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diff --git a/xpcom/threads/TaskQueue.h b/xpcom/threads/TaskQueue.h
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+++ b/xpcom/threads/TaskQueue.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 TaskQueue_h_
+#define TaskQueue_h_
+
+#include <queue>
+
+#include "mozilla/AbstractThread.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/Monitor.h"
+#include "mozilla/MozPromise.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/TaskDispatcher.h"
+#include "nsIDirectTaskDispatcher.h"
+#include "nsThreadUtils.h"
+
+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.
+//
+// A TaskQueue does not require explicit shutdown, however it provides a
+// BeginShutdown() method that places TaskQueue in a shut down state and returns
+// a promise that gets resolved once all pending tasks have completed
+class TaskQueue : public AbstractThread, public nsIDirectTaskDispatcher {
+  class EventTargetWrapper;
+
+ public:
+  explicit TaskQueue(already_AddRefed<nsIEventTarget> aTarget,
+                     bool aSupportsTailDispatch = false);
+
+  TaskQueue(already_AddRefed<nsIEventTarget> aTarget, const char* aName,
+            bool aSupportsTailDispatch = false);
+
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_NSIDIRECTTASKDISPATCHER
+
+  TaskDispatcher& TailDispatcher() override;
+
+  NS_IMETHOD Dispatch(already_AddRefed<nsIRunnable> aEvent,
+                      uint32_t aFlags) override {
+    nsCOMPtr<nsIRunnable> runnable = aEvent;
+    {
+      MonitorAutoLock mon(mQueueMonitor);
+      return DispatchLocked(/* passed by ref */ runnable, aFlags,
+                            NormalDispatch);
+    }
+    // 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.
+  }
+
+  [[nodiscard]] nsresult Dispatch(
+      already_AddRefed<nsIRunnable> aRunnable,
+      DispatchReason aReason = NormalDispatch) override {
+    nsCOMPtr<nsIRunnable> r = aRunnable;
+    {
+      MonitorAutoLock mon(mQueueMonitor);
+      return DispatchLocked(/* passed by ref */ r, NS_DISPATCH_NORMAL, aReason);
+    }
+    // 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.
+  }
+
+  // So we can access nsIEventTarget::Dispatch(nsIRunnable*, uint32_t aFlags)
+  using nsIEventTarget::Dispatch;
+
+  // 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();
+
+  // Returns true if the current thread is currently running a Runnable in
+  // the task queue.
+  bool IsCurrentThreadIn() const override;
+  using nsISerialEventTarget::IsOnCurrentThread;
+
+ 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, uint32_t aFlags,
+                          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;
+
+  typedef struct TaskStruct {
+    nsCOMPtr<nsIRunnable> event;
+    uint32_t flags;
+  } TaskStruct;
+
+  // Queue of tasks to run.
+  std::queue<TaskStruct> 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<PRThread*> mRunningThread;
+
+  // RAII class that gets instantiated for each dispatched task.
+  class AutoTaskGuard {
+   public:
+    explicit AutoTaskGuard(TaskQueue* aQueue)
+        : 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);
+      mTaskDispatcher.emplace(aQueue,
+                              /* aIsTailDispatcher = */ true);
+      mQueue->mTailDispatcher = mTaskDispatcher.ptr();
+
+      mLastCurrentThread = sCurrentThreadTLS.get();
+      sCurrentThreadTLS.set(aQueue);
+
+      MOZ_ASSERT(mQueue->mRunningThread == nullptr);
+      mQueue->mRunningThread = PR_GetCurrentThread();
+    }
+
+    ~AutoTaskGuard() {
+      mTaskDispatcher->DrainDirectTasks();
+      mTaskDispatcher.reset();
+
+      MOZ_ASSERT(mQueue->mRunningThread == PR_GetCurrentThread());
+      mQueue->mRunningThread = nullptr;
+
+      sCurrentThreadTLS.set(mLastCurrentThread);
+      mQueue->mTailDispatcher = nullptr;
+    }
+
+   private:
+    Maybe<AutoTaskDispatcher> mTaskDispatcher;
+    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;
+
+  // The name of this TaskQueue. Useful when debugging dispatch failures.
+  const char* const mName;
+
+  SimpleTaskQueue mDirectTasks;
+
+  class Runner : public Runnable {
+   public:
+    explicit Runner(TaskQueue* aQueue)
+        : Runnable("TaskQueue::Runner"), mQueue(aQueue) {}
+    NS_IMETHOD Run() override;
+
+   private:
+    RefPtr<TaskQueue> mQueue;
+  };
+};
+
+}  // namespace mozilla
+
+#endif  // TaskQueue_h_