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diff --git a/xpcom/threads/ThreadEventQueue.h b/xpcom/threads/ThreadEventQueue.h
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+++ b/xpcom/threads/ThreadEventQueue.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/. */
+
+#ifndef mozilla_ThreadEventQueue_h
+#define mozilla_ThreadEventQueue_h
+
+#include "mozilla/EventQueue.h"
+#include "mozilla/CondVar.h"
+#include "mozilla/SynchronizedEventQueue.h"
+#include "nsCOMPtr.h"
+#include "nsTArray.h"
+
+class nsIEventTarget;
+class nsISerialEventTarget;
+class nsIThreadObserver;
+
+namespace mozilla {
+
+class EventQueue;
+class ThreadEventTarget;
+
+// A ThreadEventQueue implements normal monitor-style synchronization over the
+// EventQueue. It also implements PushEventQueue and PopEventQueue for workers
+// (see the documentation below for an explanation of those). All threads use a
+// ThreadEventQueue as their event queue. Although for the main thread this
+// simply forwards events to the TaskController.
+class ThreadEventQueue final : public SynchronizedEventQueue {
+ public:
+  explicit ThreadEventQueue(UniquePtr<EventQueue> aQueue,
+                            bool aIsMainThread = false);
+
+  bool PutEvent(already_AddRefed<nsIRunnable>&& aEvent,
+                EventQueuePriority aPriority) final;
+
+  already_AddRefed<nsIRunnable> GetEvent(
+      bool aMayWait, mozilla::TimeDuration* aLastEventDelay = nullptr) final;
+  bool HasPendingEvent() final;
+
+  bool ShutdownIfNoPendingEvents() final;
+
+  void Disconnect(const MutexAutoLock& aProofOfLock) final {}
+
+  nsresult RegisterShutdownTask(nsITargetShutdownTask* aTask) final;
+  nsresult UnregisterShutdownTask(nsITargetShutdownTask* aTask) final;
+  void RunShutdownTasks() final;
+
+  already_AddRefed<nsISerialEventTarget> PushEventQueue() final;
+  void PopEventQueue(nsIEventTarget* aTarget) final;
+
+  already_AddRefed<nsIThreadObserver> GetObserver() final;
+  already_AddRefed<nsIThreadObserver> GetObserverOnThread() final;
+  void SetObserver(nsIThreadObserver* aObserver) final;
+
+  Mutex& MutexRef() { return mLock; }
+
+  size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) override;
+
+ private:
+  class NestedSink;
+
+  virtual ~ThreadEventQueue();
+
+  bool PutEventInternal(already_AddRefed<nsIRunnable>&& aEvent,
+                        EventQueuePriority aPriority, NestedSink* aQueue);
+
+  const UniquePtr<EventQueue> mBaseQueue MOZ_GUARDED_BY(mLock);
+
+  struct NestedQueueItem {
+    UniquePtr<EventQueue> mQueue;
+    RefPtr<ThreadEventTarget> mEventTarget;
+
+    NestedQueueItem(UniquePtr<EventQueue> aQueue,
+                    ThreadEventTarget* aEventTarget);
+  };
+
+  nsTArray<NestedQueueItem> mNestedQueues MOZ_GUARDED_BY(mLock);
+
+  Mutex mLock;
+  CondVar mEventsAvailable MOZ_GUARDED_BY(mLock);
+
+  bool mEventsAreDoomed MOZ_GUARDED_BY(mLock) = false;
+  nsCOMPtr<nsIThreadObserver> mObserver MOZ_GUARDED_BY(mLock);
+  nsTArray<nsCOMPtr<nsITargetShutdownTask>> mShutdownTasks
+      MOZ_GUARDED_BY(mLock);
+  bool mShutdownTasksRun MOZ_GUARDED_BY(mLock) = false;
+
+  const bool mIsMainThread;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_ThreadEventQueue_h