2abaf0-6886-11d3-9382-00104ba0fd40)]
+interface nsIRunnable : nsISupports
+{
+    /**
+     * The function implementing the task to be run.
+     */
+    void run();
+};
+
+[uuid(e75aa42a-80a9-11e6-afb5-e89d87348e2c)]
+interface nsIRunnablePriority : nsISupports
+{
+    const unsigned short PRIORITY_NORMAL = 0;
+    const unsigned short PRIORITY_HIGH = 1;
+    readonly attribute unsigned long priority;
+};
diff --git a/xpcom/threads/nsISupportsPriority.idl b/xpcom/threads/nsISupportsPriority.idl
new file mode 100644
index 000000000..579c280cf
--- /dev/null
+++ b/xpcom/threads/nsISupportsPriority.idl
@@ -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/. */
+
+#include "nsISupports.idl"
+
+/**
+ * This interface exposes the general notion of a scheduled object with a
+ * integral priority value.  Following UNIX conventions, smaller (and possibly
+ * negative) values have higher priority.
+ *
+ * This interface does not strictly define what happens when the priority of an
+ * object is changed.  An implementation of this interface is free to define
+ * the side-effects of changing the priority of an object.  In some cases,
+ * changing the priority of an object may be disallowed (resulting in an
+ * exception being thrown) or may simply be ignored.
+ */
+[scriptable, uuid(aa578b44-abd5-4c19-8b14-36d4de6fdc36)]
+interface nsISupportsPriority : nsISupports
+{
+  /**
+   * Typical priority values.
+   */
+  const long PRIORITY_HIGHEST = -20;
+  const long PRIORITY_HIGH    = -10;
+  const long PRIORITY_NORMAL  =   0;
+  const long PRIORITY_LOW     =  10;
+  const long PRIORITY_LOWEST  =  20;
+  
+  /**
+   * This attribute may be modified to change the priority of this object.  The
+   * implementation of this interface is free to truncate a given priority
+   * value to whatever limits are appropriate.  Typically, this attribute is
+   * initialized to PRIORITY_NORMAL, but implementations may choose to assign a
+   * different initial value.
+   */
+  attribute long priority;
+
+  /**
+   * This method adjusts the priority attribute by a given delta.  It helps
+   * reduce the amount of coding required to increment or decrement the value
+   * of the priority attribute.
+   */
+  void adjustPriority(in long delta);
+};
diff --git a/xpcom/threads/nsIThread.idl b/xpcom/threads/nsIThread.idl
new file mode 100644
index 000000000..fbfc8d4fb
--- /dev/null
+++ b/xpcom/threads/nsIThread.idl
@@ -0,0 +1,149 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 "nsIEventTarget.idl"
+#include "nsIIdlePeriod.idl"
+
+%{C++
+#include "mozilla/AlreadyAddRefed.h"
+%}
+
+[ptr] native PRThread(PRThread);
+
+native alreadyAddRefed_nsIIdlePeriod(already_AddRefed<nsIIdlePeriod>);
+
+/**
+ * This interface provides a high-level abstraction for an operating system
+ * thread.
+ *
+ * Threads have a built-in event queue, and a thread is an event target that
+ * can receive nsIRunnable objects (events) to be processed on the thread.
+ *
+ * See nsIThreadManager for the API used to create and locate threads.
+ */
+[scriptable, uuid(5801d193-29d1-4964-a6b7-70eb697ddf2b)]
+interface nsIThread : nsIEventTarget
+{
+  /**
+   * @returns
+   *   The NSPR thread object corresponding to this nsIThread.
+   */
+  [noscript] readonly attribute PRThread PRThread;
+
+  /**
+   * @returns
+   *  Whether or not this thread may call into JS. Used in the profiler
+   *  to avoid some unnecessary locking.
+   */
+  [noscript] attribute boolean CanInvokeJS;
+
+
+  /**
+   * Shutdown the thread.  This method prevents further dispatch of events to
+   * the thread, and it causes any pending events to run to completion before
+   * the thread joins (see PR_JoinThread) with the current thread.  During this
+   * method call, events for the current thread may be processed.
+   *
+   * This method MAY NOT be executed from the thread itself.  Instead, it is
+   * meant to be executed from another thread (usually the thread that created
+   * this thread or the main application thread).  When this function returns,
+   * the thread will be shutdown, and it will no longer be possible to dispatch
+   * events to the thread.
+   *
+   * @throws NS_ERROR_UNEXPECTED
+   *   Indicates that this method was erroneously called when this thread was
+   *   the current thread, that this thread was not created with a call to
+   *   nsIThreadManager::NewThread, or if this method was called more than once
+   *   on the thread object.
+   */
+  void shutdown();
+
+  /**
+   * This method may be called to determine if there are any events ready to be
+   * processed.  It may only be called when this thread is the current thread.
+   *
+   * Because events may be added to this thread by another thread, a "false"
+   * result does not mean that this thread has no pending events.  It only
+   * means that there were no pending events when this method was called.
+   *
+   * @returns
+   *   A boolean value that if "true" indicates that this thread has one or
+   *   more pending events.
+   *
+   * @throws NS_ERROR_UNEXPECTED
+   *   Indicates that this method was erroneously called when this thread was
+   *   not the current thread.
+   */
+  boolean hasPendingEvents();
+
+  /**
+   * Process the next event.  If there are no pending events, then this method
+   * may wait -- depending on the value of the mayWait parameter -- until an
+   * event is dispatched to this thread.  This method is re-entrant but may
+   * only be called if this thread is the current thread.
+   *
+   * @param mayWait
+   *   A boolean parameter that if "true" indicates that the method may block
+   *   the calling thread to wait for a pending event.
+   *
+   * @returns
+   *   A boolean value that if "true" indicates that an event was processed.
+   *
+   * @throws NS_ERROR_UNEXPECTED
+   *   Indicates that this method was erroneously called when this thread was
+   *   not the current thread.
+   */
+  boolean processNextEvent(in boolean mayWait);
+
+  /**
+   * Shutdown the thread asynchronously.  This method immediately prevents
+   * further dispatch of events to the thread, and it causes any pending events
+   * to run to completion before this thread joins with the current thread.
+   *
+   * UNLIKE shutdown() this does not process events on the current thread.
+   * Instead it merely ensures that the current thread continues running until
+   * this thread has shut down.
+   *
+   * This method MAY NOT be executed from the thread itself.  Instead, it is
+   * meant to be executed from another thread (usually the thread that created
+   * this thread or the main application thread).  When this function returns,
+   * the thread will continue running until it exhausts its event queue.
+   *
+   * @throws NS_ERROR_UNEXPECTED
+   *   Indicates that this method was erroneously called when this thread was
+   *   the current thread, that this thread was not created with a call to
+   *   nsIThreadManager::NewThread, or if this method was called more than once
+   *   on the thread object.
+   */
+  void asyncShutdown();
+
+  /**
+   * Register an instance of nsIIdlePeriod which works as a facade of
+   * the abstract notion of a "current idle period". The
+   * nsIIdlePeriod should always represent the "current" idle period
+   * with regard to expected work for the thread. The thread is free
+   * to use this when there are no higher prioritized tasks to process
+   * to determine if it is reasonable to schedule tasks that could run
+   * when the thread is idle. The responsibility of the registered
+   * nsIIdlePeriod is to answer with an estimated deadline at which
+   * the thread should expect that it could receive new higher
+   * priority tasks.
+   */
+  [noscript] void registerIdlePeriod(in alreadyAddRefed_nsIIdlePeriod aIdlePeriod);
+
+  /**
+   * Dispatch an event to the thread's idle queue.  This function may be called
+   * from any thread, and it may be called re-entrantly.
+   *
+   * @param event
+   *   The alreadyAddRefed<> event to dispatch.
+   *   NOTE that the event will be leaked if it fails to dispatch.
+   *
+   * @throws NS_ERROR_INVALID_ARG
+   *   Indicates that event is null.
+   */
+  [noscript] void idleDispatch(in alreadyAddRefed_nsIRunnable event);
+};
diff --git a/xpcom/threads/nsIThreadInternal.idl b/xpcom/threads/nsIThreadInternal.idl
new file mode 100644
index 000000000..e5d7cc83f
--- /dev/null
+++ b/xpcom/threads/nsIThreadInternal.idl
@@ -0,0 +1,135 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 "nsIThread.idl"
+
+interface nsIRunnable;
+interface nsIThreadObserver;
+
+/**
+ * The XPCOM thread object implements this interface, which allows a consumer
+ * to observe dispatch activity on the thread.
+ */
+[scriptable, uuid(a3a72e5f-71d9-4add-8f30-59a78fb6d5eb)]
+interface nsIThreadInternal : nsIThread
+{
+  /**
+   * Get/set the current thread observer (may be null).  This attribute may be
+   * read from any thread, but must only be set on the thread corresponding to
+   * this thread object.  The observer will be released on the thread
+   * corresponding to this thread object after all other events have been
+   * processed during a call to Shutdown.
+   */
+  attribute nsIThreadObserver observer;
+
+  /**
+   * Add an observer that will *only* receive onProcessNextEvent,
+   * beforeProcessNextEvent. and afterProcessNextEvent callbacks. Always called
+   * on the target thread, and the implementation does not have to be
+   * threadsafe. Order of callbacks is not guaranteed (i.e.
+   * afterProcessNextEvent may be called first depending on whether or not the
+   * observer is added in a nested loop). Holds a strong ref.
+   */
+  void addObserver(in nsIThreadObserver observer);
+
+  /**
+   * Remove an observer added via the addObserver call. Once removed the
+   * observer will never be called again by the thread.
+   */
+  void removeObserver(in nsIThreadObserver observer);
+
+  /**
+   * This method causes any events currently enqueued on the thread to be
+   * suppressed until PopEventQueue is called, and any event dispatched to this
+   * thread's nsIEventTarget will queue as well. Calls to PushEventQueue may be
+   * nested and must each be paired with a call to PopEventQueue in order to
+   * restore the original state of the thread. The returned nsIEventTarget may
+   * be used to push events onto the nested queue. Dispatching will be disabled
+   * once the event queue is popped. The thread will only ever process pending
+   * events for the innermost event queue. Must only be called on the target
+   * thread.
+   */
+  [noscript] nsIEventTarget pushEventQueue();
+
+  /**
+   * Revert a call to PushEventQueue. When an event queue is popped, any events
+   * remaining in the queue are appended to the elder queue. This also causes
+   * the nsIEventTarget returned from PushEventQueue to stop dispatching events.
+   * Must only be called on the target thread, and with the innermost event
+   * queue.
+   */
+  [noscript] void popEventQueue(in nsIEventTarget aInnermostTarget);
+};
+
+/**
+ * This interface provides the observer with hooks to implement a layered
+ * event queue.  For example, it is possible to overlay processing events
+ * for a GUI toolkit on top of the events for a thread:
+ *
+ *   var NativeQueue;
+ *   Observer = {
+ *     onDispatchedEvent(thread) {
+ *       NativeQueue.signal();
+ *     }
+ *     onProcessNextEvent(thread, mayWait) {
+ *       if (NativeQueue.hasNextEvent())
+ *         NativeQueue.processNextEvent();
+ *       while (mayWait && !thread.hasPendingEvent()) {
+ *         NativeQueue.wait();
+ *         NativeQueue.processNextEvent();
+ *       }
+ *     }
+ *   };
+ *
+ * NOTE: The implementation of this interface must be threadsafe.
+ * 
+ * NOTE: It is valid to change the thread's observer during a call to an
+ *       observer method.
+ *
+ * NOTE: Will be split into two interfaces soon: one for onProcessNextEvent and
+ *       afterProcessNextEvent, then another that inherits the first and adds
+ *       onDispatchedEvent.
+ */
+[uuid(cc8da053-1776-44c2-9199-b5a629d0a19d)]
+interface nsIThreadObserver : nsISupports
+{
+  /**
+   * This method is called after an event has been dispatched to the thread.
+   * This method may be called from any thread. 
+   *
+   * @param thread
+   *   The thread where the event is being dispatched.
+   */
+  void onDispatchedEvent(in nsIThreadInternal thread);
+
+  /**
+   * This method is called when nsIThread::ProcessNextEvent is called.  It does
+   * not guarantee that an event is actually going to be processed.  This method
+   * is only called on the target thread.
+   *
+   * @param thread
+   *   The thread being asked to process another event.
+   * @param mayWait
+   *   Indicates whether or not the method is allowed to block the calling
+   *   thread.  For example, this parameter is false during thread shutdown.
+   */
+  void onProcessNextEvent(in nsIThreadInternal thread, in boolean mayWait);
+
+  /**
+   * This method is called (from nsIThread::ProcessNextEvent) after an event
+   * is processed.  It does not guarantee that an event was actually processed
+   * (depends on the value of |eventWasProcessed|.  This method is only called
+   * on the target thread.  DO NOT EVER RUN SCRIPT FROM THIS CALLBACK!!!
+   *
+   * @param thread
+   *   The thread that processed another event.
+   * @param eventWasProcessed
+   *   Indicates whether an event was actually processed. May be false if the
+   *   |mayWait| flag was false when calling nsIThread::ProcessNextEvent().
+   */
+  void afterProcessNextEvent(in nsIThreadInternal thread,
+                             in bool eventWasProcessed);
+};
diff --git a/xpcom/threads/nsIThreadManager.idl b/xpcom/threads/nsIThreadManager.idl
new file mode 100644
index 000000000..9b4fc126f
--- /dev/null
+++ b/xpcom/threads/nsIThreadManager.idl
@@ -0,0 +1,68 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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"
+
+[ptr] native PRThread(PRThread);
+
+interface nsIThread;
+
+/**
+ * An interface for creating and locating nsIThread instances.
+ */
+[scriptable, uuid(1be89eca-e2f7-453b-8d38-c11ba247f6f3)]
+interface nsIThreadManager : nsISupports
+{
+  /**
+   * Default number of bytes reserved for a thread's stack, if no stack size
+   * is specified in newThread(). 0 means use platform default.
+   */
+  const unsigned long DEFAULT_STACK_SIZE = 0;
+
+  /**
+   * Create a new thread (a global, user PRThread).
+   *
+   * @param creationFlags
+   *   Reserved for future use.  Pass 0.
+   * @param stackSize
+   *   Number of bytes to reserve for the thread's stack.
+   *
+   * @returns
+   *   The newly created nsIThread object.
+   */
+  nsIThread newThread(in unsigned long creationFlags, [optional] in unsigned long stackSize);
+
+  /**
+   * Get the nsIThread object (if any) corresponding to the given PRThread.
+   * This method returns null if there is no corresponding nsIThread.
+   *
+   * @param prthread
+   *   The PRThread of the nsIThread being requested.
+   *
+   * @returns
+   *   The nsIThread object corresponding to the given PRThread or null if no
+   *   such nsIThread exists.
+   */
+  [noscript] nsIThread getThreadFromPRThread(in PRThread prthread);
+
+  /**
+   * Get the main thread.
+   */
+  readonly attribute nsIThread mainThread;
+
+  /**
+   * Get the current thread.  If the calling thread does not already have a
+   * nsIThread associated with it, then a new nsIThread will be created and
+   * associated with the current PRThread.
+   */
+  readonly attribute nsIThread currentThread;
+
+  /**
+   * This attribute is true if the calling thread is the main thread of the
+   * application process.
+   */
+  readonly attribute boolean isMainThread;
+};
diff --git a/xpcom/threads/nsIThreadPool.idl b/xpcom/threads/nsIThreadPool.idl
new file mode 100644
index 000000000..d04e8504a
--- /dev/null
+++ b/xpcom/threads/nsIThreadPool.idl
@@ -0,0 +1,88 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 "nsIEventTarget.idl"
+
+[scriptable, uuid(ef194cab-3f86-4b61-b132-e5e96a79e5d1)]
+interface nsIThreadPoolListener : nsISupports
+{
+  /**
+   * Called when a new thread is created by the thread pool. The notification
+   * happens on the newly-created thread.
+   */
+  void onThreadCreated();
+
+  /**
+   * Called when a thread is about to be destroyed by the thread pool. The
+   * notification happens on the thread that is about to be destroyed.
+   */
+  void onThreadShuttingDown();
+};
+
+/**
+ * An interface to a thread pool.  A thread pool creates a limited number of
+ * anonymous (unnamed) worker threads.  An event dispatched to the thread pool
+ * will be run on the next available worker thread.
+ */
+[scriptable, uuid(76ce99c9-8e43-489a-9789-f27cc4424965)]
+interface nsIThreadPool : nsIEventTarget
+{
+  /**
+   * Shutdown the thread pool.  This method may not be executed from any thread
+   * in the thread pool.  Instead, it is meant to be executed from another
+   * thread (usually the thread that created this thread pool).  When this
+   * function returns, the thread pool and all of its threads will be shutdown,
+   * and it will no longer be possible to dispatch tasks to the thread pool.
+   *
+   * As a side effect, events on the current thread will be processed.
+   */
+  void shutdown();
+
+  /**
+   * Get/set the maximum number of threads allowed at one time in this pool.
+   */
+  attribute unsigned long threadLimit;
+
+  /**
+   * Get/set the maximum number of idle threads kept alive.
+   */
+  attribute unsigned long idleThreadLimit;
+
+  /**
+   * Get/set the amount of time in milliseconds before an idle thread is
+   * destroyed.
+   */
+  attribute unsigned long idleThreadTimeout;
+
+  /**
+   * Get/set the number of bytes reserved for the stack of all threads in
+   * the pool. By default this is nsIThreadManager::DEFAULT_STACK_SIZE.
+   */
+  attribute unsigned long threadStackSize;
+
+  /**
+   * An optional listener that will be notified when a thread is created or
+   * destroyed in the course of the thread pool's operation.
+   *
+   * A listener will only receive notifications about threads created after the
+   * listener is set so it is recommended that the consumer set the listener
+   * before dispatching the first event. A listener that receives an
+   * onThreadCreated() notification is guaranteed to always receive the
+   * corresponding onThreadShuttingDown() notification.
+   *
+   * The thread pool takes ownership of the listener and releases it when the
+   * shutdown() method is called. Threads created after the listener is set will
+   * also take ownership of the listener so that the listener will be kept alive
+   * long enough to receive the guaranteed onThreadShuttingDown() notification.
+   */
+  attribute nsIThreadPoolListener listener;
+
+  /**
+   * Set the label for threads in the pool. All threads will be named
+   * "<aName> #<n>", where <n> is a serial number.
+   */
+  void setName(in ACString aName);
+};
diff --git a/xpcom/threads/nsITimer.idl b/xpcom/threads/nsITimer.idl
new file mode 100644
index 000000000..ade2168f2
--- /dev/null
+++ b/xpcom/threads/nsITimer.idl
@@ -0,0 +1,244 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * 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"
+
+interface nsIObserver;
+interface nsIEventTarget;
+
+%{C++
+#include "mozilla/MemoryReporting.h"
+
+/**
+ * The signature of the timer callback function passed to initWithFuncCallback.
+ * This is the function that will get called when the timer expires if the
+ * timer is initialized via initWithFuncCallback.
+ *
+ * @param aTimer the timer which has expired
+ * @param aClosure opaque parameter passed to initWithFuncCallback
+ */
+class nsITimer;
+typedef void (*nsTimerCallbackFunc) (nsITimer *aTimer, void *aClosure);
+
+/**
+ * The signature of the timer name callback function passed to
+ * initWithNameableFuncCallback.
+ * This is the function that will get called when timer profiling is enabled
+ * via the "TimerFirings" log module.
+ *
+ * @param aTimer the timer which has expired
+ * @param aClosure opaque parameter passed to initWithFuncCallback
+ * @param aBuf a buffer in which to put the name
+ * @param aLen the length of the buffer
+ */
+typedef void (*nsTimerNameCallbackFunc) (nsITimer *aTimer, void *aClosure,
+                                         char *aBuf, size_t aLen);
+%}
+
+native nsTimerCallbackFunc(nsTimerCallbackFunc);
+native nsTimerNameCallbackFunc(nsTimerNameCallbackFunc);
+
+/**
+ * The callback interface for timers.
+ */
+interface nsITimer;
+
+[function, scriptable, uuid(a796816d-7d47-4348-9ab8-c7aeb3216a7d)]
+interface nsITimerCallback : nsISupports
+{
+  /**
+   * @param aTimer the timer which has expired
+   */
+  void notify(in nsITimer timer);
+};
+
+%{C++
+// Two timer deadlines must differ by less than half the PRIntervalTime domain.
+#define DELAY_INTERVAL_LIMIT    PR_BIT(8 * sizeof(PRIntervalTime) - 1)
+%}
+
+/**
+ * nsITimer instances must be initialized by calling one of the "init" methods
+ * documented below.  You may also re-initialize (using one of the init()
+ * methods) an existing instance to avoid the overhead of destroying and
+ * creating a timer.  It is not necessary to cancel the timer in that case.
+ *
+ * By default a timer will fire on the thread that created it.  Set the .target
+ * attribute to fire on a different thread.  Once you have set a timer's .target
+ * and called one of its init functions, any further interactions with the timer
+ * (calling cancel(), changing member fields, etc) should only be done by the
+ * target thread, or races may occur with bad results like timers firing after
+ * they've been canceled, and/or not firing after re-initiatization.
+ */
+[scriptable, uuid(3de4b105-363c-482c-a409-baac83a01bfc)]
+interface nsITimer : nsISupports
+{
+  /* Timer types */
+
+  /**
+   * Type of a timer that fires once only.
+   */
+  const short TYPE_ONE_SHOT = 0;
+
+  /**
+   * After firing, a TYPE_REPEATING_SLACK timer is stopped and not restarted
+   * until its callback completes.  Specified timer period will be at least
+   * the time between when processing for last firing the callback completes
+   * and when the next firing occurs.
+   *
+   * This is the preferable repeating type for most situations.
+   */
+  const short TYPE_REPEATING_SLACK = 1;
+
+  /**
+   * TYPE_REPEATING_PRECISE is just a synonym for
+   * TYPE_REPEATING_PRECISE_CAN_SKIP. They used to be distinct, but the old
+   * TYPE_REPEATING_PRECISE kind was similar to TYPE_REPEATING_PRECISE_CAN_SKIP
+   * while also being less useful. So the distinction was removed.
+   */
+  const short TYPE_REPEATING_PRECISE = 2;
+
+  /**
+   * A TYPE_REPEATING_PRECISE_CAN_SKIP repeating timer aims to have constant
+   * period between firings.  The processing time for each timer callback
+   * should not influence the timer period.  However this timer type
+   * guarantees that it will not queue up new events to fire the callback
+   * until the previous callback event finishes firing.  If the callback
+   * takes a long time, then the next callback will be scheduled immediately
+   * afterward, but only once.  This is the only non-slack timer available.
+   */
+  const short TYPE_REPEATING_PRECISE_CAN_SKIP = 3;
+
+  /**
+   * Initialize a timer that will fire after the said delay.
+   * A user must keep a reference to this timer till it is
+   * is no longer needed or has been cancelled.
+   *
+   * @param aObserver   the callback object that observes the
+   *                    ``timer-callback'' topic with the subject being
+   *                    the timer itself when the timer fires:
+   *
+   *                    observe(nsISupports aSubject, => nsITimer
+   *                            string aTopic,        => ``timer-callback''
+   *                            wstring data          =>  null
+   *
+   * @param aDelay      delay in milliseconds for timer to fire
+   * @param aType       timer type per TYPE* consts defined above
+   */
+  void init(in nsIObserver aObserver, in unsigned long aDelay,
+            in unsigned long aType);
+
+
+  /**
+   * Initialize a timer to fire after the given millisecond interval.
+   * This version takes a function to call and a closure to pass to
+   * that function.
+   *
+   * @param aFunc      The function to invoke
+   * @param aClosure   An opaque pointer to pass to that function
+   * @param aDelay     The millisecond interval
+   * @param aType      Timer type per TYPE* consts defined above
+   */
+  [noscript] void initWithFuncCallback(in nsTimerCallbackFunc aCallback,
+                                       in voidPtr aClosure,
+                                       in unsigned long aDelay,
+                                       in unsigned long aType);
+
+  /**
+   * Initialize a timer to fire after the given millisecond interval.
+   * This version takes a function to call.
+   *
+   * @param aFunc      nsITimerCallback interface to call when timer expires
+   * @param aDelay     The millisecond interval
+   * @param aType      Timer type per TYPE* consts defined above
+   */
+  void initWithCallback(in nsITimerCallback aCallback,
+                        in unsigned long aDelay,
+                        in unsigned long aType);
+
+  /**
+   * Cancel the timer.  This method works on all types, not just on repeating
+   * timers -- you might want to cancel a TYPE_ONE_SHOT timer, and even reuse
+   * it by re-initializing it (to avoid object destruction and creation costs
+   * by conserving one timer instance).
+   */
+  void cancel();
+
+  /**
+   * Like initWithFuncCallback, but also takes a name for the timer; the name
+   * will be used when timer profiling is enabled via the "TimerFirings" log
+   * module.
+   *
+   * @param aFunc      The function to invoke
+   * @param aClosure   An opaque pointer to pass to that function
+   * @param aDelay     The millisecond interval
+   * @param aType      Timer type per TYPE* consts defined above
+   * @param aName      The timer's name
+   */
+  [noscript] void initWithNamedFuncCallback(in nsTimerCallbackFunc aCallback,
+                                            in voidPtr aClosure,
+                                            in unsigned long aDelay,
+                                            in unsigned long aType,
+                                            in string aName);
+
+  /**
+   * Like initWithNamedFuncCallback, but instead of a timer name it takes a
+   * callback that will provide a name when the timer fires.
+   *
+   * @param aFunc      The function to invoke
+   * @param aClosure   An opaque pointer to pass to that function
+   * @param aDelay     The millisecond interval
+   * @param aType      Timer type per TYPE* consts defined above
+   * @param aNameCallback  The callback function
+   */
+  [noscript] void initWithNameableFuncCallback(
+                    in nsTimerCallbackFunc aCallback,
+                    in voidPtr aClosure,
+                    in unsigned long aDelay,
+                    in unsigned long aType,
+                    in nsTimerNameCallbackFunc aNameCallback);
+
+  /**
+   * The millisecond delay of the timeout.
+   *
+   * NOTE: Re-setting the delay on a one-shot timer that has already fired
+   * doesn't restart the timer. Call one of the init() methods to restart
+   * a one-shot timer.
+   */
+  attribute unsigned long delay;
+
+  /**
+   * The timer type - one of the above TYPE_* constants.
+   */
+  attribute unsigned long type;
+
+  /**
+   * The opaque pointer pass to initWithFuncCallback.
+   */
+  [noscript] readonly attribute voidPtr closure;
+
+  /**
+   * The nsITimerCallback object passed to initWithCallback.
+   */
+  readonly attribute nsITimerCallback callback;
+
+  /**
+   * The nsIEventTarget where the callback will be dispatched. Note that this
+   * target may only be set before the call to one of the init methods above.
+   *
+   * By default the target is the thread that created the timer.
+   */
+  attribute nsIEventTarget target;
+
+%{C++
+  virtual size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const = 0;
+%}
+};
+
+%{C++
+#define NS_TIMER_CONTRACTID "@mozilla.org/timer;1"
+#define NS_TIMER_CALLBACK_TOPIC "timer-callback"
+%}
+
diff --git a/xpcom/threads/nsMemoryPressure.cpp b/xpcom/threads/nsMemoryPressure.cpp
new file mode 100644
index 000000000..fea9b0437
--- /dev/null
+++ b/xpcom/threads/nsMemoryPressure.cpp
@@ -0,0 +1,54 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "nsMemoryPressure.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Atomics.h"
+
+#include "nsThreadUtils.h"
+
+using namespace mozilla;
+
+static Atomic<int32_t, Relaxed> sMemoryPressurePending;
+static_assert(MemPressure_None == 0,
+              "Bad static initialization with the default constructor.");
+
+MemoryPressureState
+NS_GetPendingMemoryPressure()
+{
+  int32_t value = sMemoryPressurePending.exchange(MemPressure_None);
+  return MemoryPressureState(value);
+}
+
+void
+NS_DispatchEventualMemoryPressure(MemoryPressureState aState)
+{
+  /*
+   * A new memory pressure event erases an ongoing memory pressure, but an
+   * existing "new" memory pressure event takes precedence over a new "ongoing"
+   * memory pressure event.
+   */
+  switch (aState) {
+    case MemPressure_None:
+      sMemoryPressurePending = MemPressure_None;
+      break;
+    case MemPressure_New:
+      sMemoryPressurePending = MemPressure_New;
+      break;
+    case MemPressure_Ongoing:
+      sMemoryPressurePending.compareExchange(MemPressure_None,
+                                             MemPressure_Ongoing);
+      break;
+  }
+}
+
+nsresult
+NS_DispatchMemoryPressure(MemoryPressureState aState)
+{
+  NS_DispatchEventualMemoryPressure(aState);
+  nsCOMPtr<nsIRunnable> event = new Runnable;
+  return NS_DispatchToMainThread(event);
+}
diff --git a/xpcom/threads/nsMemoryPressure.h b/xpcom/threads/nsMemoryPressure.h
new file mode 100644
index 000000000..a05728c61
--- /dev/null
+++ b/xpcom/threads/nsMemoryPressure.h
@@ -0,0 +1,77 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 nsMemoryPressure_h__
+#define nsMemoryPressure_h__
+
+#include "nscore.h"
+
+enum MemoryPressureState
+{
+  /*
+   * No memory pressure.
+   */
+  MemPressure_None = 0,
+
+  /*
+   * New memory pressure deteced.
+   *
+   * On a new memory pressure, we stop everything to start cleaning
+   * aggresively the memory used, in order to free as much memory as
+   * possible.
+   */
+  MemPressure_New,
+
+  /*
+   * Repeated memory pressure.
+   *
+   * A repeated memory pressure implies to clean softly recent allocations.
+   * It is supposed to happen after a new memory pressure which already
+   * cleaned aggressivley.  So there is no need to damage the reactivity of
+   * Gecko by stopping the world again.
+   *
+   * In case of conflict with an new memory pressue, the new memory pressure
+   * takes precedence over an ongoing memory pressure.  The reason being
+   * that if no events are processed between 2 notifications (new followed
+   * by ongoing, or ongoing followed by a new) we want to be as aggresive as
+   * possible on the clean-up of the memory.  After all, we are trying to
+   * keep Gecko alive as long as possible.
+   */
+  MemPressure_Ongoing
+};
+
+/**
+ * Return and erase the latest state of the memory pressure event set by any of
+ * the corresponding dispatch function.
+ */
+MemoryPressureState
+NS_GetPendingMemoryPressure();
+
+/**
+ * This function causes the main thread to fire a memory pressure event
+ * before processing the next event, but if there are no events pending in
+ * the main thread's event queue, the memory pressure event would not be
+ * dispatched until one is enqueued. It is infallible and does not allocate
+ * any memory.
+ *
+ * You may call this function from any thread.
+ */
+void
+NS_DispatchEventualMemoryPressure(MemoryPressureState aState);
+
+/**
+ * This function causes the main thread to fire a memory pressure event
+ * before processing the next event. We wake up the main thread by adding a
+ * dummy event to its event loop, so, unlike with
+ * NS_DispatchEventualMemoryPressure, this memory-pressure event is always
+ * fired relatively quickly, even if the event loop is otherwise empty.
+ *
+ * You may call this function from any thread.
+ */
+nsresult
+NS_DispatchMemoryPressure(MemoryPressureState aState);
+
+#endif // nsMemoryPressure_h__
diff --git a/xpcom/threads/nsProcess.h b/xpcom/threads/nsProcess.h
new file mode 100644
index 000000000..140944415
--- /dev/null
+++ b/xpcom/threads/nsProcess.h
@@ -0,0 +1,82 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 _nsPROCESSWIN_H_
+#define _nsPROCESSWIN_H_
+
+#if defined(XP_WIN)
+#define PROCESSMODEL_WINAPI
+#endif
+
+#include "mozilla/Attributes.h"
+#include "mozilla/Mutex.h"
+#include "nsIProcess.h"
+#include "nsIFile.h"
+#include "nsIThread.h"
+#include "nsIObserver.h"
+#include "nsIWeakReferenceUtils.h"
+#include "nsIObserver.h"
+#include "nsString.h"
+#ifndef XP_MACOSX
+#include "prproces.h"
+#endif
+#if defined(PROCESSMODEL_WINAPI)
+#include <windows.h>
+#include <shellapi.h>
+#endif
+
+#define NS_PROCESS_CID \
+{0x7b4eeb20, 0xd781, 0x11d4, \
+   {0x8A, 0x83, 0x00, 0x10, 0xa4, 0xe0, 0xc9, 0xca}}
+
+class nsProcess final
+  : public nsIProcess
+  , public nsIObserver
+{
+public:
+
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIPROCESS
+  NS_DECL_NSIOBSERVER
+
+  nsProcess();
+
+private:
+  ~nsProcess();
+  static void Monitor(void* aArg);
+  void ProcessComplete();
+  nsresult CopyArgsAndRunProcess(bool aBlocking, const char** aArgs,
+                                 uint32_t aCount, nsIObserver* aObserver,
+                                 bool aHoldWeak);
+  nsresult CopyArgsAndRunProcessw(bool aBlocking, const char16_t** aArgs,
+                                  uint32_t aCount, nsIObserver* aObserver,
+                                  bool aHoldWeak);
+  // The 'args' array is null-terminated.
+  nsresult RunProcess(bool aBlocking, char** aArgs, nsIObserver* aObserver,
+                      bool aHoldWeak, bool aArgsUTF8);
+
+  PRThread* mThread;
+  mozilla::Mutex mLock;
+  bool mShutdown;
+  bool mBlocking;
+
+  nsCOMPtr<nsIFile> mExecutable;
+  nsString mTargetPath;
+  int32_t mPid;
+  nsCOMPtr<nsIObserver> mObserver;
+  nsWeakPtr mWeakObserver;
+
+  // These members are modified by multiple threads, any accesses should be
+  // protected with mLock.
+  int32_t mExitValue;
+#if defined(PROCESSMODEL_WINAPI)
+  HANDLE mProcess;
+#elif !defined(XP_MACOSX)
+  PRProcess* mProcess;
+#endif
+};
+
+#endif
diff --git a/xpcom/threads/nsProcessCommon.cpp b/xpcom/threads/nsProcessCommon.cpp
new file mode 100644
index 000000000..709865a09
--- /dev/null
+++ b/xpcom/threads/nsProcessCommon.cpp
@@ -0,0 +1,663 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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/. */
+
+/*****************************************************************************
+ *
+ * nsProcess is used to execute new processes and specify if you want to
+ * wait (blocking) or continue (non-blocking).
+ *
+ *****************************************************************************
+ */
+
+#include "mozilla/ArrayUtils.h"
+
+#include "nsCOMPtr.h"
+#include "nsAutoPtr.h"
+#include "nsMemory.h"
+#include "nsProcess.h"
+#include "prio.h"
+#include "prenv.h"
+#include "nsCRT.h"
+#include "nsThreadUtils.h"
+#include "nsIObserverService.h"
+#include "nsXULAppAPI.h"
+#include "mozilla/Services.h"
+
+#include <stdlib.h>
+
+#if defined(PROCESSMODEL_WINAPI)
+#include "prmem.h"
+#include "nsString.h"
+#include "nsLiteralString.h"
+#include "nsReadableUtils.h"
+#else
+#ifdef XP_MACOSX
+#include <crt_externs.h>
+#include <spawn.h>
+#include <sys/wait.h>
+#include <sys/errno.h>
+#endif
+#include <sys/types.h>
+#include <signal.h>
+#endif
+
+using namespace mozilla;
+
+#ifdef XP_MACOSX
+cpu_type_t pref_cpu_types[2] = {
+#if defined(__i386__)
+  CPU_TYPE_X86,
+#elif defined(__x86_64__)
+  CPU_TYPE_X86_64,
+#elif defined(__ppc__)
+  CPU_TYPE_POWERPC,
+#endif
+  CPU_TYPE_ANY
+};
+#endif
+
+//-------------------------------------------------------------------//
+// nsIProcess implementation
+//-------------------------------------------------------------------//
+NS_IMPL_ISUPPORTS(nsProcess, nsIProcess,
+                  nsIObserver)
+
+//Constructor
+nsProcess::nsProcess()
+  : mThread(nullptr)
+  , mLock("nsProcess.mLock")
+  , mShutdown(false)
+  , mBlocking(false)
+  , mPid(-1)
+  , mObserver(nullptr)
+  , mWeakObserver(nullptr)
+  , mExitValue(-1)
+#if !defined(XP_MACOSX)
+  , mProcess(nullptr)
+#endif
+{
+}
+
+//Destructor
+nsProcess::~nsProcess()
+{
+}
+
+NS_IMETHODIMP
+nsProcess::Init(nsIFile* aExecutable)
+{
+  if (mExecutable) {
+    return NS_ERROR_ALREADY_INITIALIZED;
+  }
+
+  if (NS_WARN_IF(!aExecutable)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+  bool isFile;
+
+  //First make sure the file exists
+  nsresult rv = aExecutable->IsFile(&isFile);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+  if (!isFile) {
+    return NS_ERROR_FAILURE;
+  }
+
+  //Store the nsIFile in mExecutable
+  mExecutable = aExecutable;
+  //Get the path because it is needed by the NSPR process creation
+#ifdef XP_WIN
+  rv = mExecutable->GetTarget(mTargetPath);
+  if (NS_FAILED(rv) || mTargetPath.IsEmpty())
+#endif
+    rv = mExecutable->GetPath(mTargetPath);
+
+  return rv;
+}
+
+
+#if defined(XP_WIN)
+// Out param `aWideCmdLine` must be PR_Freed by the caller.
+static int
+assembleCmdLine(char* const* aArgv, wchar_t** aWideCmdLine, UINT aCodePage)
+{
+  char* const* arg;
+  char* p;
+  char* q;
+  char* cmdLine;
+  int cmdLineSize;
+  int numBackslashes;
+  int i;
+  int argNeedQuotes;
+
+  /*
+   * Find out how large the command line buffer should be.
+   */
+  cmdLineSize = 0;
+  for (arg = aArgv; *arg; ++arg) {
+    /*
+     * \ and " need to be escaped by a \.  In the worst case,
+     * every character is a \ or ", so the string of length
+     * may double.  If we quote an argument, that needs two ".
+     * Finally, we need a space between arguments, and
+     * a null byte at the end of command line.
+     */
+    cmdLineSize += 2 * strlen(*arg)  /* \ and " need to be escaped */
+                   + 2               /* we quote every argument */
+                   + 1;              /* space in between, or final null */
+  }
+  p = cmdLine = (char*)PR_MALLOC(cmdLineSize * sizeof(char));
+  if (!p) {
+    return -1;
+  }
+
+  for (arg = aArgv; *arg; ++arg) {
+    /* Add a space to separates the arguments */
+    if (arg != aArgv) {
+      *p++ = ' ';
+    }
+    q = *arg;
+    numBackslashes = 0;
+    argNeedQuotes = 0;
+
+    /* If the argument contains white space, it needs to be quoted. */
+    if (strpbrk(*arg, " \f\n\r\t\v")) {
+      argNeedQuotes = 1;
+    }
+
+    if (argNeedQuotes) {
+      *p++ = '"';
+    }
+    while (*q) {
+      if (*q == '\\') {
+        numBackslashes++;
+        q++;
+      } else if (*q == '"') {
+        if (numBackslashes) {
+          /*
+           * Double the backslashes since they are followed
+           * by a quote
+           */
+          for (i = 0; i < 2 * numBackslashes; i++) {
+            *p++ = '\\';
+          }
+          numBackslashes = 0;
+        }
+        /* To escape the quote */
+        *p++ = '\\';
+        *p++ = *q++;
+      } else {
+        if (numBackslashes) {
+          /*
+           * Backslashes are not followed by a quote, so
+           * don't need to double the backslashes.
+           */
+          for (i = 0; i < numBackslashes; i++) {
+            *p++ = '\\';
+          }
+          numBackslashes = 0;
+        }
+        *p++ = *q++;
+      }
+    }
+
+    /* Now we are at the end of this argument */
+    if (numBackslashes) {
+      /*
+       * Double the backslashes if we have a quote string
+       * delimiter at the end.
+       */
+      if (argNeedQuotes) {
+        numBackslashes *= 2;
+      }
+      for (i = 0; i < numBackslashes; i++) {
+        *p++ = '\\';
+      }
+    }
+    if (argNeedQuotes) {
+      *p++ = '"';
+    }
+  }
+
+  *p = '\0';
+  int32_t numChars = MultiByteToWideChar(aCodePage, 0, cmdLine, -1, nullptr, 0);
+  *aWideCmdLine = (wchar_t*)PR_MALLOC(numChars * sizeof(wchar_t));
+  MultiByteToWideChar(aCodePage, 0, cmdLine, -1, *aWideCmdLine, numChars);
+  PR_Free(cmdLine);
+  return 0;
+}
+#endif
+
+void
+nsProcess::Monitor(void* aArg)
+{
+  RefPtr<nsProcess> process = dont_AddRef(static_cast<nsProcess*>(aArg));
+
+  if (!process->mBlocking) {
+    PR_SetCurrentThreadName("RunProcess");
+  }
+
+#if defined(PROCESSMODEL_WINAPI)
+  DWORD dwRetVal;
+  unsigned long exitCode = -1;
+
+  dwRetVal = WaitForSingleObject(process->mProcess, INFINITE);
+  if (dwRetVal != WAIT_FAILED) {
+    if (GetExitCodeProcess(process->mProcess, &exitCode) == FALSE) {
+      exitCode = -1;
+    }
+  }
+
+  // Lock in case Kill or GetExitCode are called during this
+  {
+    MutexAutoLock lock(process->mLock);
+    CloseHandle(process->mProcess);
+    process->mProcess = nullptr;
+    process->mExitValue = exitCode;
+    if (process->mShutdown) {
+      return;
+    }
+  }
+#else
+#ifdef XP_MACOSX
+  int exitCode = -1;
+  int status = 0;
+  pid_t result;
+  do {
+    result = waitpid(process->mPid, &status, 0);
+  } while (result == -1 && errno == EINTR);
+  if (result == process->mPid) {
+    if (WIFEXITED(status)) {
+      exitCode = WEXITSTATUS(status);
+    } else if (WIFSIGNALED(status)) {
+      exitCode = 256; // match NSPR's signal exit status
+    }
+  }
+#else
+  int32_t exitCode = -1;
+  if (PR_WaitProcess(process->mProcess, &exitCode) != PR_SUCCESS) {
+    exitCode = -1;
+  }
+#endif
+
+  // Lock in case Kill or GetExitCode are called during this
+  {
+    MutexAutoLock lock(process->mLock);
+#if !defined(XP_MACOSX)
+    process->mProcess = nullptr;
+#endif
+    process->mExitValue = exitCode;
+    if (process->mShutdown) {
+      return;
+    }
+  }
+#endif
+
+  // If we ran a background thread for the monitor then notify on the main
+  // thread
+  if (NS_IsMainThread()) {
+    process->ProcessComplete();
+  } else {
+    NS_DispatchToMainThread(NewRunnableMethod(process, &nsProcess::ProcessComplete));
+  }
+}
+
+void
+nsProcess::ProcessComplete()
+{
+  if (mThread) {
+    nsCOMPtr<nsIObserverService> os =
+      mozilla::services::GetObserverService();
+    if (os) {
+      os->RemoveObserver(this, "xpcom-shutdown");
+    }
+    PR_JoinThread(mThread);
+    mThread = nullptr;
+  }
+
+  const char* topic;
+  if (mExitValue < 0) {
+    topic = "process-failed";
+  } else {
+    topic = "process-finished";
+  }
+
+  mPid = -1;
+  nsCOMPtr<nsIObserver> observer;
+  if (mWeakObserver) {
+    observer = do_QueryReferent(mWeakObserver);
+  } else if (mObserver) {
+    observer = mObserver;
+  }
+  mObserver = nullptr;
+  mWeakObserver = nullptr;
+
+  if (observer) {
+    observer->Observe(NS_ISUPPORTS_CAST(nsIProcess*, this), topic, nullptr);
+  }
+}
+
+// XXXldb |aArgs| has the wrong const-ness
+NS_IMETHODIMP
+nsProcess::Run(bool aBlocking, const char** aArgs, uint32_t aCount)
+{
+  return CopyArgsAndRunProcess(aBlocking, aArgs, aCount, nullptr, false);
+}
+
+// XXXldb |aArgs| has the wrong const-ness
+NS_IMETHODIMP
+nsProcess::RunAsync(const char** aArgs, uint32_t aCount,
+                    nsIObserver* aObserver, bool aHoldWeak)
+{
+  return CopyArgsAndRunProcess(false, aArgs, aCount, aObserver, aHoldWeak);
+}
+
+nsresult
+nsProcess::CopyArgsAndRunProcess(bool aBlocking, const char** aArgs,
+                                 uint32_t aCount, nsIObserver* aObserver,
+                                 bool aHoldWeak)
+{
+  // Add one to the aCount for the program name and one for null termination.
+  char** my_argv = nullptr;
+  my_argv = (char**)moz_xmalloc(sizeof(char*) * (aCount + 2));
+  if (!my_argv) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  my_argv[0] = ToNewUTF8String(mTargetPath);
+
+  for (uint32_t i = 0; i < aCount; ++i) {
+    my_argv[i + 1] = const_cast<char*>(aArgs[i]);
+  }
+
+  my_argv[aCount + 1] = nullptr;
+
+  nsresult rv = RunProcess(aBlocking, my_argv, aObserver, aHoldWeak, false);
+
+  free(my_argv[0]);
+  free(my_argv);
+  return rv;
+}
+
+// XXXldb |aArgs| has the wrong const-ness
+NS_IMETHODIMP
+nsProcess::Runw(bool aBlocking, const char16_t** aArgs, uint32_t aCount)
+{
+  return CopyArgsAndRunProcessw(aBlocking, aArgs, aCount, nullptr, false);
+}
+
+// XXXldb |aArgs| has the wrong const-ness
+NS_IMETHODIMP
+nsProcess::RunwAsync(const char16_t** aArgs, uint32_t aCount,
+                     nsIObserver* aObserver, bool aHoldWeak)
+{
+  return CopyArgsAndRunProcessw(false, aArgs, aCount, aObserver, aHoldWeak);
+}
+
+nsresult
+nsProcess::CopyArgsAndRunProcessw(bool aBlocking, const char16_t** aArgs,
+                                  uint32_t aCount, nsIObserver* aObserver,
+                                  bool aHoldWeak)
+{
+  // Add one to the aCount for the program name and one for null termination.
+  char** my_argv = nullptr;
+  my_argv = (char**)moz_xmalloc(sizeof(char*) * (aCount + 2));
+  if (!my_argv) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  my_argv[0] = ToNewUTF8String(mTargetPath);
+
+  for (uint32_t i = 0; i < aCount; i++) {
+    my_argv[i + 1] = ToNewUTF8String(nsDependentString(aArgs[i]));
+  }
+
+  my_argv[aCount + 1] = nullptr;
+
+  nsresult rv = RunProcess(aBlocking, my_argv, aObserver, aHoldWeak, true);
+
+  for (uint32_t i = 0; i <= aCount; ++i) {
+    free(my_argv[i]);
+  }
+  free(my_argv);
+  return rv;
+}
+
+nsresult
+nsProcess::RunProcess(bool aBlocking, char** aMyArgv, nsIObserver* aObserver,
+                      bool aHoldWeak, bool aArgsUTF8)
+{
+  NS_WARNING_ASSERTION(!XRE_IsContentProcess(),
+                       "No launching of new processes in the content process");
+
+  if (NS_WARN_IF(!mExecutable)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+  if (NS_WARN_IF(mThread)) {
+    return NS_ERROR_ALREADY_INITIALIZED;
+  }
+
+  if (aObserver) {
+    if (aHoldWeak) {
+      mWeakObserver = do_GetWeakReference(aObserver);
+      if (!mWeakObserver) {
+        return NS_NOINTERFACE;
+      }
+    } else {
+      mObserver = aObserver;
+    }
+  }
+
+  mExitValue = -1;
+  mPid = -1;
+
+#if defined(PROCESSMODEL_WINAPI)
+  BOOL retVal;
+  wchar_t* cmdLine = nullptr;
+
+  // |aMyArgv| is null-terminated and always starts with the program path. If
+  // the second slot is non-null then arguments are being passed.
+  if (aMyArgv[1] && assembleCmdLine(aMyArgv + 1, &cmdLine,
+                                    aArgsUTF8 ? CP_UTF8 : CP_ACP) == -1) {
+    return NS_ERROR_FILE_EXECUTION_FAILED;
+  }
+
+  /* The SEE_MASK_NO_CONSOLE flag is important to prevent console windows
+   * from appearing. This makes behavior the same on all platforms. The flag
+   * will not have any effect on non-console applications.
+   */
+
+  // The program name in aMyArgv[0] is always UTF-8
+  NS_ConvertUTF8toUTF16 wideFile(aMyArgv[0]);
+
+  SHELLEXECUTEINFOW sinfo;
+  memset(&sinfo, 0, sizeof(SHELLEXECUTEINFOW));
+  sinfo.cbSize = sizeof(SHELLEXECUTEINFOW);
+  sinfo.hwnd   = nullptr;
+  sinfo.lpFile = wideFile.get();
+  sinfo.nShow  = SW_SHOWNORMAL;
+  sinfo.fMask  = SEE_MASK_FLAG_DDEWAIT |
+                 SEE_MASK_NO_CONSOLE |
+                 SEE_MASK_NOCLOSEPROCESS;
+
+  if (cmdLine) {
+    sinfo.lpParameters = cmdLine;
+  }
+
+  retVal = ShellExecuteExW(&sinfo);
+  if (!retVal) {
+    return NS_ERROR_FILE_EXECUTION_FAILED;
+  }
+
+  mProcess = sinfo.hProcess;
+
+  if (cmdLine) {
+    PR_Free(cmdLine);
+  }
+
+  mPid = GetProcessId(mProcess);
+#elif defined(XP_MACOSX)
+  // Initialize spawn attributes.
+  posix_spawnattr_t spawnattr;
+  if (posix_spawnattr_init(&spawnattr) != 0) {
+    return NS_ERROR_FAILURE;
+  }
+
+  // Set spawn attributes.
+  size_t attr_count = ArrayLength(pref_cpu_types);
+  size_t attr_ocount = 0;
+  if (posix_spawnattr_setbinpref_np(&spawnattr, attr_count, pref_cpu_types,
+                                    &attr_ocount) != 0 ||
+      attr_ocount != attr_count) {
+    posix_spawnattr_destroy(&spawnattr);
+    return NS_ERROR_FAILURE;
+  }
+
+  // Note: |aMyArgv| is already null-terminated as required by posix_spawnp.
+  pid_t newPid = 0;
+  int result = posix_spawnp(&newPid, aMyArgv[0], nullptr, &spawnattr, aMyArgv,
+                            *_NSGetEnviron());
+  mPid = static_cast<int32_t>(newPid);
+
+  posix_spawnattr_destroy(&spawnattr);
+
+  if (result != 0) {
+    return NS_ERROR_FAILURE;
+  }
+#else
+  mProcess = PR_CreateProcess(aMyArgv[0], aMyArgv, nullptr, nullptr);
+  if (!mProcess) {
+    return NS_ERROR_FAILURE;
+  }
+  struct MYProcess
+  {
+    uint32_t pid;
+  };
+  MYProcess* ptrProc = (MYProcess*)mProcess;
+  mPid = ptrProc->pid;
+#endif
+
+  NS_ADDREF_THIS();
+  mBlocking = aBlocking;
+  if (aBlocking) {
+    Monitor(this);
+    if (mExitValue < 0) {
+      return NS_ERROR_FILE_EXECUTION_FAILED;
+    }
+  } else {
+    mThread = PR_CreateThread(PR_SYSTEM_THREAD, Monitor, this,
+                              PR_PRIORITY_NORMAL, PR_GLOBAL_THREAD,
+                              PR_JOINABLE_THREAD, 0);
+    if (!mThread) {
+      NS_RELEASE_THIS();
+      return NS_ERROR_FAILURE;
+    }
+
+    // It isn't a failure if we just can't watch for shutdown
+    nsCOMPtr<nsIObserverService> os =
+      mozilla::services::GetObserverService();
+    if (os) {
+      os->AddObserver(this, "xpcom-shutdown", false);
+    }
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsProcess::GetIsRunning(bool* aIsRunning)
+{
+  if (mThread) {
+    *aIsRunning = true;
+  } else {
+    *aIsRunning = false;
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsProcess::GetPid(uint32_t* aPid)
+{
+  if (!mThread) {
+    return NS_ERROR_FAILURE;
+  }
+  if (mPid < 0) {
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+  *aPid = mPid;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsProcess::Kill()
+{
+  if (!mThread) {
+    return NS_ERROR_FAILURE;
+  }
+
+  {
+    MutexAutoLock lock(mLock);
+#if defined(PROCESSMODEL_WINAPI)
+    if (TerminateProcess(mProcess, 0) == 0) {
+      return NS_ERROR_FAILURE;
+    }
+#elif defined(XP_MACOSX)
+    if (kill(mPid, SIGKILL) != 0) {
+      return NS_ERROR_FAILURE;
+    }
+#else
+    if (!mProcess || (PR_KillProcess(mProcess) != PR_SUCCESS)) {
+      return NS_ERROR_FAILURE;
+    }
+#endif
+  }
+
+  // We must null out mThread if we want IsRunning to return false immediately
+  // after this call.
+  nsCOMPtr<nsIObserverService> os = mozilla::services::GetObserverService();
+  if (os) {
+    os->RemoveObserver(this, "xpcom-shutdown");
+  }
+  PR_JoinThread(mThread);
+  mThread = nullptr;
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsProcess::GetExitValue(int32_t* aExitValue)
+{
+  MutexAutoLock lock(mLock);
+
+  *aExitValue = mExitValue;
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsProcess::Observe(nsISupports* aSubject, const char* aTopic,
+                   const char16_t* aData)
+{
+  // Shutting down, drop all references
+  if (mThread) {
+    nsCOMPtr<nsIObserverService> os =
+      mozilla::services::GetObserverService();
+    if (os) {
+      os->RemoveObserver(this, "xpcom-shutdown");
+    }
+    mThread = nullptr;
+  }
+
+  mObserver = nullptr;
+  mWeakObserver = nullptr;
+
+  MutexAutoLock lock(mLock);
+  mShutdown = true;
+
+  return NS_OK;
+}
diff --git a/xpcom/threads/nsThread.cpp b/xpcom/threads/nsThread.cpp
new file mode 100644
index 000000000..63bd28ca3
--- /dev/null
+++ b/xpcom/threads/nsThread.cpp
@@ -0,0 +1,1500 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "nsThread.h"
+
+#include "base/message_loop.h"
+
+// Chromium's logging can sometimes leak through...
+#ifdef LOG
+#undef LOG
+#endif
+
+#include "mozilla/ReentrantMonitor.h"
+#include "nsMemoryPressure.h"
+#include "nsThreadManager.h"
+#include "nsIClassInfoImpl.h"
+#include "nsAutoPtr.h"
+#include "nsCOMPtr.h"
+#include "nsQueryObject.h"
+#include "pratom.h"
+#include "mozilla/CycleCollectedJSContext.h"
+#include "mozilla/Logging.h"
+#include "nsIObserverService.h"
+#include "mozilla/HangMonitor.h"
+#include "mozilla/IOInterposer.h"
+#include "mozilla/ipc/MessageChannel.h"
+#include "mozilla/ipc/BackgroundChild.h"
+#include "mozilla/Services.h"
+#include "nsXPCOMPrivate.h"
+#include "mozilla/ChaosMode.h"
+#include "mozilla/TimeStamp.h"
+#include "mozilla/Unused.h"
+#include "mozilla/dom/ScriptSettings.h"
+#include "nsIIdlePeriod.h"
+#include "nsIIncrementalRunnable.h"
+#include "nsThreadSyncDispatch.h"
+#include "LeakRefPtr.h"
+
+#ifdef MOZ_CRASHREPORTER
+#include "nsServiceManagerUtils.h"
+#include "nsICrashReporter.h"
+#include "mozilla/dom/ContentChild.h"
+#endif
+
+#ifdef XP_LINUX
+#include <sys/time.h>
+#include <sys/resource.h>
+#include <sched.h>
+#endif
+
+#define HAVE_UALARM _BSD_SOURCE || (_XOPEN_SOURCE >= 500 ||                 \
+                      _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED) &&           \
+                      !(_POSIX_C_SOURCE >= 200809L || _XOPEN_SOURCE >= 700)
+
+#if defined(XP_LINUX) && !defined(ANDROID) && defined(_GNU_SOURCE)
+#define HAVE_SCHED_SETAFFINITY
+#endif
+
+#ifdef XP_MACOSX
+#include <mach/mach.h>
+#include <mach/thread_policy.h>
+#endif
+
+#ifdef MOZ_CANARY
+# include <unistd.h>
+# include <execinfo.h>
+# include <signal.h>
+# include <fcntl.h>
+# include "nsXULAppAPI.h"
+#endif
+
+#if defined(NS_FUNCTION_TIMER) && defined(_MSC_VER)
+#include "nsTimerImpl.h"
+#include "mozilla/StackWalk.h"
+#endif
+#ifdef NS_FUNCTION_TIMER
+#include "nsCRT.h"
+#endif
+
+#ifdef MOZ_TASK_TRACER
+#include "GeckoTaskTracer.h"
+#include "TracedTaskCommon.h"
+using namespace mozilla::tasktracer;
+#endif
+
+using namespace mozilla;
+
+static LazyLogModule sThreadLog("nsThread");
+#ifdef LOG
+#undef LOG
+#endif
+#define LOG(args) MOZ_LOG(sThreadLog, mozilla::LogLevel::Debug, args)
+
+NS_DECL_CI_INTERFACE_GETTER(nsThread)
+
+//-----------------------------------------------------------------------------
+// Because we do not have our own nsIFactory, we have to implement nsIClassInfo
+// somewhat manually.
+
+class nsThreadClassInfo : public nsIClassInfo
+{
+public:
+  NS_DECL_ISUPPORTS_INHERITED  // no mRefCnt
+  NS_DECL_NSICLASSINFO
+
+  nsThreadClassInfo()
+  {
+  }
+};
+
+NS_IMETHODIMP_(MozExternalRefCountType)
+nsThreadClassInfo::AddRef()
+{
+  return 2;
+}
+NS_IMETHODIMP_(MozExternalRefCountType)
+nsThreadClassInfo::Release()
+{
+  return 1;
+}
+NS_IMPL_QUERY_INTERFACE(nsThreadClassInfo, nsIClassInfo)
+
+NS_IMETHODIMP
+nsThreadClassInfo::GetInterfaces(uint32_t* aCount, nsIID*** aArray)
+{
+  return NS_CI_INTERFACE_GETTER_NAME(nsThread)(aCount, aArray);
+}
+
+NS_IMETHODIMP
+nsThreadClassInfo::GetScriptableHelper(nsIXPCScriptable** aResult)
+{
+  *aResult = nullptr;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadClassInfo::GetContractID(char** aResult)
+{
+  *aResult = nullptr;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadClassInfo::GetClassDescription(char** aResult)
+{
+  *aResult = nullptr;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadClassInfo::GetClassID(nsCID** aResult)
+{
+  *aResult = nullptr;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadClassInfo::GetFlags(uint32_t* aResult)
+{
+  *aResult = THREADSAFE;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadClassInfo::GetClassIDNoAlloc(nsCID* aResult)
+{
+  return NS_ERROR_NOT_AVAILABLE;
+}
+
+//-----------------------------------------------------------------------------
+
+NS_IMPL_ADDREF(nsThread)
+NS_IMPL_RELEASE(nsThread)
+NS_INTERFACE_MAP_BEGIN(nsThread)
+  NS_INTERFACE_MAP_ENTRY(nsIThread)
+  NS_INTERFACE_MAP_ENTRY(nsIThreadInternal)
+  NS_INTERFACE_MAP_ENTRY(nsIEventTarget)
+  NS_INTERFACE_MAP_ENTRY(nsISupportsPriority)
+  NS_INTERFACE_MAP_ENTRY_AMBIGUOUS(nsISupports, nsIThread)
+  if (aIID.Equals(NS_GET_IID(nsIClassInfo))) {
+    static nsThreadClassInfo sThreadClassInfo;
+    foundInterface = static_cast<nsIClassInfo*>(&sThreadClassInfo);
+  } else
+NS_INTERFACE_MAP_END
+NS_IMPL_CI_INTERFACE_GETTER(nsThread, nsIThread, nsIThreadInternal,
+                            nsIEventTarget, nsISupportsPriority)
+
+//-----------------------------------------------------------------------------
+
+class nsThreadStartupEvent : public Runnable
+{
+public:
+  nsThreadStartupEvent()
+    : mMon("nsThreadStartupEvent.mMon")
+    , mInitialized(false)
+  {
+  }
+
+  // This method does not return until the thread startup object is in the
+  // completion state.
+  void Wait()
+  {
+    ReentrantMonitorAutoEnter mon(mMon);
+    while (!mInitialized) {
+      mon.Wait();
+    }
+  }
+
+  // This method needs to be public to support older compilers (xlC_r on AIX).
+  // It should be called directly as this class type is reference counted.
+  virtual ~nsThreadStartupEvent() {}
+
+private:
+  NS_IMETHOD Run() override
+  {
+    ReentrantMonitorAutoEnter mon(mMon);
+    mInitialized = true;
+    mon.Notify();
+    return NS_OK;
+  }
+
+  ReentrantMonitor mMon;
+  bool mInitialized;
+};
+//-----------------------------------------------------------------------------
+
+namespace {
+class DelayedRunnable : public Runnable,
+                        public nsITimerCallback
+{
+public:
+  DelayedRunnable(already_AddRefed<nsIThread> aTargetThread,
+                  already_AddRefed<nsIRunnable> aRunnable,
+                  uint32_t aDelay)
+    : mTargetThread(aTargetThread),
+      mWrappedRunnable(aRunnable),
+      mDelayedFrom(TimeStamp::NowLoRes()),
+      mDelay(aDelay)
+  { }
+
+  NS_DECL_ISUPPORTS_INHERITED
+
+  nsresult Init()
+  {
+    nsresult rv;
+    mTimer = do_CreateInstance(NS_TIMER_CONTRACTID, &rv);
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    MOZ_ASSERT(mTimer);
+    rv = mTimer->SetTarget(mTargetThread);
+
+    NS_ENSURE_SUCCESS(rv, rv);
+    return mTimer->InitWithCallback(this, mDelay, nsITimer::TYPE_ONE_SHOT);
+  }
+
+  nsresult DoRun()
+  {
+    nsCOMPtr<nsIRunnable> r = mWrappedRunnable.forget();
+    return r->Run();
+  }
+
+  NS_IMETHOD Run() override
+  {
+    // Already ran?
+    if (!mWrappedRunnable) {
+      return NS_OK;
+    }
+
+    // Are we too early?
+    if ((TimeStamp::NowLoRes() - mDelayedFrom).ToMilliseconds() < mDelay) {
+      return NS_OK; // Let the nsITimer run us.
+    }
+
+    mTimer->Cancel();
+    return DoRun();
+  }
+
+  NS_IMETHOD Notify(nsITimer* aTimer) override
+  {
+    // If we already ran, the timer should have been canceled.
+    MOZ_ASSERT(mWrappedRunnable);
+    MOZ_ASSERT(aTimer == mTimer);
+
+    return DoRun();
+  }
+
+private:
+  ~DelayedRunnable() {}
+
+  nsCOMPtr<nsIThread> mTargetThread;
+  nsCOMPtr<nsIRunnable> mWrappedRunnable;
+  nsCOMPtr<nsITimer> mTimer;
+  TimeStamp mDelayedFrom;
+  uint32_t mDelay;
+};
+
+NS_IMPL_ISUPPORTS_INHERITED(DelayedRunnable, Runnable, nsITimerCallback)
+
+} // anonymous namespace
+
+//-----------------------------------------------------------------------------
+
+struct nsThreadShutdownContext
+{
+  nsThreadShutdownContext(NotNull<nsThread*> aTerminatingThread,
+                          NotNull<nsThread*> aJoiningThread,
+                          bool      aAwaitingShutdownAck)
+    : mTerminatingThread(aTerminatingThread)
+    , mJoiningThread(aJoiningThread)
+    , mAwaitingShutdownAck(aAwaitingShutdownAck)
+  {
+    MOZ_COUNT_CTOR(nsThreadShutdownContext);
+  }
+  ~nsThreadShutdownContext()
+  {
+    MOZ_COUNT_DTOR(nsThreadShutdownContext);
+  }
+
+  // NB: This will be the last reference.
+  NotNull<RefPtr<nsThread>> mTerminatingThread;
+  NotNull<nsThread*> mJoiningThread;
+  bool mAwaitingShutdownAck;
+};
+
+// This event is responsible for notifying nsThread::Shutdown that it is time
+// to call PR_JoinThread. It implements nsICancelableRunnable so that it can
+// run on a DOM Worker thread (where all events must implement
+// nsICancelableRunnable.)
+class nsThreadShutdownAckEvent : public CancelableRunnable
+{
+public:
+  explicit nsThreadShutdownAckEvent(NotNull<nsThreadShutdownContext*> aCtx)
+    : mShutdownContext(aCtx)
+  {
+  }
+  NS_IMETHOD Run() override
+  {
+    mShutdownContext->mTerminatingThread->ShutdownComplete(mShutdownContext);
+    return NS_OK;
+  }
+  nsresult Cancel() override
+  {
+    return Run();
+  }
+private:
+  virtual ~nsThreadShutdownAckEvent() { }
+
+  NotNull<nsThreadShutdownContext*> mShutdownContext;
+};
+
+// This event is responsible for setting mShutdownContext
+class nsThreadShutdownEvent : public Runnable
+{
+public:
+  nsThreadShutdownEvent(NotNull<nsThread*> aThr,
+                        NotNull<nsThreadShutdownContext*> aCtx)
+    : mThread(aThr)
+    , mShutdownContext(aCtx)
+  {
+  }
+  NS_IMETHOD Run() override
+  {
+    mThread->mShutdownContext = mShutdownContext;
+    MessageLoop::current()->Quit();
+    return NS_OK;
+  }
+private:
+  NotNull<RefPtr<nsThread>> mThread;
+  NotNull<nsThreadShutdownContext*> mShutdownContext;
+};
+
+//-----------------------------------------------------------------------------
+
+static void
+SetThreadAffinity(unsigned int cpu)
+{
+#ifdef HAVE_SCHED_SETAFFINITY
+  cpu_set_t cpus;
+  CPU_ZERO(&cpus);
+  CPU_SET(cpu, &cpus);
+  sched_setaffinity(0, sizeof(cpus), &cpus);
+  // Don't assert sched_setaffinity's return value because it intermittently (?)
+  // fails with EINVAL on Linux x64 try runs.
+#elif defined(XP_MACOSX)
+  // OS X does not provide APIs to pin threads to specific processors, but you
+  // can tag threads as belonging to the same "affinity set" and the OS will try
+  // to run them on the same processor. To run threads on different processors,
+  // tag them as belonging to different affinity sets. Tag 0, the default, means
+  // "no affinity" so let's pretend each CPU has its own tag `cpu+1`.
+  thread_affinity_policy_data_t policy;
+  policy.affinity_tag = cpu + 1;
+  MOZ_ALWAYS_TRUE(thread_policy_set(mach_thread_self(), THREAD_AFFINITY_POLICY,
+                                    &policy.affinity_tag, 1) == KERN_SUCCESS);
+#elif defined(XP_WIN)
+  MOZ_ALWAYS_TRUE(SetThreadIdealProcessor(GetCurrentThread(), cpu) != -1);
+#endif
+}
+
+static void
+SetupCurrentThreadForChaosMode()
+{
+  if (!ChaosMode::isActive(ChaosFeature::ThreadScheduling)) {
+    return;
+  }
+
+#ifdef XP_LINUX
+  // PR_SetThreadPriority doesn't really work since priorities >
+  // PR_PRIORITY_NORMAL can't be set by non-root users. Instead we'll just use
+  // setpriority(2) to set random 'nice values'. In regular Linux this is only
+  // a dynamic adjustment so it still doesn't really do what we want, but tools
+  // like 'rr' can be more aggressive about honoring these values.
+  // Some of these calls may fail due to trying to lower the priority
+  // (e.g. something may have already called setpriority() for this thread).
+  // This makes it hard to have non-main threads with higher priority than the
+  // main thread, but that's hard to fix. Tools like rr can choose to honor the
+  // requested values anyway.
+  // Use just 4 priorities so there's a reasonable chance of any two threads
+  // having equal priority.
+  setpriority(PRIO_PROCESS, 0, ChaosMode::randomUint32LessThan(4));
+#else
+  // We should set the affinity here but NSPR doesn't provide a way to expose it.
+  uint32_t priority = ChaosMode::randomUint32LessThan(PR_PRIORITY_LAST + 1);
+  PR_SetThreadPriority(PR_GetCurrentThread(), PRThreadPriority(priority));
+#endif
+
+  // Force half the threads to CPU 0 so they compete for CPU
+  if (ChaosMode::randomUint32LessThan(2)) {
+    SetThreadAffinity(0);
+  }
+}
+
+/*static*/ void
+nsThread::ThreadFunc(void* aArg)
+{
+  using mozilla::ipc::BackgroundChild;
+
+  nsThread* self = static_cast<nsThread*>(aArg);  // strong reference
+  self->mThread = PR_GetCurrentThread();
+  SetupCurrentThreadForChaosMode();
+
+  // Inform the ThreadManager
+  nsThreadManager::get().RegisterCurrentThread(*self);
+
+  mozilla::IOInterposer::RegisterCurrentThread();
+
+  // Wait for and process startup event
+  nsCOMPtr<nsIRunnable> event;
+  {
+    MutexAutoLock lock(self->mLock);
+    if (!self->mEvents->GetEvent(true, getter_AddRefs(event), lock)) {
+      NS_WARNING("failed waiting for thread startup event");
+      return;
+    }
+  }
+  event->Run();  // unblocks nsThread::Init
+  event = nullptr;
+
+  {
+    // Scope for MessageLoop.
+    nsAutoPtr<MessageLoop> loop(
+      new MessageLoop(MessageLoop::TYPE_MOZILLA_NONMAINTHREAD, self));
+
+    // Now, process incoming events...
+    loop->Run();
+
+    BackgroundChild::CloseForCurrentThread();
+
+    // NB: The main thread does not shut down here!  It shuts down via
+    // nsThreadManager::Shutdown.
+
+    // Do NS_ProcessPendingEvents but with special handling to set
+    // mEventsAreDoomed atomically with the removal of the last event. The key
+    // invariant here is that we will never permit PutEvent to succeed if the
+    // event would be left in the queue after our final call to
+    // NS_ProcessPendingEvents. We also have to keep processing events as long
+    // as we have outstanding mRequestedShutdownContexts.
+    while (true) {
+      // Check and see if we're waiting on any threads.
+      self->WaitForAllAsynchronousShutdowns();
+
+      {
+        MutexAutoLock lock(self->mLock);
+        if (!self->mEvents->HasPendingEvent(lock)) {
+          // No events in the queue, so we will stop now. Don't let any more
+          // events be added, since they won't be processed. It is critical
+          // that no PutEvent can occur between testing that the event queue is
+          // empty and setting mEventsAreDoomed!
+          self->mEventsAreDoomed = true;
+          break;
+        }
+      }
+      NS_ProcessPendingEvents(self);
+    }
+  }
+
+  mozilla::IOInterposer::UnregisterCurrentThread();
+
+  // Inform the threadmanager that this thread is going away
+  nsThreadManager::get().UnregisterCurrentThread(*self);
+
+  // Dispatch shutdown ACK
+  NotNull<nsThreadShutdownContext*> context =
+    WrapNotNull(self->mShutdownContext);
+  MOZ_ASSERT(context->mTerminatingThread == self);
+  event = do_QueryObject(new nsThreadShutdownAckEvent(context));
+  context->mJoiningThread->Dispatch(event, NS_DISPATCH_NORMAL);
+
+  // Release any observer of the thread here.
+  self->SetObserver(nullptr);
+
+#ifdef MOZ_TASK_TRACER
+  FreeTraceInfo();
+#endif
+
+  NS_RELEASE(self);
+}
+
+//-----------------------------------------------------------------------------
+
+#ifdef MOZ_CRASHREPORTER
+// Tell the crash reporter to save a memory report if our heuristics determine
+// that an OOM failure is likely to occur soon.
+// Memory usage will not be checked more than every 30 seconds or saved more
+// than every 3 minutes
+// If |aShouldSave == kForceReport|, a report will be saved regardless of
+// whether the process is low on memory or not. However, it will still not be
+// saved if a report was saved less than 3 minutes ago.
+bool
+nsThread::SaveMemoryReportNearOOM(ShouldSaveMemoryReport aShouldSave)
+{
+  // Keep an eye on memory usage (cheap, ~7ms) somewhat frequently,
+  // but save memory reports (expensive, ~75ms) less frequently.
+  const size_t kLowMemoryCheckSeconds = 30;
+  const size_t kLowMemorySaveSeconds = 3 * 60;
+
+  static TimeStamp nextCheck = TimeStamp::NowLoRes()
+    + TimeDuration::FromSeconds(kLowMemoryCheckSeconds);
+  static bool recentlySavedReport = false; // Keeps track of whether a report
+                                           // was saved last time we checked
+
+  // Are we checking again too soon?
+  TimeStamp now = TimeStamp::NowLoRes();
+  if ((aShouldSave == ShouldSaveMemoryReport::kMaybeReport ||
+      recentlySavedReport) && now < nextCheck) {
+    return false;
+  }
+
+  bool needMemoryReport = (aShouldSave == ShouldSaveMemoryReport::kForceReport);
+#ifdef XP_WIN // XXX implement on other platforms as needed
+  // If the report is forced there is no need to check whether it is necessary
+  if (aShouldSave != ShouldSaveMemoryReport::kForceReport) {
+    const size_t LOWMEM_THRESHOLD_VIRTUAL = 200 * 1024 * 1024;
+    MEMORYSTATUSEX statex;
+    statex.dwLength = sizeof(statex);
+    if (GlobalMemoryStatusEx(&statex)) {
+      if (statex.ullAvailVirtual < LOWMEM_THRESHOLD_VIRTUAL) {
+        needMemoryReport = true;
+      }
+    }
+  }
+#endif
+
+  if (needMemoryReport) {
+    if (XRE_IsContentProcess()) {
+      dom::ContentChild* cc = dom::ContentChild::GetSingleton();
+      if (cc) {
+        cc->SendNotifyLowMemory();
+      }
+    } else {
+      nsCOMPtr<nsICrashReporter> cr =
+        do_GetService("@mozilla.org/toolkit/crash-reporter;1");
+      if (cr) {
+        cr->SaveMemoryReport();
+      }
+    }
+    recentlySavedReport = true;
+    nextCheck = now + TimeDuration::FromSeconds(kLowMemorySaveSeconds);
+  } else {
+    recentlySavedReport = false;
+    nextCheck = now + TimeDuration::FromSeconds(kLowMemoryCheckSeconds);
+  }
+
+  return recentlySavedReport;
+}
+#endif
+
+#ifdef MOZ_CANARY
+int sCanaryOutputFD = -1;
+#endif
+
+nsThread::nsThread(MainThreadFlag aMainThread, uint32_t aStackSize)
+  : mLock("nsThread.mLock")
+  , mScriptObserver(nullptr)
+  , mEvents(WrapNotNull(&mEventsRoot))
+  , mEventsRoot(mLock)
+  , mIdleEventsAvailable(mLock, "[nsThread.mEventsAvailable]")
+  , mIdleEvents(mIdleEventsAvailable, nsEventQueue::eNormalQueue)
+  , mPriority(PRIORITY_NORMAL)
+  , mThread(nullptr)
+  , mNestedEventLoopDepth(0)
+  , mStackSize(aStackSize)
+  , mShutdownContext(nullptr)
+  , mShutdownRequired(false)
+  , mEventsAreDoomed(false)
+  , mIsMainThread(aMainThread)
+  , mCanInvokeJS(false)
+{
+}
+
+nsThread::~nsThread()
+{
+  NS_ASSERTION(mRequestedShutdownContexts.IsEmpty(),
+               "shouldn't be waiting on other threads to shutdown");
+#ifdef DEBUG
+  // We deliberately leak these so they can be tracked by the leak checker.
+  // If you're having nsThreadShutdownContext leaks, you can set:
+  //   XPCOM_MEM_LOG_CLASSES=nsThreadShutdownContext
+  // during a test run and that will at least tell you what thread is
+  // requesting shutdown on another, which can be helpful for diagnosing
+  // the leak.
+  for (size_t i = 0; i < mRequestedShutdownContexts.Length(); ++i) {
+    Unused << mRequestedShutdownContexts[i].forget();
+  }
+#endif
+}
+
+nsresult
+nsThread::Init()
+{
+  // spawn thread and wait until it is fully setup
+  RefPtr<nsThreadStartupEvent> startup = new nsThreadStartupEvent();
+
+  NS_ADDREF_THIS();
+
+  mIdlePeriod = new IdlePeriod();
+
+  mShutdownRequired = true;
+
+  // ThreadFunc is responsible for setting mThread
+  if (!PR_CreateThread(PR_USER_THREAD, ThreadFunc, this,
+                       PR_PRIORITY_NORMAL, PR_GLOBAL_THREAD,
+                       PR_JOINABLE_THREAD, mStackSize)) {
+    NS_RELEASE_THIS();
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  // ThreadFunc will wait for this event to be run before it tries to access
+  // mThread.  By delaying insertion of this event into the queue, we ensure
+  // that mThread is set properly.
+  {
+    MutexAutoLock lock(mLock);
+    mEventsRoot.PutEvent(startup, lock); // retain a reference
+  }
+
+  // Wait for thread to call ThreadManager::SetupCurrentThread, which completes
+  // initialization of ThreadFunc.
+  startup->Wait();
+  return NS_OK;
+}
+
+nsresult
+nsThread::InitCurrentThread()
+{
+  mThread = PR_GetCurrentThread();
+  SetupCurrentThreadForChaosMode();
+
+  mIdlePeriod = new IdlePeriod();
+
+  nsThreadManager::get().RegisterCurrentThread(*this);
+  return NS_OK;
+}
+
+nsresult
+nsThread::PutEvent(nsIRunnable* aEvent, nsNestedEventTarget* aTarget)
+{
+  nsCOMPtr<nsIRunnable> event(aEvent);
+  return PutEvent(event.forget(), aTarget);
+}
+
+nsresult
+nsThread::PutEvent(already_AddRefed<nsIRunnable> aEvent, nsNestedEventTarget* aTarget)
+{
+  // We want to leak the reference when we fail to dispatch it, so that
+  // we won't release the event in a wrong thread.
+  LeakRefPtr<nsIRunnable> event(Move(aEvent));
+  nsCOMPtr<nsIThreadObserver> obs;
+
+  {
+    MutexAutoLock lock(mLock);
+    nsChainedEventQueue* queue = aTarget ? aTarget->mQueue : &mEventsRoot;
+    if (!queue || (queue == &mEventsRoot && mEventsAreDoomed)) {
+      NS_WARNING("An event was posted to a thread that will never run it (rejected)");
+      return NS_ERROR_UNEXPECTED;
+    }
+    queue->PutEvent(event.take(), lock);
+
+    // Make sure to grab the observer before dropping the lock, otherwise the
+    // event that we just placed into the queue could run and eventually delete
+    // this nsThread before the calling thread is scheduled again. We would then
+    // crash while trying to access a dead nsThread.
+    obs = mObserver;
+  }
+
+  if (obs) {
+    obs->OnDispatchedEvent(this);
+  }
+
+  return NS_OK;
+}
+
+nsresult
+nsThread::DispatchInternal(already_AddRefed<nsIRunnable> aEvent, uint32_t aFlags,
+                           nsNestedEventTarget* aTarget)
+{
+  // We want to leak the reference when we fail to dispatch it, so that
+  // we won't release the event in a wrong thread.
+  LeakRefPtr<nsIRunnable> event(Move(aEvent));
+  if (NS_WARN_IF(!event)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  if (gXPCOMThreadsShutDown && MAIN_THREAD != mIsMainThread && !aTarget) {
+    NS_ASSERTION(false, "Failed Dispatch after xpcom-shutdown-threads");
+    return NS_ERROR_ILLEGAL_DURING_SHUTDOWN;
+  }
+
+#ifdef MOZ_TASK_TRACER
+  nsCOMPtr<nsIRunnable> tracedRunnable = CreateTracedRunnable(event.take());
+  (static_cast<TracedRunnable*>(tracedRunnable.get()))->DispatchTask();
+  // XXX tracedRunnable will always leaked when we fail to disptch.
+  event = tracedRunnable.forget();
+#endif
+
+  if (aFlags & DISPATCH_SYNC) {
+    nsThread* thread = nsThreadManager::get().GetCurrentThread();
+    if (NS_WARN_IF(!thread)) {
+      return NS_ERROR_NOT_AVAILABLE;
+    }
+
+    // XXX we should be able to do something better here... we should
+    //     be able to monitor the slot occupied by this event and use
+    //     that to tell us when the event has been processed.
+
+    RefPtr<nsThreadSyncDispatch> wrapper =
+      new nsThreadSyncDispatch(thread, event.take());
+    nsresult rv = PutEvent(wrapper, aTarget); // hold a ref
+    // Don't wait for the event to finish if we didn't dispatch it...
+    if (NS_FAILED(rv)) {
+      // PutEvent leaked the wrapper runnable object on failure, so we
+      // explicitly release this object once for that. Note that this
+      // object will be released again soon because it exits the scope.
+      wrapper.get()->Release();
+      return rv;
+    }
+
+    // Allows waiting; ensure no locks are held that would deadlock us!
+    while (wrapper->IsPending()) {
+      NS_ProcessNextEvent(thread, true);
+    }
+    return NS_OK;
+  }
+
+  NS_ASSERTION(aFlags == NS_DISPATCH_NORMAL ||
+               aFlags == NS_DISPATCH_AT_END, "unexpected dispatch flags");
+  return PutEvent(event.take(), aTarget);
+}
+
+bool
+nsThread::nsChainedEventQueue::GetEvent(bool aMayWait, nsIRunnable** aEvent,
+                                        mozilla::MutexAutoLock& aProofOfLock)
+{
+  bool retVal = false;
+  do {
+    if (mProcessSecondaryQueueRunnable) {
+      MOZ_ASSERT(mSecondaryQueue->HasPendingEvent(aProofOfLock));
+      retVal = mSecondaryQueue->GetEvent(aMayWait, aEvent, aProofOfLock);
+      MOZ_ASSERT(*aEvent);
+      mProcessSecondaryQueueRunnable = false;
+      return retVal;
+    }
+
+    // We don't want to wait if mSecondaryQueue has some events.
+    bool reallyMayWait =
+      aMayWait && !mSecondaryQueue->HasPendingEvent(aProofOfLock);
+    retVal =
+      mNormalQueue->GetEvent(reallyMayWait, aEvent, aProofOfLock);
+
+    // Let's see if we should next time process an event from the secondary
+    // queue.
+    mProcessSecondaryQueueRunnable =
+      mSecondaryQueue->HasPendingEvent(aProofOfLock);
+
+    if (*aEvent) {
+      // We got an event, return early.
+      return retVal;
+    }
+  } while(aMayWait || mProcessSecondaryQueueRunnable);
+
+  return retVal;
+}
+
+//-----------------------------------------------------------------------------
+// nsIEventTarget
+
+NS_IMETHODIMP
+nsThread::DispatchFromScript(nsIRunnable* aEvent, uint32_t aFlags)
+{
+  nsCOMPtr<nsIRunnable> event(aEvent);
+  return Dispatch(event.forget(), aFlags);
+}
+
+NS_IMETHODIMP
+nsThread::Dispatch(already_AddRefed<nsIRunnable> aEvent, uint32_t aFlags)
+{
+  LOG(("THRD(%p) Dispatch [%p %x]\n", this, /* XXX aEvent */nullptr, aFlags));
+
+  return DispatchInternal(Move(aEvent), aFlags, nullptr);
+}
+
+NS_IMETHODIMP
+nsThread::DelayedDispatch(already_AddRefed<nsIRunnable> aEvent, uint32_t aDelayMs)
+{
+  NS_ENSURE_TRUE(!!aDelayMs, NS_ERROR_UNEXPECTED);
+
+  RefPtr<DelayedRunnable> r = new DelayedRunnable(Move(do_AddRef(this)),
+                                                  Move(aEvent),
+                                                  aDelayMs);
+  nsresult rv = r->Init();
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  return DispatchInternal(r.forget(), 0, nullptr);
+}
+
+NS_IMETHODIMP
+nsThread::IsOnCurrentThread(bool* aResult)
+{
+  *aResult = (PR_GetCurrentThread() == mThread);
+  return NS_OK;
+}
+
+//-----------------------------------------------------------------------------
+// nsIThread
+
+NS_IMETHODIMP
+nsThread::GetPRThread(PRThread** aResult)
+{
+  *aResult = mThread;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThread::GetCanInvokeJS(bool* aResult)
+{
+  *aResult = mCanInvokeJS;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThread::SetCanInvokeJS(bool aCanInvokeJS)
+{
+  mCanInvokeJS = aCanInvokeJS;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThread::AsyncShutdown()
+{
+  LOG(("THRD(%p) async shutdown\n", this));
+
+  // XXX If we make this warn, then we hit that warning at xpcom shutdown while
+  //     shutting down a thread in a thread pool.  That happens b/c the thread
+  //     in the thread pool is already shutdown by the thread manager.
+  if (!mThread) {
+    return NS_OK;
+  }
+
+  return !!ShutdownInternal(/* aSync = */ false) ? NS_OK : NS_ERROR_UNEXPECTED;
+}
+
+nsThreadShutdownContext*
+nsThread::ShutdownInternal(bool aSync)
+{
+  MOZ_ASSERT(mThread);
+  MOZ_ASSERT(mThread != PR_GetCurrentThread());
+  if (NS_WARN_IF(mThread == PR_GetCurrentThread())) {
+    return nullptr;
+  }
+
+  // Prevent multiple calls to this method
+  {
+    MutexAutoLock lock(mLock);
+    if (!mShutdownRequired) {
+      return nullptr;
+    }
+    mShutdownRequired = false;
+  }
+
+  NotNull<nsThread*> currentThread =
+    WrapNotNull(nsThreadManager::get().GetCurrentThread());
+
+  nsAutoPtr<nsThreadShutdownContext>& context =
+    *currentThread->mRequestedShutdownContexts.AppendElement();
+  context = new nsThreadShutdownContext(WrapNotNull(this), currentThread, aSync);
+
+  // Set mShutdownContext and wake up the thread in case it is waiting for
+  // events to process.
+  nsCOMPtr<nsIRunnable> event =
+    new nsThreadShutdownEvent(WrapNotNull(this), WrapNotNull(context.get()));
+  // XXXroc What if posting the event fails due to OOM?
+  PutEvent(event.forget(), nullptr);
+
+  // We could still end up with other events being added after the shutdown
+  // task, but that's okay because we process pending events in ThreadFunc
+  // after setting mShutdownContext just before exiting.
+  return context;
+}
+
+void
+nsThread::ShutdownComplete(NotNull<nsThreadShutdownContext*> aContext)
+{
+  MOZ_ASSERT(mThread);
+  MOZ_ASSERT(aContext->mTerminatingThread == this);
+
+  if (aContext->mAwaitingShutdownAck) {
+    // We're in a synchronous shutdown, so tell whatever is up the stack that
+    // we're done and unwind the stack so it can call us again.
+    aContext->mAwaitingShutdownAck = false;
+    return;
+  }
+
+  // Now, it should be safe to join without fear of dead-locking.
+
+  PR_JoinThread(mThread);
+  mThread = nullptr;
+
+  // We hold strong references to our event observers, and once the thread is
+  // shut down the observers can't easily unregister themselves. Do it here
+  // to avoid leaking.
+  ClearObservers();
+
+#ifdef DEBUG
+  {
+    MutexAutoLock lock(mLock);
+    MOZ_ASSERT(!mObserver, "Should have been cleared at shutdown!");
+  }
+#endif
+
+  // Delete aContext.
+  MOZ_ALWAYS_TRUE(
+    aContext->mJoiningThread->mRequestedShutdownContexts.RemoveElement(aContext));
+}
+
+void
+nsThread::WaitForAllAsynchronousShutdowns()
+{
+  while (mRequestedShutdownContexts.Length()) {
+    NS_ProcessNextEvent(this, true);
+  }
+}
+
+NS_IMETHODIMP
+nsThread::Shutdown()
+{
+  LOG(("THRD(%p) sync shutdown\n", this));
+
+  // XXX If we make this warn, then we hit that warning at xpcom shutdown while
+  //     shutting down a thread in a thread pool.  That happens b/c the thread
+  //     in the thread pool is already shutdown by the thread manager.
+  if (!mThread) {
+    return NS_OK;
+  }
+
+  nsThreadShutdownContext* maybeContext = ShutdownInternal(/* aSync = */ true);
+  NS_ENSURE_TRUE(maybeContext, NS_ERROR_UNEXPECTED);
+  NotNull<nsThreadShutdownContext*> context = WrapNotNull(maybeContext);
+
+  // Process events on the current thread until we receive a shutdown ACK.
+  // Allows waiting; ensure no locks are held that would deadlock us!
+  while (context->mAwaitingShutdownAck) {
+    NS_ProcessNextEvent(context->mJoiningThread, true);
+  }
+
+  ShutdownComplete(context);
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThread::HasPendingEvents(bool* aResult)
+{
+  if (NS_WARN_IF(PR_GetCurrentThread() != mThread)) {
+    return NS_ERROR_NOT_SAME_THREAD;
+  }
+
+  {
+    MutexAutoLock lock(mLock);
+    *aResult = mEvents->HasPendingEvent(lock);
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThread::RegisterIdlePeriod(already_AddRefed<nsIIdlePeriod> aIdlePeriod)
+{
+  if (NS_WARN_IF(PR_GetCurrentThread() != mThread)) {
+    return NS_ERROR_NOT_SAME_THREAD;
+  }
+
+  MutexAutoLock lock(mLock);
+  mIdlePeriod = aIdlePeriod;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThread::IdleDispatch(already_AddRefed<nsIRunnable> aEvent)
+{
+  // Currently the only supported idle dispatch is from the same
+  // thread. To support idle dispatch from another thread we need to
+  // support waking threads that are waiting for an event queue that
+  // isn't mIdleEvents.
+  MOZ_ASSERT(PR_GetCurrentThread() == mThread);
+
+  MutexAutoLock lock(mLock);
+  LeakRefPtr<nsIRunnable> event(Move(aEvent));
+
+  if (NS_WARN_IF(!event)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  if (mEventsAreDoomed) {
+    NS_WARNING("An idle event was posted to a thread that will never run it (rejected)");
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  mIdleEvents.PutEvent(event.take(), lock);
+  return NS_OK;
+}
+
+#ifdef MOZ_CANARY
+void canary_alarm_handler(int signum);
+
+class Canary
+{
+  //XXX ToDo: support nested loops
+public:
+  Canary()
+  {
+    if (sCanaryOutputFD > 0 && EventLatencyIsImportant()) {
+      signal(SIGALRM, canary_alarm_handler);
+      ualarm(15000, 0);
+    }
+  }
+
+  ~Canary()
+  {
+    if (sCanaryOutputFD != 0 && EventLatencyIsImportant()) {
+      ualarm(0, 0);
+    }
+  }
+
+  static bool EventLatencyIsImportant()
+  {
+    return NS_IsMainThread() && XRE_IsParentProcess();
+  }
+};
+
+void canary_alarm_handler(int signum)
+{
+  void* array[30];
+  const char msg[29] = "event took too long to run:\n";
+  // use write to be safe in the signal handler
+  write(sCanaryOutputFD, msg, sizeof(msg));
+  backtrace_symbols_fd(array, backtrace(array, 30), sCanaryOutputFD);
+}
+
+#endif
+
+#define NOTIFY_EVENT_OBSERVERS(func_, params_)                                 \
+  PR_BEGIN_MACRO                                                               \
+    if (!mEventObservers.IsEmpty()) {                                          \
+      nsAutoTObserverArray<NotNull<nsCOMPtr<nsIThreadObserver>>, 2>::ForwardIterator \
+        iter_(mEventObservers);                                                \
+      nsCOMPtr<nsIThreadObserver> obs_;                                        \
+      while (iter_.HasMore()) {                                                \
+        obs_ = iter_.GetNext();                                                \
+        obs_ -> func_ params_ ;                                                \
+      }                                                                        \
+    }                                                                          \
+  PR_END_MACRO
+
+void
+nsThread::GetIdleEvent(nsIRunnable** aEvent, MutexAutoLock& aProofOfLock)
+{
+  MOZ_ASSERT(PR_GetCurrentThread() == mThread);
+  MOZ_ASSERT(aEvent);
+
+  TimeStamp idleDeadline;
+  {
+    MutexAutoUnlock unlock(mLock);
+    mIdlePeriod->GetIdlePeriodHint(&idleDeadline);
+  }
+
+  if (!idleDeadline || idleDeadline < TimeStamp::Now()) {
+    aEvent = nullptr;
+    return;
+  }
+
+  mIdleEvents.GetEvent(false, aEvent, aProofOfLock);
+
+  if (*aEvent) {
+    nsCOMPtr<nsIIncrementalRunnable> incrementalEvent(do_QueryInterface(*aEvent));
+    if (incrementalEvent) {
+      incrementalEvent->SetDeadline(idleDeadline);
+    }
+  }
+}
+
+void
+nsThread::GetEvent(bool aWait, nsIRunnable** aEvent, MutexAutoLock& aProofOfLock)
+{
+  MOZ_ASSERT(PR_GetCurrentThread() == mThread);
+  MOZ_ASSERT(aEvent);
+
+  // We'll try to get an event to execute in three stages.
+  // [1] First we just try to get it from the regular queue without waiting.
+  mEvents->GetEvent(false, aEvent, aProofOfLock);
+
+  // [2] If we didn't get an event from the regular queue, try to
+  // get one from the idle queue
+  if (!*aEvent) {
+    // Since events in mEvents have higher priority than idle
+    // events, we will only consider idle events when there are no
+    // pending events in mEvents. We will for the same reason never
+    // wait for an idle event, since a higher priority event might
+    // appear at any time.
+    GetIdleEvent(aEvent, aProofOfLock);
+  }
+
+  // [3] If we neither got an event from the regular queue nor the
+  // idle queue, then if we should wait for events we block on the
+  // main queue until an event is available.
+  // If we are shutting down, then do not wait for new events.
+  if (!*aEvent && aWait) {
+    mEvents->GetEvent(aWait, aEvent, aProofOfLock);
+  }
+}
+
+NS_IMETHODIMP
+nsThread::ProcessNextEvent(bool aMayWait, bool* aResult)
+{
+  LOG(("THRD(%p) ProcessNextEvent [%u %u]\n", this, aMayWait,
+       mNestedEventLoopDepth));
+
+  if (NS_WARN_IF(PR_GetCurrentThread() != mThread)) {
+    return NS_ERROR_NOT_SAME_THREAD;
+  }
+
+  // The toplevel event loop normally blocks waiting for the next event, but
+  // if we're trying to shut this thread down, we must exit the event loop when
+  // the event queue is empty.
+  // This only applys to the toplevel event loop! Nested event loops (e.g.
+  // during sync dispatch) are waiting for some state change and must be able
+  // to block even if something has requested shutdown of the thread. Otherwise
+  // we'll just busywait as we endlessly look for an event, fail to find one,
+  // and repeat the nested event loop since its state change hasn't happened yet.
+  bool reallyWait = aMayWait && (mNestedEventLoopDepth > 0 || !ShuttingDown());
+
+  if (mIsMainThread == MAIN_THREAD) {
+    DoMainThreadSpecificProcessing(reallyWait);
+  }
+
+  ++mNestedEventLoopDepth;
+
+  // We only want to create an AutoNoJSAPI on threads that actually do DOM stuff
+  // (including workers).  Those are exactly the threads that have an
+  // mScriptObserver.
+  Maybe<dom::AutoNoJSAPI> noJSAPI;
+  bool callScriptObserver = !!mScriptObserver;
+  if (callScriptObserver) {
+    noJSAPI.emplace();
+    mScriptObserver->BeforeProcessTask(reallyWait);
+  }
+
+  nsCOMPtr<nsIThreadObserver> obs = mObserver;
+  if (obs) {
+    obs->OnProcessNextEvent(this, reallyWait);
+  }
+
+  NOTIFY_EVENT_OBSERVERS(OnProcessNextEvent, (this, reallyWait));
+
+#ifdef MOZ_CANARY
+  Canary canary;
+#endif
+  nsresult rv = NS_OK;
+
+  {
+    // Scope for |event| to make sure that its destructor fires while
+    // mNestedEventLoopDepth has been incremented, since that destructor can
+    // also do work.
+    nsCOMPtr<nsIRunnable> event;
+    {
+      MutexAutoLock lock(mLock);
+      GetEvent(reallyWait, getter_AddRefs(event), lock);
+    }
+
+    *aResult = (event.get() != nullptr);
+
+    if (event) {
+      LOG(("THRD(%p) running [%p]\n", this, event.get()));
+      if (MAIN_THREAD == mIsMainThread) {
+        HangMonitor::NotifyActivity();
+      }
+      event->Run();
+    } else if (aMayWait) {
+      MOZ_ASSERT(ShuttingDown(),
+                 "This should only happen when shutting down");
+      rv = NS_ERROR_UNEXPECTED;
+    }
+  }
+
+  NOTIFY_EVENT_OBSERVERS(AfterProcessNextEvent, (this, *aResult));
+
+  if (obs) {
+    obs->AfterProcessNextEvent(this, *aResult);
+  }
+
+  if (callScriptObserver) {
+    if (mScriptObserver) {
+      mScriptObserver->AfterProcessTask(mNestedEventLoopDepth);
+    }
+    noJSAPI.reset();
+  }
+
+  --mNestedEventLoopDepth;
+
+  return rv;
+}
+
+//-----------------------------------------------------------------------------
+// nsISupportsPriority
+
+NS_IMETHODIMP
+nsThread::GetPriority(int32_t* aPriority)
+{
+  *aPriority = mPriority;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThread::SetPriority(int32_t aPriority)
+{
+  if (NS_WARN_IF(!mThread)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  // NSPR defines the following four thread priorities:
+  //   PR_PRIORITY_LOW
+  //   PR_PRIORITY_NORMAL
+  //   PR_PRIORITY_HIGH
+  //   PR_PRIORITY_URGENT
+  // We map the priority values defined on nsISupportsPriority to these values.
+
+  mPriority = aPriority;
+
+  PRThreadPriority pri;
+  if (mPriority <= PRIORITY_HIGHEST) {
+    pri = PR_PRIORITY_URGENT;
+  } else if (mPriority < PRIORITY_NORMAL) {
+    pri = PR_PRIORITY_HIGH;
+  } else if (mPriority > PRIORITY_NORMAL) {
+    pri = PR_PRIORITY_LOW;
+  } else {
+    pri = PR_PRIORITY_NORMAL;
+  }
+  // If chaos mode is active, retain the randomly chosen priority
+  if (!ChaosMode::isActive(ChaosFeature::ThreadScheduling)) {
+    PR_SetThreadPriority(mThread, pri);
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThread::AdjustPriority(int32_t aDelta)
+{
+  return SetPriority(mPriority + aDelta);
+}
+
+//-----------------------------------------------------------------------------
+// nsIThreadInternal
+
+NS_IMETHODIMP
+nsThread::GetObserver(nsIThreadObserver** aObs)
+{
+  MutexAutoLock lock(mLock);
+  NS_IF_ADDREF(*aObs = mObserver);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThread::SetObserver(nsIThreadObserver* aObs)
+{
+  if (NS_WARN_IF(PR_GetCurrentThread() != mThread)) {
+    return NS_ERROR_NOT_SAME_THREAD;
+  }
+
+  MutexAutoLock lock(mLock);
+  mObserver = aObs;
+  return NS_OK;
+}
+
+uint32_t
+nsThread::RecursionDepth() const
+{
+  MOZ_ASSERT(PR_GetCurrentThread() == mThread);
+  return mNestedEventLoopDepth;
+}
+
+NS_IMETHODIMP
+nsThread::AddObserver(nsIThreadObserver* aObserver)
+{
+  if (NS_WARN_IF(!aObserver)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+  if (NS_WARN_IF(PR_GetCurrentThread() != mThread)) {
+    return NS_ERROR_NOT_SAME_THREAD;
+  }
+
+  NS_WARNING_ASSERTION(!mEventObservers.Contains(aObserver),
+                       "Adding an observer twice!");
+
+  if (!mEventObservers.AppendElement(WrapNotNull(aObserver))) {
+    NS_WARNING("Out of memory!");
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThread::RemoveObserver(nsIThreadObserver* aObserver)
+{
+  if (NS_WARN_IF(PR_GetCurrentThread() != mThread)) {
+    return NS_ERROR_NOT_SAME_THREAD;
+  }
+
+  if (aObserver && !mEventObservers.RemoveElement(aObserver)) {
+    NS_WARNING("Removing an observer that was never added!");
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThread::PushEventQueue(nsIEventTarget** aResult)
+{
+  if (NS_WARN_IF(PR_GetCurrentThread() != mThread)) {
+    return NS_ERROR_NOT_SAME_THREAD;
+  }
+
+  NotNull<nsChainedEventQueue*> queue =
+    WrapNotNull(new nsChainedEventQueue(mLock));
+  queue->mEventTarget = new nsNestedEventTarget(WrapNotNull(this), queue);
+
+  {
+    MutexAutoLock lock(mLock);
+    queue->mNext = mEvents;
+    mEvents = queue;
+  }
+
+  NS_ADDREF(*aResult = queue->mEventTarget);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThread::PopEventQueue(nsIEventTarget* aInnermostTarget)
+{
+  if (NS_WARN_IF(PR_GetCurrentThread() != mThread)) {
+    return NS_ERROR_NOT_SAME_THREAD;
+  }
+
+  if (NS_WARN_IF(!aInnermostTarget)) {
+    return NS_ERROR_NULL_POINTER;
+  }
+
+  // Don't delete or release anything while holding the lock.
+  nsAutoPtr<nsChainedEventQueue> queue;
+  RefPtr<nsNestedEventTarget> target;
+
+  {
+    MutexAutoLock lock(mLock);
+
+    // Make sure we're popping the innermost event target.
+    if (NS_WARN_IF(mEvents->mEventTarget != aInnermostTarget)) {
+      return NS_ERROR_UNEXPECTED;
+    }
+
+    MOZ_ASSERT(mEvents != &mEventsRoot);
+
+    queue = mEvents;
+    mEvents = WrapNotNull(mEvents->mNext);
+
+    nsCOMPtr<nsIRunnable> event;
+    while (queue->GetEvent(false, getter_AddRefs(event), lock)) {
+      mEvents->PutEvent(event.forget(), lock);
+    }
+
+    // Don't let the event target post any more events.
+    queue->mEventTarget.swap(target);
+    target->mQueue = nullptr;
+  }
+
+  return NS_OK;
+}
+
+void
+nsThread::SetScriptObserver(mozilla::CycleCollectedJSContext* aScriptObserver)
+{
+  if (!aScriptObserver) {
+    mScriptObserver = nullptr;
+    return;
+  }
+
+  MOZ_ASSERT(!mScriptObserver);
+  mScriptObserver = aScriptObserver;
+}
+
+void
+nsThread::DoMainThreadSpecificProcessing(bool aReallyWait)
+{
+  MOZ_ASSERT(mIsMainThread == MAIN_THREAD);
+
+  ipc::CancelCPOWs();
+
+  if (aReallyWait) {
+    HangMonitor::Suspend();
+  }
+
+  // Fire a memory pressure notification, if one is pending.
+  if (!ShuttingDown()) {
+    MemoryPressureState mpPending = NS_GetPendingMemoryPressure();
+    if (mpPending != MemPressure_None) {
+      nsCOMPtr<nsIObserverService> os = services::GetObserverService();
+
+      // Use no-forward to prevent the notifications from being transferred to
+      // the children of this process.
+      NS_NAMED_LITERAL_STRING(lowMem, "low-memory-no-forward");
+      NS_NAMED_LITERAL_STRING(lowMemOngoing, "low-memory-ongoing-no-forward");
+
+      if (os) {
+        os->NotifyObservers(nullptr, "memory-pressure",
+                            mpPending == MemPressure_New ? lowMem.get() :
+                            lowMemOngoing.get());
+      } else {
+        NS_WARNING("Can't get observer service!");
+      }
+    }
+  }
+
+#ifdef MOZ_CRASHREPORTER
+  if (!ShuttingDown()) {
+    SaveMemoryReportNearOOM(ShouldSaveMemoryReport::kMaybeReport);
+  }
+#endif
+}
+
+//-----------------------------------------------------------------------------
+
+NS_IMPL_ISUPPORTS(nsThread::nsNestedEventTarget, nsIEventTarget)
+
+NS_IMETHODIMP
+nsThread::nsNestedEventTarget::DispatchFromScript(nsIRunnable* aEvent, uint32_t aFlags)
+{
+  nsCOMPtr<nsIRunnable> event(aEvent);
+  return Dispatch(event.forget(), aFlags);
+}
+
+NS_IMETHODIMP
+nsThread::nsNestedEventTarget::Dispatch(already_AddRefed<nsIRunnable> aEvent, uint32_t aFlags)
+{
+  LOG(("THRD(%p) Dispatch [%p %x] to nested loop %p\n", mThread.get().get(),
+       /*XXX aEvent*/ nullptr, aFlags, this));
+
+  return mThread->DispatchInternal(Move(aEvent), aFlags, this);
+}
+
+NS_IMETHODIMP
+nsThread::nsNestedEventTarget::DelayedDispatch(already_AddRefed<nsIRunnable>, uint32_t)
+{
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+NS_IMETHODIMP
+nsThread::nsNestedEventTarget::IsOnCurrentThread(bool* aResult)
+{
+  return mThread->IsOnCurrentThread(aResult);
+}
diff --git a/xpcom/threads/nsThread.h b/xpcom/threads/nsThread.h
new file mode 100644
index 000000000..836123747
--- /dev/null
+++ b/xpcom/threads/nsThread.h
@@ -0,0 +1,284 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 nsThread_h__
+#define nsThread_h__
+
+#include "mozilla/Mutex.h"
+#include "nsIIdlePeriod.h"
+#include "nsIThreadInternal.h"
+#include "nsISupportsPriority.h"
+#include "nsEventQueue.h"
+#include "nsThreadUtils.h"
+#include "nsString.h"
+#include "nsTObserverArray.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/NotNull.h"
+#include "nsAutoPtr.h"
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/UniquePtr.h"
+
+namespace mozilla {
+class CycleCollectedJSContext;
+}
+
+using mozilla::NotNull;
+
+// A native thread
+class nsThread
+  : public nsIThreadInternal
+  , public nsISupportsPriority
+{
+public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIEVENTTARGET
+  NS_DECL_NSITHREAD
+  NS_DECL_NSITHREADINTERNAL
+  NS_DECL_NSISUPPORTSPRIORITY
+  using nsIEventTarget::Dispatch;
+
+  enum MainThreadFlag
+  {
+    MAIN_THREAD,
+    NOT_MAIN_THREAD
+  };
+
+  nsThread(MainThreadFlag aMainThread, uint32_t aStackSize);
+
+  // Initialize this as a wrapper for a new PRThread.
+  nsresult Init();
+
+  // Initialize this as a wrapper for the current PRThread.
+  nsresult InitCurrentThread();
+
+  // The PRThread corresponding to this thread.
+  PRThread* GetPRThread()
+  {
+    return mThread;
+  }
+
+  // If this flag is true, then the nsThread was created using
+  // nsIThreadManager::NewThread.
+  bool ShutdownRequired()
+  {
+    return mShutdownRequired;
+  }
+
+  // Clear the observer list.
+  void ClearObservers()
+  {
+    mEventObservers.Clear();
+  }
+
+  void
+  SetScriptObserver(mozilla::CycleCollectedJSContext* aScriptObserver);
+
+  uint32_t
+  RecursionDepth() const;
+
+  void ShutdownComplete(NotNull<struct nsThreadShutdownContext*> aContext);
+
+  void WaitForAllAsynchronousShutdowns();
+
+#ifdef MOZ_CRASHREPORTER
+  enum class ShouldSaveMemoryReport
+  {
+    kMaybeReport,
+    kForceReport
+  };
+
+  static bool SaveMemoryReportNearOOM(ShouldSaveMemoryReport aShouldSave);
+#endif
+
+private:
+  void DoMainThreadSpecificProcessing(bool aReallyWait);
+
+  void GetIdleEvent(nsIRunnable** aEvent, mozilla::MutexAutoLock& aProofOfLock);
+  void GetEvent(bool aWait, nsIRunnable** aEvent,
+                mozilla::MutexAutoLock& aProofOfLock);
+
+protected:
+  class nsChainedEventQueue;
+
+  class nsNestedEventTarget;
+  friend class nsNestedEventTarget;
+
+  friend class nsThreadShutdownEvent;
+
+  virtual ~nsThread();
+
+  bool ShuttingDown()
+  {
+    return mShutdownContext != nullptr;
+  }
+
+  static void ThreadFunc(void* aArg);
+
+  // Helper
+  already_AddRefed<nsIThreadObserver> GetObserver()
+  {
+    nsIThreadObserver* obs;
+    nsThread::GetObserver(&obs);
+    return already_AddRefed<nsIThreadObserver>(obs);
+  }
+
+  // Wrappers for event queue methods:
+  nsresult PutEvent(nsIRunnable* aEvent, nsNestedEventTarget* aTarget);
+  nsresult PutEvent(already_AddRefed<nsIRunnable> aEvent,
+                    nsNestedEventTarget* aTarget);
+
+  nsresult DispatchInternal(already_AddRefed<nsIRunnable> aEvent,
+                            uint32_t aFlags, nsNestedEventTarget* aTarget);
+
+  struct nsThreadShutdownContext* ShutdownInternal(bool aSync);
+
+  // Wrapper for nsEventQueue that supports chaining.
+  class nsChainedEventQueue
+  {
+  public:
+    explicit nsChainedEventQueue(mozilla::Mutex& aLock)
+      : mNext(nullptr)
+      , mEventsAvailable(aLock, "[nsChainedEventQueue.mEventsAvailable]")
+      , mProcessSecondaryQueueRunnable(false)
+    {
+      mNormalQueue =
+        mozilla::MakeUnique<nsEventQueue>(mEventsAvailable,
+                                          nsEventQueue::eSharedCondVarQueue);
+      // Both queues need to use the same CondVar!
+      mSecondaryQueue =
+        mozilla::MakeUnique<nsEventQueue>(mEventsAvailable,
+                                          nsEventQueue::eSharedCondVarQueue);
+    }
+
+    bool GetEvent(bool aMayWait, nsIRunnable** aEvent,
+                  mozilla::MutexAutoLock& aProofOfLock);
+
+    void PutEvent(nsIRunnable* aEvent, mozilla::MutexAutoLock& aProofOfLock)
+    {
+      RefPtr<nsIRunnable> event(aEvent);
+      PutEvent(event.forget(), aProofOfLock);
+    }
+
+    void PutEvent(already_AddRefed<nsIRunnable> aEvent,
+                  mozilla::MutexAutoLock& aProofOfLock)
+    {
+      RefPtr<nsIRunnable> event(aEvent);
+      nsCOMPtr<nsIRunnablePriority> runnablePrio =
+        do_QueryInterface(event);
+      uint32_t prio = nsIRunnablePriority::PRIORITY_NORMAL;
+      if (runnablePrio) {
+        runnablePrio->GetPriority(&prio);
+      }
+      MOZ_ASSERT(prio == nsIRunnablePriority::PRIORITY_NORMAL ||
+                 prio == nsIRunnablePriority::PRIORITY_HIGH);
+      if (prio == nsIRunnablePriority::PRIORITY_NORMAL) {
+        mNormalQueue->PutEvent(event.forget(), aProofOfLock);
+      } else {
+        mSecondaryQueue->PutEvent(event.forget(), aProofOfLock);
+      }
+    }
+
+    bool HasPendingEvent(mozilla::MutexAutoLock& aProofOfLock)
+    {
+      return mNormalQueue->HasPendingEvent(aProofOfLock) ||
+             mSecondaryQueue->HasPendingEvent(aProofOfLock);
+    }
+
+    nsChainedEventQueue* mNext;
+    RefPtr<nsNestedEventTarget> mEventTarget;
+
+  private:
+    mozilla::CondVar mEventsAvailable;
+    mozilla::UniquePtr<nsEventQueue> mNormalQueue;
+    mozilla::UniquePtr<nsEventQueue> mSecondaryQueue;
+
+    // Try to process one high priority runnable after each normal
+    // priority runnable. This gives the processing model HTML spec has for
+    // 'Update the rendering' in the case only vsync messages are in the
+    // secondary queue and prevents starving the normal queue.
+    bool mProcessSecondaryQueueRunnable;
+  };
+
+  class nsNestedEventTarget final : public nsIEventTarget
+  {
+  public:
+    NS_DECL_THREADSAFE_ISUPPORTS
+    NS_DECL_NSIEVENTTARGET
+
+    nsNestedEventTarget(NotNull<nsThread*> aThread,
+                        NotNull<nsChainedEventQueue*> aQueue)
+      : mThread(aThread)
+      , mQueue(aQueue)
+
+
+
+    {
+    }
+
+    NotNull<RefPtr<nsThread>> mThread;
+
+    // This is protected by mThread->mLock.
+    nsChainedEventQueue* mQueue;
+
+  private:
+    ~nsNestedEventTarget()
+    {
+    }
+  };
+
+  // This lock protects access to mObserver, mEvents, mIdleEvents,
+  // mIdlePeriod and mEventsAreDoomed.  All of those fields are only
+  // modified on the thread itself (never from another thread).  This
+  // means that we can avoid holding the lock while using mObserver
+  // and mEvents on the thread itself.  When calling PutEvent on
+  // mEvents, we have to hold the lock to synchronize with
+  // PopEventQueue.
+  mozilla::Mutex mLock;
+
+  nsCOMPtr<nsIThreadObserver> mObserver;
+  mozilla::CycleCollectedJSContext* mScriptObserver;
+
+  // Only accessed on the target thread.
+  nsAutoTObserverArray<NotNull<nsCOMPtr<nsIThreadObserver>>, 2> mEventObservers;
+
+  NotNull<nsChainedEventQueue*> mEvents;  // never null
+  nsChainedEventQueue mEventsRoot;
+
+  // mIdlePeriod keeps track of the current idle period. If at any
+  // time the main event queue is empty, calling
+  // mIdlePeriod->GetIdlePeriodHint() will give an estimate of when
+  // the current idle period will end.
+  nsCOMPtr<nsIIdlePeriod> mIdlePeriod;
+  mozilla::CondVar mIdleEventsAvailable;
+  nsEventQueue mIdleEvents;
+
+  int32_t   mPriority;
+  PRThread* mThread;
+  uint32_t  mNestedEventLoopDepth;
+  uint32_t  mStackSize;
+
+  // The shutdown context for ourselves.
+  struct nsThreadShutdownContext* mShutdownContext;
+  // The shutdown contexts for any other threads we've asked to shut down.
+  nsTArray<nsAutoPtr<struct nsThreadShutdownContext>> mRequestedShutdownContexts;
+
+  bool mShutdownRequired;
+  // Set to true when events posted to this thread will never run.
+  bool mEventsAreDoomed;
+  MainThreadFlag mIsMainThread;
+
+  // Set to true if this thread creates a JSRuntime.
+  bool mCanInvokeJS;
+};
+
+#if defined(XP_UNIX) && !defined(ANDROID) && !defined(DEBUG) && HAVE_UALARM \
+  && defined(_GNU_SOURCE)
+# define MOZ_CANARY
+
+extern int sCanaryOutputFD;
+#endif
+
+#endif  // nsThread_h__
diff --git a/xpcom/threads/nsThreadManager.cpp b/xpcom/threads/nsThreadManager.cpp
new file mode 100644
index 000000000..d1eb84b8f
--- /dev/null
+++ b/xpcom/threads/nsThreadManager.cpp
@@ -0,0 +1,342 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "nsThreadManager.h"
+#include "nsThread.h"
+#include "nsThreadUtils.h"
+#include "nsIClassInfoImpl.h"
+#include "nsTArray.h"
+#include "nsAutoPtr.h"
+#include "mozilla/ThreadLocal.h"
+#ifdef MOZ_CANARY
+#include <fcntl.h>
+#include <unistd.h>
+#endif
+
+#include "MainThreadIdlePeriod.h"
+
+using namespace mozilla;
+
+static MOZ_THREAD_LOCAL(bool) sTLSIsMainThread;
+
+bool
+NS_IsMainThread()
+{
+  return sTLSIsMainThread.get();
+}
+
+void
+NS_SetMainThread()
+{
+  if (!sTLSIsMainThread.init()) {
+    MOZ_CRASH();
+  }
+  sTLSIsMainThread.set(true);
+  MOZ_ASSERT(NS_IsMainThread());
+}
+
+typedef nsTArray<NotNull<RefPtr<nsThread>>> nsThreadArray;
+
+//-----------------------------------------------------------------------------
+
+static void
+ReleaseObject(void* aData)
+{
+  static_cast<nsISupports*>(aData)->Release();
+}
+
+// statically allocated instance
+NS_IMETHODIMP_(MozExternalRefCountType)
+nsThreadManager::AddRef()
+{
+  return 2;
+}
+NS_IMETHODIMP_(MozExternalRefCountType)
+nsThreadManager::Release()
+{
+  return 1;
+}
+NS_IMPL_CLASSINFO(nsThreadManager, nullptr,
+                  nsIClassInfo::THREADSAFE | nsIClassInfo::SINGLETON,
+                  NS_THREADMANAGER_CID)
+NS_IMPL_QUERY_INTERFACE_CI(nsThreadManager, nsIThreadManager)
+NS_IMPL_CI_INTERFACE_GETTER(nsThreadManager, nsIThreadManager)
+
+//-----------------------------------------------------------------------------
+
+nsresult
+nsThreadManager::Init()
+{
+  // Child processes need to initialize the thread manager before they
+  // initialize XPCOM in order to set up the crash reporter. This leads to
+  // situations where we get initialized twice.
+  if (mInitialized) {
+    return NS_OK;
+  }
+
+  if (PR_NewThreadPrivateIndex(&mCurThreadIndex, ReleaseObject) == PR_FAILURE) {
+    return NS_ERROR_FAILURE;
+  }
+
+
+#ifdef MOZ_CANARY
+  const int flags = O_WRONLY | O_APPEND | O_CREAT | O_NONBLOCK;
+  const mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH;
+  char* env_var_flag = getenv("MOZ_KILL_CANARIES");
+  sCanaryOutputFD =
+    env_var_flag ? (env_var_flag[0] ? open(env_var_flag, flags, mode) :
+                                      STDERR_FILENO) :
+                   0;
+#endif
+
+  // Setup "main" thread
+  mMainThread = new nsThread(nsThread::MAIN_THREAD, 0);
+
+  nsresult rv = mMainThread->InitCurrentThread();
+  if (NS_FAILED(rv)) {
+    mMainThread = nullptr;
+    return rv;
+  }
+
+  {
+    nsCOMPtr<nsIIdlePeriod> idlePeriod = new MainThreadIdlePeriod();
+    mMainThread->RegisterIdlePeriod(idlePeriod.forget());
+  }
+
+  // We need to keep a pointer to the current thread, so we can satisfy
+  // GetIsMainThread calls that occur post-Shutdown.
+  mMainThread->GetPRThread(&mMainPRThread);
+
+  mInitialized = true;
+  return NS_OK;
+}
+
+void
+nsThreadManager::Shutdown()
+{
+  MOZ_ASSERT(NS_IsMainThread(), "shutdown not called from main thread");
+
+  // Prevent further access to the thread manager (no more new threads!)
+  //
+  // What happens if shutdown happens before NewThread completes?
+  // We Shutdown() the new thread, and return error if we've started Shutdown
+  // between when NewThread started, and when the thread finished initializing
+  // and registering with ThreadManager.
+  //
+  mInitialized = false;
+
+  // Empty the main thread event queue before we begin shutting down threads.
+  NS_ProcessPendingEvents(mMainThread);
+
+  // We gather the threads from the hashtable into a list, so that we avoid
+  // holding the hashtable lock while calling nsIThread::Shutdown.
+  nsThreadArray threads;
+  {
+    OffTheBooksMutexAutoLock lock(mLock);
+    for (auto iter = mThreadsByPRThread.Iter(); !iter.Done(); iter.Next()) {
+      RefPtr<nsThread>& thread = iter.Data();
+      threads.AppendElement(WrapNotNull(thread));
+      iter.Remove();
+    }
+  }
+
+  // It's tempting to walk the list of threads here and tell them each to stop
+  // accepting new events, but that could lead to badness if one of those
+  // threads is stuck waiting for a response from another thread.  To do it
+  // right, we'd need some way to interrupt the threads.
+  //
+  // Instead, we process events on the current thread while waiting for threads
+  // to shutdown.  This means that we have to preserve a mostly functioning
+  // world until such time as the threads exit.
+
+  // Shutdown all threads that require it (join with threads that we created).
+  for (uint32_t i = 0; i < threads.Length(); ++i) {
+    NotNull<nsThread*> thread = threads[i];
+    if (thread->ShutdownRequired()) {
+      thread->Shutdown();
+    }
+  }
+
+  // NB: It's possible that there are events in the queue that want to *start*
+  // an asynchronous shutdown. But we have already shutdown the threads above,
+  // so there's no need to worry about them. We only have to wait for all
+  // in-flight asynchronous thread shutdowns to complete.
+  mMainThread->WaitForAllAsynchronousShutdowns();
+
+  // In case there are any more events somehow...
+  NS_ProcessPendingEvents(mMainThread);
+
+  // There are no more background threads at this point.
+
+  // Clear the table of threads.
+  {
+    OffTheBooksMutexAutoLock lock(mLock);
+    mThreadsByPRThread.Clear();
+  }
+
+  // Normally thread shutdown clears the observer for the thread, but since the
+  // main thread is special we do it manually here after we're sure all events
+  // have been processed.
+  mMainThread->SetObserver(nullptr);
+  mMainThread->ClearObservers();
+
+  // Release main thread object.
+  mMainThread = nullptr;
+
+  // Remove the TLS entry for the main thread.
+  PR_SetThreadPrivate(mCurThreadIndex, nullptr);
+}
+
+void
+nsThreadManager::RegisterCurrentThread(nsThread& aThread)
+{
+  MOZ_ASSERT(aThread.GetPRThread() == PR_GetCurrentThread(), "bad aThread");
+
+  OffTheBooksMutexAutoLock lock(mLock);
+
+  ++mCurrentNumberOfThreads;
+  if (mCurrentNumberOfThreads > mHighestNumberOfThreads) {
+    mHighestNumberOfThreads = mCurrentNumberOfThreads;
+  }
+
+  mThreadsByPRThread.Put(aThread.GetPRThread(), &aThread);  // XXX check OOM?
+
+  aThread.AddRef();  // for TLS entry
+  PR_SetThreadPrivate(mCurThreadIndex, &aThread);
+}
+
+void
+nsThreadManager::UnregisterCurrentThread(nsThread& aThread)
+{
+  MOZ_ASSERT(aThread.GetPRThread() == PR_GetCurrentThread(), "bad aThread");
+
+  OffTheBooksMutexAutoLock lock(mLock);
+
+  --mCurrentNumberOfThreads;
+  mThreadsByPRThread.Remove(aThread.GetPRThread());
+
+  PR_SetThreadPrivate(mCurThreadIndex, nullptr);
+  // Ref-count balanced via ReleaseObject
+}
+
+nsThread*
+nsThreadManager::GetCurrentThread()
+{
+  // read thread local storage
+  void* data = PR_GetThreadPrivate(mCurThreadIndex);
+  if (data) {
+    return static_cast<nsThread*>(data);
+  }
+
+  if (!mInitialized) {
+    return nullptr;
+  }
+
+  // OK, that's fine.  We'll dynamically create one :-)
+  RefPtr<nsThread> thread = new nsThread(nsThread::NOT_MAIN_THREAD, 0);
+  if (!thread || NS_FAILED(thread->InitCurrentThread())) {
+    return nullptr;
+  }
+
+  return thread.get();  // reference held in TLS
+}
+
+NS_IMETHODIMP
+nsThreadManager::NewThread(uint32_t aCreationFlags,
+                           uint32_t aStackSize,
+                           nsIThread** aResult)
+{
+  // Note: can be called from arbitrary threads
+  
+  // No new threads during Shutdown
+  if (NS_WARN_IF(!mInitialized)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  RefPtr<nsThread> thr = new nsThread(nsThread::NOT_MAIN_THREAD, aStackSize);
+  nsresult rv = thr->Init();  // Note: blocks until the new thread has been set up
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  // At this point, we expect that the thread has been registered in mThreadByPRThread;
+  // however, it is possible that it could have also been replaced by now, so
+  // we cannot really assert that it was added.  Instead, kill it if we entered
+  // Shutdown() during/before Init()
+
+  if (NS_WARN_IF(!mInitialized)) {
+    if (thr->ShutdownRequired()) {
+      thr->Shutdown(); // ok if it happens multiple times
+    }
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  thr.forget(aResult);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadManager::GetThreadFromPRThread(PRThread* aThread, nsIThread** aResult)
+{
+  // Keep this functioning during Shutdown
+  if (NS_WARN_IF(!mMainThread)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+  if (NS_WARN_IF(!aThread)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  RefPtr<nsThread> temp;
+  {
+    OffTheBooksMutexAutoLock lock(mLock);
+    mThreadsByPRThread.Get(aThread, getter_AddRefs(temp));
+  }
+
+  NS_IF_ADDREF(*aResult = temp);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadManager::GetMainThread(nsIThread** aResult)
+{
+  // Keep this functioning during Shutdown
+  if (NS_WARN_IF(!mMainThread)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+  NS_ADDREF(*aResult = mMainThread);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadManager::GetCurrentThread(nsIThread** aResult)
+{
+  // Keep this functioning during Shutdown
+  if (NS_WARN_IF(!mMainThread)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+  *aResult = GetCurrentThread();
+  if (!*aResult) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+  NS_ADDREF(*aResult);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadManager::GetIsMainThread(bool* aResult)
+{
+  // This method may be called post-Shutdown
+
+  *aResult = (PR_GetCurrentThread() == mMainPRThread);
+  return NS_OK;
+}
+
+uint32_t
+nsThreadManager::GetHighestNumberOfThreads()
+{
+  OffTheBooksMutexAutoLock lock(mLock);
+  return mHighestNumberOfThreads;
+}
diff --git a/xpcom/threads/nsThreadManager.h b/xpcom/threads/nsThreadManager.h
new file mode 100644
index 000000000..64ccc9bc9
--- /dev/null
+++ b/xpcom/threads/nsThreadManager.h
@@ -0,0 +1,89 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 nsThreadManager_h__
+#define nsThreadManager_h__
+
+#include "mozilla/Mutex.h"
+#include "nsIThreadManager.h"
+#include "nsRefPtrHashtable.h"
+#include "nsThread.h"
+
+class nsIRunnable;
+
+class nsThreadManager : public nsIThreadManager
+{
+public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSITHREADMANAGER
+
+  static nsThreadManager& get()
+  {
+    static nsThreadManager sInstance;
+    return sInstance;
+  }
+
+  nsresult Init();
+
+  // Shutdown all threads.  This function should only be called on the main
+  // thread of the application process.
+  void Shutdown();
+
+  // Called by nsThread to inform the ThreadManager it exists.  This method
+  // must be called when the given thread is the current thread.
+  void RegisterCurrentThread(nsThread& aThread);
+
+  // Called by nsThread to inform the ThreadManager it is going away.  This
+  // method must be called when the given thread is the current thread.
+  void UnregisterCurrentThread(nsThread& aThread);
+
+  // Returns the current thread.  Returns null if OOM or if ThreadManager isn't
+  // initialized.
+  nsThread* GetCurrentThread();
+
+  // Returns the maximal number of threads that have been in existence
+  // simultaneously during the execution of the thread manager.
+  uint32_t GetHighestNumberOfThreads();
+
+  // This needs to be public in order to support static instantiation of this
+  // class with older compilers (e.g., egcs-2.91.66).
+  ~nsThreadManager()
+  {
+  }
+
+private:
+  nsThreadManager()
+    : mCurThreadIndex(0)
+    , mMainPRThread(nullptr)
+    , mLock("nsThreadManager.mLock")
+    , mInitialized(false)
+    , mCurrentNumberOfThreads(1)
+    , mHighestNumberOfThreads(1)
+  {
+  }
+
+  nsRefPtrHashtable<nsPtrHashKey<PRThread>, nsThread> mThreadsByPRThread;
+  unsigned            mCurThreadIndex;  // thread-local-storage index
+  RefPtr<nsThread>  mMainThread;
+  PRThread*         mMainPRThread;
+  mozilla::OffTheBooksMutex mLock;  // protects tables
+  mozilla::Atomic<bool> mInitialized;
+
+  // The current number of threads
+  uint32_t            mCurrentNumberOfThreads;
+  // The highest number of threads encountered so far during the session
+  uint32_t            mHighestNumberOfThreads;
+};
+
+#define NS_THREADMANAGER_CID                       \
+{ /* 7a4204c6-e45a-4c37-8ebb-6709a22c917c */       \
+  0x7a4204c6,                                      \
+  0xe45a,                                          \
+  0x4c37,                                          \
+  {0x8e, 0xbb, 0x67, 0x09, 0xa2, 0x2c, 0x91, 0x7c} \
+}
+
+#endif  // nsThreadManager_h__
diff --git a/xpcom/threads/nsThreadPool.cpp b/xpcom/threads/nsThreadPool.cpp
new file mode 100644
index 000000000..241fad39d
--- /dev/null
+++ b/xpcom/threads/nsThreadPool.cpp
@@ -0,0 +1,449 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "nsIClassInfoImpl.h"
+#include "nsThreadPool.h"
+#include "nsThreadManager.h"
+#include "nsThread.h"
+#include "nsMemory.h"
+#include "nsAutoPtr.h"
+#include "prinrval.h"
+#include "mozilla/Logging.h"
+#include "nsThreadSyncDispatch.h"
+
+using namespace mozilla;
+
+static LazyLogModule sThreadPoolLog("nsThreadPool");
+#ifdef LOG
+#undef LOG
+#endif
+#define LOG(args) MOZ_LOG(sThreadPoolLog, mozilla::LogLevel::Debug, args)
+
+// DESIGN:
+//  o  Allocate anonymous threads.
+//  o  Use nsThreadPool::Run as the main routine for each thread.
+//  o  Each thread waits on the event queue's monitor, checking for
+//     pending events and rescheduling itself as an idle thread.
+
+#define DEFAULT_THREAD_LIMIT 4
+#define DEFAULT_IDLE_THREAD_LIMIT 1
+#define DEFAULT_IDLE_THREAD_TIMEOUT PR_SecondsToInterval(60)
+
+NS_IMPL_ADDREF(nsThreadPool)
+NS_IMPL_RELEASE(nsThreadPool)
+NS_IMPL_CLASSINFO(nsThreadPool, nullptr, nsIClassInfo::THREADSAFE,
+                  NS_THREADPOOL_CID)
+NS_IMPL_QUERY_INTERFACE_CI(nsThreadPool, nsIThreadPool, nsIEventTarget,
+                           nsIRunnable)
+NS_IMPL_CI_INTERFACE_GETTER(nsThreadPool, nsIThreadPool, nsIEventTarget)
+
+nsThreadPool::nsThreadPool()
+  : mMutex("[nsThreadPool.mMutex]")
+  , mEventsAvailable(mMutex, "[nsThreadPool.mEventsAvailable]")
+  , mEvents(mEventsAvailable, nsEventQueue::eNormalQueue)
+  , mThreadLimit(DEFAULT_THREAD_LIMIT)
+  , mIdleThreadLimit(DEFAULT_IDLE_THREAD_LIMIT)
+  , mIdleThreadTimeout(DEFAULT_IDLE_THREAD_TIMEOUT)
+  , mIdleCount(0)
+  , mStackSize(nsIThreadManager::DEFAULT_STACK_SIZE)
+  , mShutdown(false)
+{
+  LOG(("THRD-P(%p) constructor!!!\n", this));
+}
+
+nsThreadPool::~nsThreadPool()
+{
+  // Threads keep a reference to the nsThreadPool until they return from Run()
+  // after removing themselves from mThreads.
+  MOZ_ASSERT(mThreads.IsEmpty());
+}
+
+nsresult
+nsThreadPool::PutEvent(nsIRunnable* aEvent)
+{
+  nsCOMPtr<nsIRunnable> event(aEvent);
+  return PutEvent(event.forget(), 0);
+}
+
+nsresult
+nsThreadPool::PutEvent(already_AddRefed<nsIRunnable> aEvent, uint32_t aFlags)
+{
+  // Avoid spawning a new thread while holding the event queue lock...
+
+  bool spawnThread = false;
+  uint32_t stackSize = 0;
+  {
+    MutexAutoLock lock(mMutex);
+
+    if (NS_WARN_IF(mShutdown)) {
+      return NS_ERROR_NOT_AVAILABLE;
+    }
+    LOG(("THRD-P(%p) put [%d %d %d]\n", this, mIdleCount, mThreads.Count(),
+         mThreadLimit));
+    MOZ_ASSERT(mIdleCount <= (uint32_t)mThreads.Count(), "oops");
+
+    // Make sure we have a thread to service this event.
+    if (mThreads.Count() < (int32_t)mThreadLimit &&
+        !(aFlags & NS_DISPATCH_AT_END) &&
+        // Spawn a new thread if we don't have enough idle threads to serve
+        // pending events immediately.
+        mEvents.Count(lock) >= mIdleCount) {
+      spawnThread = true;
+    }
+
+    mEvents.PutEvent(Move(aEvent), lock);
+    stackSize = mStackSize;
+  }
+
+  LOG(("THRD-P(%p) put [spawn=%d]\n", this, spawnThread));
+  if (!spawnThread) {
+    return NS_OK;
+  }
+
+  nsCOMPtr<nsIThread> thread;
+  nsThreadManager::get().NewThread(0, stackSize, getter_AddRefs(thread));
+  if (NS_WARN_IF(!thread)) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  bool killThread = false;
+  {
+    MutexAutoLock lock(mMutex);
+    if (mThreads.Count() < (int32_t)mThreadLimit) {
+      mThreads.AppendObject(thread);
+    } else {
+      killThread = true;  // okay, we don't need this thread anymore
+    }
+  }
+  LOG(("THRD-P(%p) put [%p kill=%d]\n", this, thread.get(), killThread));
+  if (killThread) {
+    // We never dispatched any events to the thread, so we can shut it down
+    // asynchronously without worrying about anything.
+    ShutdownThread(thread);
+  } else {
+    thread->Dispatch(this, NS_DISPATCH_NORMAL);
+  }
+
+  return NS_OK;
+}
+
+void
+nsThreadPool::ShutdownThread(nsIThread* aThread)
+{
+  LOG(("THRD-P(%p) shutdown async [%p]\n", this, aThread));
+
+  // This is either called by a threadpool thread that is out of work, or
+  // a thread that attempted to create a threadpool thread and raced in
+  // such a way that the newly created thread is no longer necessary.
+  // In the first case, we must go to another thread to shut aThread down
+  // (because it is the current thread).  In the second case, we cannot
+  // synchronously shut down the current thread (because then Dispatch() would
+  // spin the event loop, and that could blow up the world), and asynchronous
+  // shutdown requires this thread have an event loop (and it may not, see bug
+  // 10204784).  The simplest way to cover all cases is to asynchronously
+  // shutdown aThread from the main thread.
+  NS_DispatchToMainThread(NewRunnableMethod(aThread,
+                                            &nsIThread::AsyncShutdown));
+}
+
+NS_IMETHODIMP
+nsThreadPool::Run()
+{
+  mThreadNaming.SetThreadPoolName(mName);
+
+  LOG(("THRD-P(%p) enter %s\n", this, mName.BeginReading()));
+
+  nsCOMPtr<nsIThread> current;
+  nsThreadManager::get().GetCurrentThread(getter_AddRefs(current));
+
+  bool shutdownThreadOnExit = false;
+  bool exitThread = false;
+  bool wasIdle = false;
+  PRIntervalTime idleSince;
+
+  nsCOMPtr<nsIThreadPoolListener> listener;
+  {
+    MutexAutoLock lock(mMutex);
+    listener = mListener;
+  }
+
+  if (listener) {
+    listener->OnThreadCreated();
+  }
+
+  do {
+    nsCOMPtr<nsIRunnable> event;
+    {
+      MutexAutoLock lock(mMutex);
+
+      if (!mEvents.GetPendingEvent(getter_AddRefs(event), lock)) {
+        PRIntervalTime now     = PR_IntervalNow();
+        PRIntervalTime timeout = PR_MillisecondsToInterval(mIdleThreadTimeout);
+
+        // If we are shutting down, then don't keep any idle threads
+        if (mShutdown) {
+          exitThread = true;
+        } else {
+          if (wasIdle) {
+            // if too many idle threads or idle for too long, then bail.
+            if (mIdleCount > mIdleThreadLimit ||
+                (mIdleThreadTimeout != UINT32_MAX && (now - idleSince) >= timeout)) {
+              exitThread = true;
+            }
+          } else {
+            // if would be too many idle threads...
+            if (mIdleCount == mIdleThreadLimit) {
+              exitThread = true;
+            } else {
+              ++mIdleCount;
+              idleSince = now;
+              wasIdle = true;
+            }
+          }
+        }
+
+        if (exitThread) {
+          if (wasIdle) {
+            --mIdleCount;
+          }
+          shutdownThreadOnExit = mThreads.RemoveObject(current);
+        } else {
+          PRIntervalTime delta = timeout - (now - idleSince);
+          LOG(("THRD-P(%p) %s waiting [%d]\n", this, mName.BeginReading(), delta));
+          mEvents.Wait(delta);
+          LOG(("THRD-P(%p) done waiting\n", this));
+        }
+      } else if (wasIdle) {
+        wasIdle = false;
+        --mIdleCount;
+      }
+    }
+    if (event) {
+      LOG(("THRD-P(%p) %s running [%p]\n", this, mName.BeginReading(), event.get()));
+      event->Run();
+    }
+  } while (!exitThread);
+
+  if (listener) {
+    listener->OnThreadShuttingDown();
+  }
+
+  if (shutdownThreadOnExit) {
+    ShutdownThread(current);
+  }
+
+  LOG(("THRD-P(%p) leave\n", this));
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadPool::DispatchFromScript(nsIRunnable* aEvent, uint32_t aFlags)
+{
+  nsCOMPtr<nsIRunnable> event(aEvent);
+  return Dispatch(event.forget(), aFlags);
+}
+
+NS_IMETHODIMP
+nsThreadPool::Dispatch(already_AddRefed<nsIRunnable> aEvent, uint32_t aFlags)
+{
+  LOG(("THRD-P(%p) dispatch [%p %x]\n", this, /* XXX aEvent*/ nullptr, aFlags));
+
+  if (NS_WARN_IF(mShutdown)) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+
+  if (aFlags & DISPATCH_SYNC) {
+    nsCOMPtr<nsIThread> thread;
+    nsThreadManager::get().GetCurrentThread(getter_AddRefs(thread));
+    if (NS_WARN_IF(!thread)) {
+      return NS_ERROR_NOT_AVAILABLE;
+    }
+
+    RefPtr<nsThreadSyncDispatch> wrapper =
+      new nsThreadSyncDispatch(thread, Move(aEvent));
+    PutEvent(wrapper);
+
+    while (wrapper->IsPending()) {
+      NS_ProcessNextEvent(thread);
+    }
+  } else {
+    NS_ASSERTION(aFlags == NS_DISPATCH_NORMAL ||
+                 aFlags == NS_DISPATCH_AT_END, "unexpected dispatch flags");
+    PutEvent(Move(aEvent), aFlags);
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadPool::DelayedDispatch(already_AddRefed<nsIRunnable>, uint32_t)
+{
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+NS_IMETHODIMP
+nsThreadPool::IsOnCurrentThread(bool* aResult)
+{
+  MutexAutoLock lock(mMutex);
+  if (NS_WARN_IF(mShutdown)) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+
+  nsIThread* thread = NS_GetCurrentThread();
+  for (uint32_t i = 0; i < static_cast<uint32_t>(mThreads.Count()); ++i) {
+    if (mThreads[i] == thread) {
+      *aResult = true;
+      return NS_OK;
+    }
+  }
+  *aResult = false;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadPool::Shutdown()
+{
+  nsCOMArray<nsIThread> threads;
+  nsCOMPtr<nsIThreadPoolListener> listener;
+  {
+    MutexAutoLock lock(mMutex);
+    mShutdown = true;
+    mEvents.NotifyAll();
+
+    threads.AppendObjects(mThreads);
+    mThreads.Clear();
+
+    // Swap in a null listener so that we release the listener at the end of
+    // this method. The listener will be kept alive as long as the other threads
+    // that were created when it was set.
+    mListener.swap(listener);
+  }
+
+  // It's important that we shutdown the threads while outside the event queue
+  // monitor.  Otherwise, we could end up dead-locking.
+
+  for (int32_t i = 0; i < threads.Count(); ++i) {
+    threads[i]->Shutdown();
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadPool::GetThreadLimit(uint32_t* aValue)
+{
+  *aValue = mThreadLimit;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadPool::SetThreadLimit(uint32_t aValue)
+{
+  MutexAutoLock lock(mMutex);
+  LOG(("THRD-P(%p) thread limit [%u]\n", this, aValue));
+  mThreadLimit = aValue;
+  if (mIdleThreadLimit > mThreadLimit) {
+    mIdleThreadLimit = mThreadLimit;
+  }
+
+  if (static_cast<uint32_t>(mThreads.Count()) > mThreadLimit) {
+    mEvents.NotifyAll();  // wake up threads so they observe this change
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadPool::GetIdleThreadLimit(uint32_t* aValue)
+{
+  *aValue = mIdleThreadLimit;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadPool::SetIdleThreadLimit(uint32_t aValue)
+{
+  MutexAutoLock lock(mMutex);
+  LOG(("THRD-P(%p) idle thread limit [%u]\n", this, aValue));
+  mIdleThreadLimit = aValue;
+  if (mIdleThreadLimit > mThreadLimit) {
+    mIdleThreadLimit = mThreadLimit;
+  }
+
+  // Do we need to kill some idle threads?
+  if (mIdleCount > mIdleThreadLimit) {
+    mEvents.NotifyAll();  // wake up threads so they observe this change
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadPool::GetIdleThreadTimeout(uint32_t* aValue)
+{
+  *aValue = mIdleThreadTimeout;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadPool::SetIdleThreadTimeout(uint32_t aValue)
+{
+  MutexAutoLock lock(mMutex);
+  uint32_t oldTimeout = mIdleThreadTimeout;
+  mIdleThreadTimeout = aValue;
+
+  // Do we need to notify any idle threads that their sleep time has shortened?
+  if (mIdleThreadTimeout < oldTimeout && mIdleCount > 0) {
+    mEvents.NotifyAll();  // wake up threads so they observe this change
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadPool::GetThreadStackSize(uint32_t* aValue)
+{
+  MutexAutoLock lock(mMutex);
+  *aValue = mStackSize;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadPool::SetThreadStackSize(uint32_t aValue)
+{
+  MutexAutoLock lock(mMutex);
+  mStackSize = aValue;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadPool::GetListener(nsIThreadPoolListener** aListener)
+{
+  MutexAutoLock lock(mMutex);
+  NS_IF_ADDREF(*aListener = mListener);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadPool::SetListener(nsIThreadPoolListener* aListener)
+{
+  nsCOMPtr<nsIThreadPoolListener> swappedListener(aListener);
+  {
+    MutexAutoLock lock(mMutex);
+    mListener.swap(swappedListener);
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadPool::SetName(const nsACString& aName)
+{
+  {
+    MutexAutoLock lock(mMutex);
+    if (mThreads.Count()) {
+      return NS_ERROR_NOT_AVAILABLE;
+    }
+  }
+
+  mName = aName;
+  return NS_OK;
+}
diff --git a/xpcom/threads/nsThreadPool.h b/xpcom/threads/nsThreadPool.h
new file mode 100644
index 000000000..47a4bd1ff
--- /dev/null
+++ b/xpcom/threads/nsThreadPool.h
@@ -0,0 +1,65 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 nsThreadPool_h__
+#define nsThreadPool_h__
+
+#include "nsIThreadPool.h"
+#include "nsIThread.h"
+#include "nsIRunnable.h"
+#include "nsEventQueue.h"
+#include "nsCOMArray.h"
+#include "nsCOMPtr.h"
+#include "nsThreadUtils.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/Monitor.h"
+
+class nsThreadPool final
+  : public nsIThreadPool
+  , public nsIRunnable
+{
+public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIEVENTTARGET
+  NS_DECL_NSITHREADPOOL
+  NS_DECL_NSIRUNNABLE
+  using nsIEventTarget::Dispatch;
+
+  nsThreadPool();
+
+private:
+  ~nsThreadPool();
+
+  void ShutdownThread(nsIThread* aThread);
+  nsresult PutEvent(nsIRunnable* aEvent);
+  nsresult PutEvent(already_AddRefed<nsIRunnable> aEvent, uint32_t aFlags);
+
+  nsCOMArray<nsIThread> mThreads;
+  mozilla::Mutex        mMutex;
+  mozilla::CondVar      mEventsAvailable;
+  nsEventQueue          mEvents;
+  uint32_t              mThreadLimit;
+  uint32_t              mIdleThreadLimit;
+  uint32_t              mIdleThreadTimeout;
+  uint32_t              mIdleCount;
+  uint32_t              mStackSize;
+  nsCOMPtr<nsIThreadPoolListener> mListener;
+  bool                  mShutdown;
+  nsCString             mName;
+  nsThreadPoolNaming    mThreadNaming;
+};
+
+#define NS_THREADPOOL_CID                          \
+{ /* 547ec2a8-315e-4ec4-888e-6e4264fe90eb */       \
+  0x547ec2a8,                                      \
+  0x315e,                                          \
+  0x4ec4,                                          \
+  {0x88, 0x8e, 0x6e, 0x42, 0x64, 0xfe, 0x90, 0xeb} \
+}
+
+#endif  // nsThreadPool_h__
diff --git a/xpcom/threads/nsThreadSyncDispatch.h b/xpcom/threads/nsThreadSyncDispatch.h
new file mode 100644
index 000000000..ae5e85464
--- /dev/null
+++ b/xpcom/threads/nsThreadSyncDispatch.h
@@ -0,0 +1,50 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 nsThreadSyncDispatch_h_
+#define nsThreadSyncDispatch_h_
+
+#include "nsThreadUtils.h"
+#include "LeakRefPtr.h"
+#include "mozilla/DebugOnly.h"
+
+class nsThreadSyncDispatch : public mozilla::Runnable
+{
+public:
+  nsThreadSyncDispatch(nsIThread* aOrigin, already_AddRefed<nsIRunnable>&& aTask)
+    : mOrigin(aOrigin)
+    , mSyncTask(mozilla::Move(aTask))
+  {
+  }
+
+  bool IsPending()
+  {
+    return !!mSyncTask;
+  }
+
+private:
+  NS_IMETHOD Run() override
+  {
+    if (nsIRunnable* task = mSyncTask.get()) {
+      mozilla::DebugOnly<nsresult> result = task->Run();
+      MOZ_ASSERT(NS_SUCCEEDED(result),
+                 "task in sync dispatch should not fail");
+      // We must release the task here to ensure that when the original
+      // thread is unblocked, this task has been released.
+      mSyncTask.release();
+      // unblock the origin thread
+      mOrigin->Dispatch(this, NS_DISPATCH_NORMAL);
+    }
+    return NS_OK;
+  }
+
+  nsCOMPtr<nsIThread> mOrigin;
+  // The task is leaked by default when Run() is not called, because
+  // otherwise we may release it in an incorrect thread.
+  mozilla::LeakRefPtr<nsIRunnable> mSyncTask;
+};
+
+#endif // nsThreadSyncDispatch_h_
diff --git a/xpcom/threads/nsTimerImpl.cpp b/xpcom/threads/nsTimerImpl.cpp
new file mode 100644
index 000000000..bc2d338e0
--- /dev/null
+++ b/xpcom/threads/nsTimerImpl.cpp
@@ -0,0 +1,658 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "nsTimerImpl.h"
+#include "TimerThread.h"
+#include "nsAutoPtr.h"
+#include "nsThreadManager.h"
+#include "nsThreadUtils.h"
+#include "pratom.h"
+#include "GeckoProfiler.h"
+#include "mozilla/Atomics.h"
+#include "mozilla/IntegerPrintfMacros.h"
+#include "mozilla/Logging.h"
+#ifdef MOZ_TASK_TRACER
+#include "GeckoTaskTracerImpl.h"
+using namespace mozilla::tasktracer;
+#endif
+
+#ifdef XP_WIN
+#include <process.h>
+#ifndef getpid
+#define getpid _getpid
+#endif
+#else
+#include <unistd.h>
+#endif
+
+using mozilla::Atomic;
+using mozilla::LogLevel;
+using mozilla::TimeDuration;
+using mozilla::TimeStamp;
+
+static TimerThread*     gThread = nullptr;
+
+// This module prints info about the precision of timers.
+static mozilla::LazyLogModule sTimerLog("nsTimerImpl");
+
+mozilla::LogModule*
+GetTimerLog()
+{
+  return sTimerLog;
+}
+
+// This module prints info about which timers are firing, which is useful for
+// wakeups for the purposes of power profiling. Set the following environment
+// variable before starting the browser.
+//
+//   MOZ_LOG=TimerFirings:4
+//
+// Then a line will be printed for every timer that fires. The name used for a
+// |Callback::Type::Function| timer depends on the circumstances.
+//
+// - If it was explicitly named (e.g. it was initialized with
+//   InitWithNamedFuncCallback()) then that explicit name will be shown.
+//
+// - Otherwise, if we are on a platform that supports function name lookup
+//   (Mac or Linux) then the looked-up name will be shown with a
+//   "[from dladdr]" annotation. On Mac the looked-up name will be immediately
+//   useful. On Linux it'll need post-processing with
+//   tools/rb/fix_linux_stack.py.
+//
+// - Otherwise, no name will be printed. If many timers hit this case then
+//   you'll need to re-run the workload on a Mac to find out which timers they
+//   are, and then give them explicit names.
+//
+// If you redirect this output to a file called "out", you can then
+// post-process it with a command something like the following.
+//
+//   cat out | grep timer | sort | uniq -c | sort -r -n
+//
+// This will show how often each unique line appears, with the most common ones
+// first.
+//
+// More detailed docs are here:
+// https://developer.mozilla.org/en-US/docs/Mozilla/Performance/TimerFirings_logging
+//
+static mozilla::LazyLogModule sTimerFiringsLog("TimerFirings");
+
+mozilla::LogModule*
+GetTimerFiringsLog()
+{
+  return sTimerFiringsLog;
+}
+
+#include <math.h>
+
+double nsTimerImpl::sDeltaSumSquared = 0;
+double nsTimerImpl::sDeltaSum = 0;
+double nsTimerImpl::sDeltaNum = 0;
+
+static void
+myNS_MeanAndStdDev(double n, double sumOfValues, double sumOfSquaredValues,
+                   double* meanResult, double* stdDevResult)
+{
+  double mean = 0.0, var = 0.0, stdDev = 0.0;
+  if (n > 0.0 && sumOfValues >= 0) {
+    mean = sumOfValues / n;
+    double temp = (n * sumOfSquaredValues) - (sumOfValues * sumOfValues);
+    if (temp < 0.0 || n <= 1) {
+      var = 0.0;
+    } else {
+      var = temp / (n * (n - 1));
+    }
+    // for some reason, Windows says sqrt(0.0) is "-1.#J" (?!) so do this:
+    stdDev = var != 0.0 ? sqrt(var) : 0.0;
+  }
+  *meanResult = mean;
+  *stdDevResult = stdDev;
+}
+
+NS_IMPL_QUERY_INTERFACE(nsTimer, nsITimer)
+NS_IMPL_ADDREF(nsTimer)
+
+NS_IMETHODIMP_(MozExternalRefCountType)
+nsTimer::Release(void)
+{
+  nsrefcnt count = --mRefCnt;
+  NS_LOG_RELEASE(this, count, "nsTimer");
+
+  if (count == 1) {
+    // Last ref, held by nsTimerImpl. Make sure the cycle is broken.
+    // If there is a nsTimerEvent in a queue for this timer, the nsTimer will
+    // live until that event pops, otherwise the nsTimerImpl will go away and
+    // the nsTimer along with it.
+    mImpl->Cancel();
+    mImpl = nullptr;
+  } else if (count == 0) {
+    delete this;
+  }
+
+  return count;
+}
+
+nsTimerImpl::nsTimerImpl(nsITimer* aTimer) :
+  mGeneration(0),
+  mDelay(0),
+  mITimer(aTimer),
+  mMutex("nsTimerImpl::mMutex")
+{
+  // XXXbsmedberg: shouldn't this be in Init()?
+  mEventTarget = static_cast<nsIEventTarget*>(NS_GetCurrentThread());
+}
+
+//static
+nsresult
+nsTimerImpl::Startup()
+{
+  nsresult rv;
+
+  gThread = new TimerThread();
+
+  NS_ADDREF(gThread);
+  rv = gThread->InitLocks();
+
+  if (NS_FAILED(rv)) {
+    NS_RELEASE(gThread);
+  }
+
+  return rv;
+}
+
+void
+nsTimerImpl::Shutdown()
+{
+  if (MOZ_LOG_TEST(GetTimerLog(), LogLevel::Debug)) {
+    double mean = 0, stddev = 0;
+    myNS_MeanAndStdDev(sDeltaNum, sDeltaSum, sDeltaSumSquared, &mean, &stddev);
+
+    MOZ_LOG(GetTimerLog(), LogLevel::Debug,
+           ("sDeltaNum = %f, sDeltaSum = %f, sDeltaSumSquared = %f\n",
+            sDeltaNum, sDeltaSum, sDeltaSumSquared));
+    MOZ_LOG(GetTimerLog(), LogLevel::Debug,
+           ("mean: %fms, stddev: %fms\n", mean, stddev));
+  }
+
+  if (!gThread) {
+    return;
+  }
+
+  gThread->Shutdown();
+  NS_RELEASE(gThread);
+}
+
+
+nsresult
+nsTimerImpl::InitCommon(uint32_t aDelay, uint32_t aType)
+{
+  mMutex.AssertCurrentThreadOwns();
+  nsresult rv;
+
+  if (NS_WARN_IF(!gThread)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+  if (!mEventTarget) {
+    NS_ERROR("mEventTarget is NULL");
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  rv = gThread->Init();
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  gThread->RemoveTimer(this);
+  ++mGeneration;
+
+  mType = (uint8_t)aType;
+  mDelay = aDelay;
+  mTimeout = TimeStamp::Now() + TimeDuration::FromMilliseconds(mDelay);
+
+  return gThread->AddTimer(this);
+}
+
+nsresult
+nsTimerImpl::InitWithFuncCallbackCommon(nsTimerCallbackFunc aFunc,
+                                        void* aClosure,
+                                        uint32_t aDelay,
+                                        uint32_t aType,
+                                        Callback::Name aName)
+{
+  if (NS_WARN_IF(!aFunc)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  Callback cb; // Goes out of scope after the unlock, prevents deadlock
+  cb.mType = Callback::Type::Function;
+  cb.mCallback.c = aFunc;
+  cb.mClosure = aClosure;
+  cb.mName = aName;
+
+  MutexAutoLock lock(mMutex);
+  cb.swap(mCallback);
+
+  return InitCommon(aDelay, aType);
+}
+
+NS_IMETHODIMP
+nsTimerImpl::InitWithFuncCallback(nsTimerCallbackFunc aFunc,
+                                  void* aClosure,
+                                  uint32_t aDelay,
+                                  uint32_t aType)
+{
+  Callback::Name name(Callback::Nothing);
+  return InitWithFuncCallbackCommon(aFunc, aClosure, aDelay, aType, name);
+}
+
+NS_IMETHODIMP
+nsTimerImpl::InitWithNamedFuncCallback(nsTimerCallbackFunc aFunc,
+                                       void* aClosure,
+                                       uint32_t aDelay,
+                                       uint32_t aType,
+                                       const char* aNameString)
+{
+  Callback::Name name(aNameString);
+  return InitWithFuncCallbackCommon(aFunc, aClosure, aDelay, aType, name);
+}
+
+NS_IMETHODIMP
+nsTimerImpl::InitWithNameableFuncCallback(nsTimerCallbackFunc aFunc,
+                                          void* aClosure,
+                                          uint32_t aDelay,
+                                          uint32_t aType,
+                                          nsTimerNameCallbackFunc aNameFunc)
+{
+  Callback::Name name(aNameFunc);
+  return InitWithFuncCallbackCommon(aFunc, aClosure, aDelay, aType, name);
+}
+
+NS_IMETHODIMP
+nsTimerImpl::InitWithCallback(nsITimerCallback* aCallback,
+                              uint32_t aDelay,
+                              uint32_t aType)
+{
+  if (NS_WARN_IF(!aCallback)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  Callback cb; // Goes out of scope after the unlock, prevents deadlock
+  cb.mType = Callback::Type::Interface;
+  cb.mCallback.i = aCallback;
+  NS_ADDREF(cb.mCallback.i);
+
+  MutexAutoLock lock(mMutex);
+  cb.swap(mCallback);
+
+  return InitCommon(aDelay, aType);
+}
+
+NS_IMETHODIMP
+nsTimerImpl::Init(nsIObserver* aObserver, uint32_t aDelay, uint32_t aType)
+{
+  if (NS_WARN_IF(!aObserver)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  Callback cb; // Goes out of scope after the unlock, prevents deadlock
+  cb.mType = Callback::Type::Observer;
+  cb.mCallback.o = aObserver;
+  NS_ADDREF(cb.mCallback.o);
+
+  MutexAutoLock lock(mMutex);
+  cb.swap(mCallback);
+
+  return InitCommon(aDelay, aType);
+}
+
+NS_IMETHODIMP
+nsTimerImpl::Cancel()
+{
+  Callback cb;
+
+  MutexAutoLock lock(mMutex);
+
+  if (gThread) {
+    gThread->RemoveTimer(this);
+  }
+
+  cb.swap(mCallback);
+  ++mGeneration;
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsTimerImpl::SetDelay(uint32_t aDelay)
+{
+  MutexAutoLock lock(mMutex);
+  if (GetCallback().mType == Callback::Type::Unknown && !IsRepeating()) {
+    // This may happen if someone tries to re-use a one-shot timer
+    // by re-setting delay instead of reinitializing the timer.
+    NS_ERROR("nsITimer->SetDelay() called when the "
+             "one-shot timer is not set up.");
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  bool reAdd = false;
+  if (gThread) {
+    reAdd = NS_SUCCEEDED(gThread->RemoveTimer(this));
+  }
+
+  mDelay = aDelay;
+  mTimeout = TimeStamp::Now() + TimeDuration::FromMilliseconds(mDelay);
+
+  if (reAdd) {
+    gThread->AddTimer(this);
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsTimerImpl::GetDelay(uint32_t* aDelay)
+{
+  MutexAutoLock lock(mMutex);
+  *aDelay = mDelay;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsTimerImpl::SetType(uint32_t aType)
+{
+  MutexAutoLock lock(mMutex);
+  mType = (uint8_t)aType;
+  // XXX if this is called, we should change the actual type.. this could effect
+  // repeating timers.  we need to ensure in Fire() that if mType has changed
+  // during the callback that we don't end up with the timer in the queue twice.
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsTimerImpl::GetType(uint32_t* aType)
+{
+  MutexAutoLock lock(mMutex);
+  *aType = mType;
+  return NS_OK;
+}
+
+
+NS_IMETHODIMP
+nsTimerImpl::GetClosure(void** aClosure)
+{
+  MutexAutoLock lock(mMutex);
+  *aClosure = GetCallback().mClosure;
+  return NS_OK;
+}
+
+
+NS_IMETHODIMP
+nsTimerImpl::GetCallback(nsITimerCallback** aCallback)
+{
+  MutexAutoLock lock(mMutex);
+  if (GetCallback().mType == Callback::Type::Interface) {
+    NS_IF_ADDREF(*aCallback = GetCallback().mCallback.i);
+  } else {
+    *aCallback = nullptr;
+  }
+
+  return NS_OK;
+}
+
+
+NS_IMETHODIMP
+nsTimerImpl::GetTarget(nsIEventTarget** aTarget)
+{
+  MutexAutoLock lock(mMutex);
+  NS_IF_ADDREF(*aTarget = mEventTarget);
+  return NS_OK;
+}
+
+
+NS_IMETHODIMP
+nsTimerImpl::SetTarget(nsIEventTarget* aTarget)
+{
+  MutexAutoLock lock(mMutex);
+  if (NS_WARN_IF(mCallback.mType != Callback::Type::Unknown)) {
+    return NS_ERROR_ALREADY_INITIALIZED;
+  }
+
+  if (aTarget) {
+    mEventTarget = aTarget;
+  } else {
+    mEventTarget = static_cast<nsIEventTarget*>(NS_GetCurrentThread());
+  }
+  return NS_OK;
+}
+
+
+void
+nsTimerImpl::Fire(int32_t aGeneration)
+{
+  uint8_t oldType;
+  uint32_t oldDelay;
+  TimeStamp oldTimeout;
+
+  {
+    // Don't fire callbacks or fiddle with refcounts when the mutex is locked.
+    // If some other thread Cancels/Inits after this, they're just too late.
+    MutexAutoLock lock(mMutex);
+    if (aGeneration != mGeneration) {
+      return;
+    }
+
+    mCallbackDuringFire.swap(mCallback);
+    oldType = mType;
+    oldDelay = mDelay;
+    oldTimeout = mTimeout;
+  }
+
+  PROFILER_LABEL("Timer", "Fire",
+                 js::ProfileEntry::Category::OTHER);
+
+  TimeStamp now = TimeStamp::Now();
+  if (MOZ_LOG_TEST(GetTimerLog(), LogLevel::Debug)) {
+    TimeDuration   delta = now - oldTimeout;
+    int32_t       d = delta.ToMilliseconds(); // delta in ms
+    sDeltaSum += abs(d);
+    sDeltaSumSquared += double(d) * double(d);
+    sDeltaNum++;
+
+    MOZ_LOG(GetTimerLog(), LogLevel::Debug,
+           ("[this=%p] expected delay time %4ums\n", this, oldDelay));
+    MOZ_LOG(GetTimerLog(), LogLevel::Debug,
+           ("[this=%p] actual delay time   %4dms\n", this, oldDelay + d));
+    MOZ_LOG(GetTimerLog(), LogLevel::Debug,
+           ("[this=%p] (mType is %d)       -------\n", this, oldType));
+    MOZ_LOG(GetTimerLog(), LogLevel::Debug,
+           ("[this=%p]     delta           %4dms\n", this, d));
+  }
+
+  if (MOZ_LOG_TEST(GetTimerFiringsLog(), LogLevel::Debug)) {
+    LogFiring(mCallbackDuringFire, oldType, oldDelay);
+  }
+
+  switch (mCallbackDuringFire.mType) {
+    case Callback::Type::Function:
+      mCallbackDuringFire.mCallback.c(mITimer, mCallbackDuringFire.mClosure);
+      break;
+    case Callback::Type::Interface:
+      mCallbackDuringFire.mCallback.i->Notify(mITimer);
+      break;
+    case Callback::Type::Observer:
+      mCallbackDuringFire.mCallback.o->Observe(mITimer, NS_TIMER_CALLBACK_TOPIC,
+                                               nullptr);
+      break;
+    default:
+      ;
+  }
+
+  Callback trash; // Swap into here to dispose of callback after the unlock
+  MutexAutoLock lock(mMutex);
+  if (aGeneration == mGeneration && IsRepeating()) {
+    // Repeating timer has not been re-init or canceled; reschedule
+    mCallbackDuringFire.swap(mCallback);
+    TimeDuration delay = TimeDuration::FromMilliseconds(mDelay);
+    if (mType == nsITimer::TYPE_REPEATING_SLACK) {
+      mTimeout = TimeStamp::Now() + delay;
+    } else {
+      mTimeout = mTimeout + delay;
+    }
+    if (gThread) {
+      gThread->AddTimer(this);
+    }
+  }
+
+  mCallbackDuringFire.swap(trash);
+
+  MOZ_LOG(GetTimerLog(), LogLevel::Debug,
+         ("[this=%p] Took %fms to fire timer callback\n",
+          this, (TimeStamp::Now() - now).ToMilliseconds()));
+}
+
+#if defined(HAVE_DLADDR) && defined(HAVE___CXA_DEMANGLE)
+#define USE_DLADDR 1
+#endif
+
+#ifdef USE_DLADDR
+#include <cxxabi.h>
+#include <dlfcn.h>
+#endif
+
+// See the big comment above GetTimerFiringsLog() to understand this code.
+void
+nsTimerImpl::LogFiring(const Callback& aCallback, uint8_t aType, uint32_t aDelay)
+{
+  const char* typeStr;
+  switch (aType) {
+    case nsITimer::TYPE_ONE_SHOT:                   typeStr = "ONE_SHOT"; break;
+    case nsITimer::TYPE_REPEATING_SLACK:            typeStr = "SLACK   "; break;
+    case nsITimer::TYPE_REPEATING_PRECISE:          /* fall through */
+    case nsITimer::TYPE_REPEATING_PRECISE_CAN_SKIP: typeStr = "PRECISE "; break;
+    default:                              MOZ_CRASH("bad type");
+  }
+
+  switch (aCallback.mType) {
+    case Callback::Type::Function: {
+      bool needToFreeName = false;
+      const char* annotation = "";
+      const char* name;
+      static const size_t buflen = 1024;
+      char buf[buflen];
+
+      if (aCallback.mName.is<Callback::NameString>()) {
+        name = aCallback.mName.as<Callback::NameString>();
+
+      } else if (aCallback.mName.is<Callback::NameFunc>()) {
+        aCallback.mName.as<Callback::NameFunc>()(
+            mITimer, aCallback.mClosure, buf, buflen);
+        name = buf;
+
+      } else {
+        MOZ_ASSERT(aCallback.mName.is<Callback::NameNothing>());
+#ifdef USE_DLADDR
+        annotation = "[from dladdr] ";
+
+        Dl_info info;
+        void* addr = reinterpret_cast<void*>(aCallback.mCallback.c);
+        if (dladdr(addr, &info) == 0) {
+          name = "???[dladdr: failed]";
+
+        } else if (info.dli_sname) {
+          int status;
+          name = abi::__cxa_demangle(info.dli_sname, nullptr, nullptr, &status);
+          if (status == 0) {
+            // Success. Because we didn't pass in a buffer to __cxa_demangle it
+            // allocates its own one with malloc() which we must free() later.
+            MOZ_ASSERT(name);
+            needToFreeName = true;
+          } else if (status == -1) {
+            name = "???[__cxa_demangle: OOM]";
+          } else if (status == -2) {
+            name = "???[__cxa_demangle: invalid mangled name]";
+          } else if (status == -3) {
+            name = "???[__cxa_demangle: invalid argument]";
+          } else {
+            name = "???[__cxa_demangle: unexpected status value]";
+          }
+
+        } else if (info.dli_fname) {
+          // The "#0: " prefix is necessary for fix_linux_stack.py to interpret
+          // this string as something to convert.
+          snprintf(buf, buflen, "#0: ???[%s +0x%" PRIxPTR "]\n",
+                   info.dli_fname, uintptr_t(addr) - uintptr_t(info.dli_fbase));
+          name = buf;
+
+        } else {
+          name = "???[dladdr: no symbol or shared object obtained]";
+        }
+#else
+        name = "???[dladdr is unimplemented or doesn't work well on this OS]";
+#endif
+      }
+
+      MOZ_LOG(GetTimerFiringsLog(), LogLevel::Debug,
+              ("[%d]    fn timer (%s %5d ms): %s%s\n",
+               getpid(), typeStr, aDelay, annotation, name));
+
+      if (needToFreeName) {
+        free(const_cast<char*>(name));
+      }
+
+      break;
+    }
+
+    case Callback::Type::Interface: {
+      MOZ_LOG(GetTimerFiringsLog(), LogLevel::Debug,
+              ("[%d] iface timer (%s %5d ms): %p\n",
+               getpid(), typeStr, aDelay, aCallback.mCallback.i));
+      break;
+    }
+
+    case Callback::Type::Observer: {
+      MOZ_LOG(GetTimerFiringsLog(), LogLevel::Debug,
+              ("[%d]   obs timer (%s %5d ms): %p\n",
+               getpid(), typeStr, aDelay, aCallback.mCallback.o));
+      break;
+    }
+
+    case Callback::Type::Unknown:
+    default: {
+      MOZ_LOG(GetTimerFiringsLog(), LogLevel::Debug,
+              ("[%d]   ??? timer (%s, %5d ms)\n",
+               getpid(), typeStr, aDelay));
+      break;
+    }
+  }
+}
+
+nsTimer::~nsTimer()
+{
+}
+
+size_t
+nsTimer::SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const
+{
+  return aMallocSizeOf(this);
+}
+
+/* static */
+const nsTimerImpl::Callback::NameNothing nsTimerImpl::Callback::Nothing = 0;
+
+#ifdef MOZ_TASK_TRACER
+void
+nsTimerImpl::GetTLSTraceInfo()
+{
+  mTracedTask.GetTLSTraceInfo();
+}
+
+TracedTaskCommon
+nsTimerImpl::GetTracedTask()
+{
+  return mTracedTask;
+}
+
+#endif
+
diff --git a/xpcom/threads/nsTimerImpl.h b/xpcom/threads/nsTimerImpl.h
new file mode 100644
index 000000000..5c731fbb4
--- /dev/null
+++ b/xpcom/threads/nsTimerImpl.h
@@ -0,0 +1,207 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 nsTimerImpl_h___
+#define nsTimerImpl_h___
+
+#include "nsITimer.h"
+#include "nsIEventTarget.h"
+#include "nsIObserver.h"
+
+#include "nsCOMPtr.h"
+
+#include "mozilla/Attributes.h"
+#include "mozilla/Logging.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/TimeStamp.h"
+#include "mozilla/Variant.h"
+
+#ifdef MOZ_TASK_TRACER
+#include "TracedTaskCommon.h"
+#endif
+
+extern mozilla::LogModule* GetTimerLog();
+
+#define NS_TIMER_CID \
+{ /* 5ff24248-1dd2-11b2-8427-fbab44f29bc8 */         \
+     0x5ff24248,                                     \
+     0x1dd2,                                         \
+     0x11b2,                                         \
+    {0x84, 0x27, 0xfb, 0xab, 0x44, 0xf2, 0x9b, 0xc8} \
+}
+
+// TimerThread, nsTimerEvent, and nsTimer have references to these. nsTimer has
+// a separate lifecycle so we can Cancel() the underlying timer when the user of
+// the nsTimer has let go of its last reference.
+class nsTimerImpl
+{
+  ~nsTimerImpl() {}
+public:
+  typedef mozilla::TimeStamp TimeStamp;
+
+  explicit nsTimerImpl(nsITimer* aTimer);
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(nsTimerImpl)
+  NS_DECL_NON_VIRTUAL_NSITIMER
+
+  static nsresult Startup();
+  static void Shutdown();
+
+  void Fire(int32_t aGeneration);
+
+#ifdef MOZ_TASK_TRACER
+  void GetTLSTraceInfo();
+  mozilla::tasktracer::TracedTaskCommon GetTracedTask();
+#endif
+
+  int32_t GetGeneration()
+  {
+    return mGeneration;
+  }
+
+  nsresult InitCommon(uint32_t aDelay, uint32_t aType);
+
+  struct Callback {
+    Callback() :
+      mType(Type::Unknown),
+      mName(Nothing),
+      mClosure(nullptr)
+    {
+      mCallback.c = nullptr;
+    }
+
+    Callback(const Callback& other) = delete;
+    Callback& operator=(const Callback& other) = delete;
+
+    ~Callback()
+    {
+      if (mType == Type::Interface) {
+        NS_RELEASE(mCallback.i);
+      } else if (mType == Type::Observer) {
+        NS_RELEASE(mCallback.o);
+      }
+    }
+
+    void swap(Callback& other)
+    {
+      std::swap(mType, other.mType);
+      std::swap(mCallback, other.mCallback);
+      std::swap(mName, other.mName);
+      std::swap(mClosure, other.mClosure);
+    }
+
+    enum class Type : uint8_t {
+      Unknown = 0,
+      Interface = 1,
+      Function = 2,
+      Observer = 3,
+    };
+    Type mType;
+
+    union CallbackUnion
+    {
+      nsTimerCallbackFunc c;
+      // These refcounted references are managed manually, as they are in a union
+      nsITimerCallback* MOZ_OWNING_REF i;
+      nsIObserver* MOZ_OWNING_REF o;
+    } mCallback;
+
+    // |Name| is a tagged union type representing one of (a) nothing, (b) a
+    // string, or (c) a function. mozilla::Variant doesn't naturally handle the
+    // "nothing" case, so we define a dummy type and value (which is unused and
+    // so the exact value doesn't matter) for it.
+    typedef const int NameNothing;
+    typedef const char* NameString;
+    typedef nsTimerNameCallbackFunc NameFunc;
+    typedef mozilla::Variant<NameNothing, NameString, NameFunc> Name;
+    static const NameNothing Nothing;
+    Name mName;
+
+    void*                 mClosure;
+  };
+
+  Callback& GetCallback()
+  {
+    mMutex.AssertCurrentThreadOwns();
+    if (mCallback.mType == Callback::Type::Unknown) {
+      return mCallbackDuringFire;
+    }
+
+    return mCallback;
+  }
+
+  bool IsRepeating() const
+  {
+    static_assert(nsITimer::TYPE_ONE_SHOT < nsITimer::TYPE_REPEATING_SLACK,
+                  "invalid ordering of timer types!");
+    static_assert(
+        nsITimer::TYPE_REPEATING_SLACK < nsITimer::TYPE_REPEATING_PRECISE,
+        "invalid ordering of timer types!");
+    static_assert(
+        nsITimer::TYPE_REPEATING_PRECISE <
+          nsITimer::TYPE_REPEATING_PRECISE_CAN_SKIP,
+        "invalid ordering of timer types!");
+    return mType >= nsITimer::TYPE_REPEATING_SLACK;
+  }
+
+  nsCOMPtr<nsIEventTarget> mEventTarget;
+
+  void LogFiring(const Callback& aCallback, uint8_t aType, uint32_t aDelay);
+
+  nsresult InitWithFuncCallbackCommon(nsTimerCallbackFunc aFunc,
+                                      void* aClosure,
+                                      uint32_t aDelay,
+                                      uint32_t aType,
+                                      Callback::Name aName);
+
+  // These members are set by the initiating thread, when the timer's type is
+  // changed and during the period where it fires on that thread.
+  uint8_t               mType;
+
+  // The generation number of this timer, re-generated each time the timer is
+  // initialized so one-shot timers can be canceled and re-initialized by the
+  // arming thread without any bad race conditions.
+  // Updated only after this timer has been removed from the timer thread.
+  int32_t               mGeneration;
+
+  uint32_t              mDelay;
+  // Updated only after this timer has been removed from the timer thread.
+  TimeStamp             mTimeout;
+
+#ifdef MOZ_TASK_TRACER
+  mozilla::tasktracer::TracedTaskCommon mTracedTask;
+#endif
+
+  static double         sDeltaSum;
+  static double         sDeltaSumSquared;
+  static double         sDeltaNum;
+  const RefPtr<nsITimer>      mITimer;
+  mozilla::Mutex mMutex;
+  Callback              mCallback;
+  Callback              mCallbackDuringFire;
+};
+
+class nsTimer final : public nsITimer
+{
+  virtual ~nsTimer();
+public:
+  nsTimer() : mImpl(new nsTimerImpl(this)) {}
+
+  friend class TimerThread;
+  friend class nsTimerEvent;
+  friend struct TimerAdditionComparator;
+
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_FORWARD_SAFE_NSITIMER(mImpl);
+
+  virtual size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const override;
+
+private:
+  // nsTimerImpl holds a strong ref to us. When our refcount goes to 1, we will
+  // null this to break the cycle.
+  RefPtr<nsTimerImpl> mImpl;
+};
+
+#endif /* nsTimerImpl_h___ */