gecko-dev/xpcom/threads/ThrottledEventQueue.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 "ThrottledEventQueue.h"

#include "mozilla/Atomics.h"
#include "mozilla/ClearOnShutdown.h"
#include "mozilla/EventQueue.h"
#include "mozilla/Mutex.h"
#include "mozilla/Unused.h"
#include "nsThreadUtils.h"

namespace mozilla {

namespace {}  // anonymous namespace

// The ThrottledEventQueue is designed with inner and outer objects:
//
//       XPCOM code     base event target
//            |               |
//            v               v
//        +-------+       +--------+
//        | Outer |   +-->|executor|
//        +-------+   |   +--------+
//            |       |       |
//            |   +-------+   |
//            +-->| Inner |<--+
//                +-------+
//
// Client code references the outer nsIEventTarget which in turn references
// an inner object, which actually holds the queue of runnables.
//
// Whenever the queue is non-empty (and not paused), it keeps an "executor"
// runnable dispatched to the base event target. Each time the executor is run,
// it draws the next event from Inner's queue and runs it. If that queue has
// more events, the executor is dispatched to the base again.
//
// The executor holds a strong reference to the Inner object. This means that if
// the outer object is dereferenced and destroyed, the Inner object will remain
// live for as long as the executor exists - that is, until the Inner's queue is
// empty.
//
// A Paused ThrottledEventQueue does not enqueue an executor when new events are
// added. Any executor previously queued on the base event target draws no
// events from a Paused ThrottledEventQueue, and returns without re-enqueueing
// itself. Since there is no executor keeping the Inner object alive until its
// queue is empty, dropping a Paused ThrottledEventQueue may drop the Inner
// while it still owns events. This is the correct behavior: if there are no
// references to it, it will never be Resumed, and thus it will never dispatch
// events again.
//
// Resuming a ThrottledEventQueue must dispatch an executor, so calls to Resume
// are fallible for the same reasons as calls to Dispatch.
//
// The xpcom shutdown process drains the main thread's event queue several
// times, so if a ThrottledEventQueue is being driven by the main thread, it
// should get emptied out by the time we reach the "eventq shutdown" phase.
class ThrottledEventQueue::Inner final : public nsISupports {
  // 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, public nsIRunnablePriority {
    // The Inner whose runnables we execute. mInner->mExecutor points
    // to this executor, forming a reference loop.
    RefPtr<Inner> mInner;

    ~Executor() = default;

   public:
    explicit Executor(Inner* aInner)
        : Runnable("ThrottledEventQueue::Inner::Executor"), mInner(aInner) {}

    NS_DECL_ISUPPORTS_INHERITED

    NS_IMETHODIMP
    Run() override {
      mInner->ExecuteRunnable();
      return NS_OK;
    }

    NS_IMETHODIMP
    GetPriority(uint32_t* aPriority) override {
      *aPriority = mInner->mPriority;
      return NS_OK;
    }

#ifdef MOZ_COLLECTING_RUNNABLE_TELEMETRY
    NS_IMETHODIMP
    GetName(nsACString& aName) override { return mInner->CurrentName(aName); }
#endif
  };

  mutable Mutex mMutex;
  mutable CondVar mIdleCondVar;

  // As-of-yet unexecuted runnables queued on this ThrottledEventQueue.
  //
  // Used from any thread; protected by mMutex. Signals mIdleCondVar when
  // emptied.
  EventQueue mEventQueue;

  // The event target we dispatch our events (actually, just our Executor) to.
  //
  // Written only during construction. Readable by any thread without locking.
  nsCOMPtr<nsISerialEventTarget> mBaseTarget;

  // The Executor that we dispatch to mBaseTarget to draw runnables from our
  // queue. mExecutor->mInner points to this Inner, forming a reference loop.
  //
  // Used from any thread; protected by mMutex.
  nsCOMPtr<nsIRunnable> mExecutor;

  const uint32_t mPriority;

  // True if this queue is currently paused.
  // Used from any thread; protected by mMutex.
  bool mIsPaused;

  explicit Inner(nsISerialEventTarget* aBaseTarget, uint32_t aPriority)
      : mMutex("ThrottledEventQueue"),
        mIdleCondVar(mMutex, "ThrottledEventQueue:Idle"),
        mBaseTarget(aBaseTarget),
        mPriority(aPriority),
        mIsPaused(false) {}

  ~Inner() {
#ifdef DEBUG
    MutexAutoLock lock(mMutex);

    // As long as an executor exists, it had better keep us alive, since it's
    // going to call ExecuteRunnable on us.
    MOZ_ASSERT(!mExecutor);

    // If we have any events in our queue, there should be an executor queued
    // for them, and that should have kept us alive. The exception is that, if
    // we're paused, we don't enqueue an executor.
    MOZ_ASSERT(mEventQueue.IsEmpty(lock) || IsPaused(lock));

    // Some runnables are only safe to drop on the main thread, so if our queue
    // isn't empty, we'd better be on the main thread.
    MOZ_ASSERT_IF(!mEventQueue.IsEmpty(lock), NS_IsMainThread());
#endif
  }

  // Make sure an executor has been queued on our base target. If we already
  // have one, do nothing; otherwise, create and dispatch it.
  nsresult EnsureExecutor(MutexAutoLock& lock) {
    if (mExecutor) return NS_OK;

    // 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;
    }

    return NS_OK;
  }

  nsresult CurrentName(nsACString& aName) {
    nsCOMPtr<nsIRunnable> event;

#ifdef DEBUG
    bool currentThread = false;
    mBaseTarget->IsOnCurrentThread(&currentThread);
    MOZ_ASSERT(currentThread);
#endif

    {
      MutexAutoLock lock(mMutex);

      // We only check the name of an executor runnable when we know there is
      // something in the queue, so this should never fail.
      event = mEventQueue.PeekEvent(lock);
      MOZ_ALWAYS_TRUE(event);
    }

    if (nsCOMPtr<nsINamed> named = do_QueryInterface(event)) {
      nsresult rv = named->GetName(aName);
      return rv;
    }

    aName.AssignLiteral("non-nsINamed ThrottledEventQueue runnable");
    return NS_OK;
  }

  void ExecuteRunnable() {
    // Any thread
    nsCOMPtr<nsIRunnable> event;

#ifdef DEBUG
    bool currentThread = false;
    mBaseTarget->IsOnCurrentThread(&currentThread);
    MOZ_ASSERT(currentThread);
#endif

    {
      MutexAutoLock lock(mMutex);

      // Normally, a paused queue doesn't dispatch any executor, but we might
      // have been paused after the executor was already in flight. There's no
      // way to yank the executor out of the base event target, so we just check
      // for a paused queue here and return without running anything. We'll
      // create a new executor when we're resumed.
      if (IsPaused(lock)) {
        // Note, this breaks a ref cycle.
        mExecutor = nullptr;
        return;
      }

      // We only dispatch an executor runnable when we know there is something
      // in the queue, so this should never fail.
      event = mEventQueue.GetEvent(nullptr, lock);
      MOZ_ASSERT(event);

      // 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.HasReadyEvent(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.
      else {
        // Break the Executor::mInner / Inner::mExecutor reference loop.
        mExecutor = nullptr;
        mIdleCondVar.NotifyAll();
      }
    }

    // Execute the event now that we have unlocked.
    Unused << event->Run();
  }

 public:
  static already_AddRefed<Inner> Create(nsISerialEventTarget* aBaseTarget,
                                        uint32_t aPriority) {
    MOZ_ASSERT(NS_IsMainThread());
    MOZ_ASSERT(ClearOnShutdown_Internal::sCurrentShutdownPhase ==
               ShutdownPhase::NotInShutdown);

    RefPtr<Inner> ref = new Inner(aBaseTarget, aPriority);
    return ref.forget();
  }

  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 || IsPaused(lock)) {
      mIdleCondVar.Wait();
    }
  }

  bool IsPaused() const {
    MutexAutoLock lock(mMutex);
    return IsPaused(lock);
  }

  bool IsPaused(const MutexAutoLock& aProofOfLock) const { return mIsPaused; }

  nsresult SetIsPaused(bool aIsPaused) {
    MutexAutoLock lock(mMutex);

    // If we will be unpaused, and we have events in our queue, make sure we
    // have an executor queued on the base event target to run them. Do this
    // before we actually change mIsPaused, since this is fallible.
    if (!aIsPaused && !mEventQueue.IsEmpty(lock)) {
      nsresult rv = EnsureExecutor(lock);
      if (NS_FAILED(rv)) {
        return rv;
      }
    }

    mIsPaused = aIsPaused;
    return NS_OK;
  }

  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 (!IsPaused(lock)) {
      // Make sure we have an executor in flight to process events. This is
      // fallible, so do it first. Our lock will prevent the executor from
      // accessing the event queue before we add the event below.
      nsresult rv = EnsureExecutor(lock);
      if (NS_FAILED(rv)) return rv;
    }

    // Only add the event to the underlying queue if are able to
    // dispatch to our base target.
    mEventQueue.PutEvent(std::move(aEvent), EventQueuePriority::Normal, 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;
  }

  bool IsOnCurrentThread() { return mBaseTarget->IsOnCurrentThread(); }

  NS_DECL_THREADSAFE_ISUPPORTS
};

NS_IMPL_ISUPPORTS(ThrottledEventQueue::Inner, nsISupports);

NS_IMPL_ISUPPORTS_INHERITED(ThrottledEventQueue::Inner::Executor, Runnable,
                            nsIRunnablePriority)

NS_IMPL_ISUPPORTS(ThrottledEventQueue, ThrottledEventQueue, nsIEventTarget,
                  nsISerialEventTarget);

ThrottledEventQueue::ThrottledEventQueue(already_AddRefed<Inner> aInner)
    : mInner(aInner) {
  MOZ_ASSERT(mInner);
}

already_AddRefed<ThrottledEventQueue> ThrottledEventQueue::Create(
    nsISerialEventTarget* aBaseTarget, uint32_t aPriority) {
  MOZ_ASSERT(NS_IsMainThread());
  MOZ_ASSERT(aBaseTarget);

  RefPtr<Inner> inner = Inner::Create(aBaseTarget, aPriority);

  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(); }

nsresult ThrottledEventQueue::SetIsPaused(bool aIsPaused) {
  return mInner->SetIsPaused(aIsPaused);
}

bool ThrottledEventQueue::IsPaused() const { return mInner->IsPaused(); }

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(std::move(aEvent), aFlags);
}

NS_IMETHODIMP
ThrottledEventQueue::DelayedDispatch(already_AddRefed<nsIRunnable> aEvent,
                                     uint32_t aFlags) {
  return mInner->DelayedDispatch(std::move(aEvent), aFlags);
}

NS_IMETHODIMP
ThrottledEventQueue::IsOnCurrentThread(bool* aResult) {
  *aResult = mInner->IsOnCurrentThread();
  return NS_OK;
}

NS_IMETHODIMP_(bool)
ThrottledEventQueue::IsOnCurrentThreadInfallible() {
  return mInner->IsOnCurrentThread();
}

}  // namespace mozilla