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diff --git a/xpcom/threads/TaskDispatcher.h b/xpcom/threads/TaskDispatcher.h
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+/* -*- 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