gecko-dev/xpcom/threads/TaskQueue.cpp

/* -*- 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 "nsISerialEventTarget.h"
#include "nsThreadUtils.h"

namespace mozilla {

class TaskQueue::EventTargetWrapper final : public nsISerialEventTarget {
  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,
                                      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_IMETHOD_(bool)
  IsOnCurrentThreadInfallible() override {
    return mTaskQueue->mTarget->IsOnCurrentThread();
  }

  NS_DECL_THREADSAFE_ISUPPORTS
};

NS_IMPL_ISUPPORTS(TaskQueue::EventTargetWrapper, nsIEventTarget,
                  nsISerialEventTarget)

TaskQueue::TaskQueue(already_AddRefed<nsIEventTarget> aTarget,
                     const char* aName, bool aRequireTailDispatch)
    : AbstractThread(aRequireTailDispatch),
      mTarget(aTarget),
      mQueueMonitor("TaskQueue::Queue"),
      mTailDispatcher(nullptr),
      mIsRunning(false),
      mIsShutdown(false),
      mName(aName) {}

TaskQueue::TaskQueue(already_AddRefed<nsIEventTarget> aTarget,
                     bool aSupportsTailDispatch)
    : TaskQueue(std::move(aTarget), "Unnamed", aSupportsTailDispatch) {}

TaskQueue::~TaskQueue() {
  // No one is referencing this TaskQueue anymore, meaning no tasks can be
  // pending as all Runner hold a reference to this 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,
                                   DispatchReason aReason) {
  mQueueMonitor.AssertCurrentThreadOwns();
  if (mIsShutdown) {
    return NS_ERROR_FAILURE;
  }

  AbstractThread* currentThread;
  if (aReason != TailDispatch && (currentThread = GetCurrent()) &&
      RequiresTailDispatch(currentThread)) {
    return currentThread->TailDispatcher().AddTask(this, aRunnable.forget());
  }

  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();
}

bool TaskQueue::IsCurrentThreadIn() const {
  bool in = mRunningThread == GetCurrentPhysicalThread();
  return in;
}

already_AddRefed<nsISerialEventTarget> TaskQueue::WrapAsEventTarget() {
  nsCOMPtr<nsISerialEventTarget> 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.empty()) {
      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.empty()) {
      // 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