gecko-dev/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 "mozilla/Monitor.h"
#include "mozilla/MozPromise.h"
#include "mozilla/RefPtr.h"
#include "mozilla/TaskDispatcher.h"
#include "mozilla/unused.h"
#include <queue>
#include "mozilla/SharedThreadPool.h"
#include "nsThreadUtils.h"
class nsIRunnable;
namespace mozilla {
class SharedThreadPool;
typedef MozPromise<bool, bool, false> ShutdownPromise;
// Abstracts executing runnables in order in a thread pool. 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.
class TaskQueue : public AbstractThread {
public:
explicit TaskQueue(already_AddRefed<SharedThreadPool> aPool, bool aSupportsTailDispatch = false);
TaskDispatcher& TailDispatcher() override;
TaskQueue* AsTaskQueue() override { return this; }
void Dispatch(already_AddRefed<nsIRunnable> aRunnable,
DispatchFailureHandling aFailureHandling = AssertDispatchSuccess,
DispatchReason aReason = NormalDispatch) override
{
nsCOMPtr<nsIRunnable> r = aRunnable;
{
MonitorAutoLock mon(mQueueMonitor);
nsresult rv = DispatchLocked(/* passed by ref */r, AbortIfFlushing, aFailureHandling, aReason);
MOZ_DIAGNOSTIC_ASSERT(aFailureHandling == DontAssertDispatchSuccess || NS_SUCCEEDED(rv));
Unused << rv;
}
// 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.
}
// 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 threadpool 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() override;
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();
enum DispatchMode { AbortIfFlushing, IgnoreFlushing };
nsresult DispatchLocked(nsCOMPtr<nsIRunnable>& aRunnable,
DispatchMode aMode,
DispatchFailureHandling aFailureHandling,
DispatchReason aReason = NormalDispatch);
void MaybeResolveShutdown()
{
mQueueMonitor.AssertCurrentThreadOwns();
if (mIsShutdown && !mIsRunning) {
mShutdownPromise.ResolveIfExists(true, __func__);
mPool = nullptr;
}
}
RefPtr<SharedThreadPool> mPool;
// Monitor that protects the queue and mIsRunning;
Monitor mQueueMonitor;
// Queue of tasks to run.
std::queue<nsCOMPtr<nsIRunnable>> 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<nsIThread*> mRunningThread;
// RAII class that gets instantiated for each dispatched task.
class AutoTaskGuard : public AutoTaskDispatcher
{
public:
explicit AutoTaskGuard(TaskQueue* aQueue)
: AutoTaskDispatcher(/* aIsTailDispatcher = */ true), mQueue(aQueue)
{
// NB: We don't hold the lock to aQueue here. Don't do anything that
// might require it.
MOZ_ASSERT(!mQueue->mTailDispatcher);
mQueue->mTailDispatcher = this;
MOZ_ASSERT(sCurrentThreadTLS.get() == nullptr);
sCurrentThreadTLS.set(aQueue);
MOZ_ASSERT(mQueue->mRunningThread == nullptr);
mQueue->mRunningThread = NS_GetCurrentThread();
}
~AutoTaskGuard()
{
DrainDirectTasks();
MOZ_ASSERT(mQueue->mRunningThread == NS_GetCurrentThread());
mQueue->mRunningThread = nullptr;
sCurrentThreadTLS.set(nullptr);
mQueue->mTailDispatcher = nullptr;
}
private:
TaskQueue* mQueue;
};
TaskDispatcher* mTailDispatcher;
// True if we've dispatched an event to the pool to execute events from
// the queue.
bool mIsRunning;
// True if we've started our shutdown process.
bool mIsShutdown;
MozPromiseHolder<ShutdownPromise> mShutdownPromise;
// True if we're flushing; we reject new tasks if we're flushing.
bool mIsFlushing;
class Runner : public Runnable {
public:
explicit Runner(TaskQueue* aQueue)
: mQueue(aQueue)
{
}
NS_METHOD Run() override;
private:
RefPtr<TaskQueue> mQueue;
};
};
} // namespace mozilla
#endif // TaskQueue_h_