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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 */
#ifndef TaskQueue_h_
#define TaskQueue_h_
#include "mozilla/AbstractThread.h"
#include "mozilla/Maybe.h"
#include "mozilla/Monitor.h"
#include "mozilla/MozPromise.h"
#include "mozilla/Queue.h"
#include "mozilla/RefPtr.h"
#include "mozilla/TaskDispatcher.h"
#include "mozilla/ThreadSafeWeakPtr.h"
#include "nsIDirectTaskDispatcher.h"
#include "nsThreadUtils.h"
namespace mozilla {
typedef MozPromise<bool, bool, false> ShutdownPromise;
class TaskQueueTrackerEntry;
// 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 final : public AbstractThread,
public nsIDirectTaskDispatcher,
public SupportsThreadSafeWeakPtr<TaskQueue> {
class EventTargetWrapper;
static RefPtr<TaskQueue> Create(already_AddRefed<nsIEventTarget> aTarget,
const char* aName,
bool aSupportsTailDispatch = false);
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,
// 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;
NS_IMETHOD RegisterShutdownTask(nsITargetShutdownTask* aTask) override;
NS_IMETHOD UnregisterShutdownTask(nsITargetShutdownTask* aTask) override;
using CancelPromise = MozPromise<bool, bool, false>;
// 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;
friend class SupportsThreadSafeWeakPtr<TaskQueue>;
TaskQueue(already_AddRefed<nsIEventTarget> aTarget, const char* aName,
bool aSupportsTailDispatch);
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();
nsCOMPtr<nsIEventTarget> mTarget MOZ_GUARDED_BY(mQueueMonitor);
// Handle for this TaskQueue being registered with our target if it implements
// TaskQueueTracker.
UniquePtr<TaskQueueTrackerEntry> mTrackerEntry MOZ_GUARDED_BY(mQueueMonitor);
// Monitor that protects the queue, mIsRunning, mIsShutdown and
// mShutdownTasks;
Monitor mQueueMonitor;
typedef struct TaskStruct {
nsCOMPtr<nsIRunnable> event;
uint32_t flags;
} TaskStruct;
// Queue of tasks to run.
Queue<TaskStruct> mTasks MOZ_GUARDED_BY(mQueueMonitor);
// List of tasks to run during shutdown.
nsTArray<nsCOMPtr<nsITargetShutdownTask>> mShutdownTasks
// 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 {
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.
/* aIsTailDispatcher = */ true);
mQueue->mTailDispatcher = mTaskDispatcher.ptr();
mLastCurrentThread = sCurrentThreadTLS.get();
MOZ_ASSERT(mQueue->mRunningThread == nullptr);
mQueue->mRunningThread = PR_GetCurrentThread();
~AutoTaskGuard() {
MOZ_ASSERT(mQueue->mRunningThread == PR_GetCurrentThread());
mQueue->mRunningThread = nullptr;
mQueue->mTailDispatcher = nullptr;
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 MOZ_GUARDED_BY(mQueueMonitor);
// True if we've started our shutdown process.
bool mIsShutdown MOZ_GUARDED_BY(mQueueMonitor);
MozPromiseHolder<ShutdownPromise> mShutdownPromise
// The name of this TaskQueue. Useful when debugging dispatch failures.
const char* const mName;
SimpleTaskQueue mDirectTasks;
class Runner : public Runnable {
explicit Runner(TaskQueue* aQueue)
: Runnable("TaskQueue::Runner"), mQueue(aQueue) {}
NS_IMETHOD Run() override;
RefPtr<TaskQueue> mQueue;
{ \
0x765c4b56, 0xd5f6, 0x4a9f, { \
0x91, 0xcf, 0x51, 0x47, 0xb3, 0xc1, 0x7e, 0xa6 \
} \
// XPCOM "interface" which may be implemented by nsIEventTarget implementations
// which want to keep track of what TaskQueue instances are currently targeting
// them. This may be used to asynchronously shutdown TaskQueues targeting a
// threadpool or other event target before the threadpool goes away.
// This explicitly TaskQueue-aware tracker is used instead of
// `nsITargetShutdownTask` as the operations required to shut down a TaskQueue
// are asynchronous, which is not a requirement of that interface.
class TaskQueueTracker : public nsISupports {
// Get a strong reference to every TaskQueue currently tracked by this
// TaskQueueTracker. May be called from any thraed.
nsTArray<RefPtr<TaskQueue>> GetAllTrackedTaskQueues();
virtual ~TaskQueueTracker();
friend class TaskQueueTrackerEntry;
Mutex mMutex{"TaskQueueTracker"};
LinkedList<TaskQueueTrackerEntry> mEntries MOZ_GUARDED_BY(mMutex);
} // namespace mozilla
#endif // TaskQueue_h_