mirror of
https://github.com/classilla/tenfourfox.git
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261 lines
8.7 KiB
C++
261 lines
8.7 KiB
C++
/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim:set ts=2 sw=2 sts=2 et cindent: */
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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#if !defined(TaskDispatcher_h_)
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#define TaskDispatcher_h_
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#include "mozilla/AbstractThread.h"
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#include "mozilla/UniquePtr.h"
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#include "mozilla/unused.h"
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#include "nsISupportsImpl.h"
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#include "nsTArray.h"
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#include "nsThreadUtils.h"
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#include <queue>
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namespace mozilla {
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/*
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* A classic approach to cross-thread communication is to dispatch asynchronous
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* runnables to perform updates on other threads. This generally works well, but
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* there are sometimes reasons why we might want to delay the actual dispatch of
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* these tasks until a specified moment. At present, this is primarily useful to
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* ensure that mirrored state gets updated atomically - but there may be other
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* applications as well.
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*
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* TaskDispatcher is a general abstract class that accepts tasks and dispatches
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* them at some later point. These groups of tasks are per-target-thread, and
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* contain separate queues for several kinds of tasks (see comments below). - "state change tasks" (which
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* run first, and are intended to be used to update the value held by mirrors),
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* and regular tasks, which are other arbitrary operations that the are gated
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* to run after all the state changes have completed.
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*/
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class TaskDispatcher
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{
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public:
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TaskDispatcher() {}
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virtual ~TaskDispatcher() {}
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// Direct tasks are run directly (rather than dispatched asynchronously) when
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// the tail dispatcher fires. A direct task may cause other tasks to be added
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// to the tail dispatcher.
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virtual void AddDirectTask(already_AddRefed<nsIRunnable> aRunnable) = 0;
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// State change tasks are dispatched asynchronously always run before regular
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// tasks. They are intended to be used to update the value held by mirrors
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// before any other dispatched tasks are run on the target thread.
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virtual void AddStateChangeTask(AbstractThread* aThread,
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already_AddRefed<nsIRunnable> aRunnable) = 0;
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// Regular tasks are dispatched asynchronously, and run after state change
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// tasks.
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virtual void AddTask(AbstractThread* aThread,
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already_AddRefed<nsIRunnable> aRunnable,
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AbstractThread::DispatchFailureHandling aFailureHandling = AbstractThread::AssertDispatchSuccess) = 0;
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virtual void DispatchTasksFor(AbstractThread* aThread) = 0;
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virtual bool HasTasksFor(AbstractThread* aThread) = 0;
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virtual void DrainDirectTasks() = 0;
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};
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/*
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* AutoTaskDispatcher is a stack-scoped TaskDispatcher implementation that fires
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* its queued tasks when it is popped off the stack.
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*/
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class AutoTaskDispatcher : public TaskDispatcher
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{
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public:
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explicit AutoTaskDispatcher(bool aIsTailDispatcher = false) : mIsTailDispatcher(aIsTailDispatcher) {}
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~AutoTaskDispatcher()
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{
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// Given that direct tasks may trigger other code that uses the tail
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// dispatcher, it's better to avoid processing them in the tail dispatcher's
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// destructor. So we require TailDispatchers to manually invoke
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// DrainDirectTasks before the AutoTaskDispatcher gets destroyed. In truth,
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// this is only necessary in the case where this AutoTaskDispatcher can be
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// accessed by the direct tasks it dispatches (true for TailDispatchers, but
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// potentially not true for other hypothetical AutoTaskDispatchers). Feel
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// free to loosen this restriction to apply only to mIsTailDispatcher if a
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// use-case requires it.
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MOZ_ASSERT(mDirectTasks.empty());
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for (size_t i = 0; i < mTaskGroups.Length(); ++i) {
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DispatchTaskGroup(Move(mTaskGroups[i]));
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}
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}
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void DrainDirectTasks() override
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{
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while (!mDirectTasks.empty()) {
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nsCOMPtr<nsIRunnable> r = mDirectTasks.front();
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mDirectTasks.pop();
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r->Run();
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}
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}
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void AddDirectTask(already_AddRefed<nsIRunnable> aRunnable) override
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{
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mDirectTasks.push(Move(aRunnable));
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}
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void AddStateChangeTask(AbstractThread* aThread,
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already_AddRefed<nsIRunnable> aRunnable) override
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{
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EnsureTaskGroup(aThread).mStateChangeTasks.AppendElement(aRunnable);
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}
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void AddTask(AbstractThread* aThread,
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already_AddRefed<nsIRunnable> aRunnable,
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AbstractThread::DispatchFailureHandling aFailureHandling) override
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{
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PerThreadTaskGroup& group = EnsureTaskGroup(aThread);
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group.mRegularTasks.AppendElement(aRunnable);
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// The task group needs to assert dispatch success if any of the runnables
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// it's dispatching want to assert it.
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if (aFailureHandling == AbstractThread::AssertDispatchSuccess) {
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group.mFailureHandling = AbstractThread::AssertDispatchSuccess;
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}
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}
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bool HasTasksFor(AbstractThread* aThread) override
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{
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return !!GetTaskGroup(aThread) || (aThread == AbstractThread::GetCurrent() && !mDirectTasks.empty());
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}
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void DispatchTasksFor(AbstractThread* aThread) override
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{
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for (size_t i = 0; i < mTaskGroups.Length(); ++i) {
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if (mTaskGroups[i]->mThread == aThread) {
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DispatchTaskGroup(Move(mTaskGroups[i]));
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mTaskGroups.RemoveElementAt(i);
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return;
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}
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}
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}
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private:
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struct PerThreadTaskGroup
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{
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public:
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explicit PerThreadTaskGroup(AbstractThread* aThread)
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: mThread(aThread), mFailureHandling(AbstractThread::DontAssertDispatchSuccess)
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{
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MOZ_COUNT_CTOR(PerThreadTaskGroup);
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}
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~PerThreadTaskGroup() { MOZ_COUNT_DTOR(PerThreadTaskGroup); }
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RefPtr<AbstractThread> mThread;
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nsTArray<nsCOMPtr<nsIRunnable>> mStateChangeTasks;
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nsTArray<nsCOMPtr<nsIRunnable>> mRegularTasks;
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AbstractThread::DispatchFailureHandling mFailureHandling;
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};
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class TaskGroupRunnable : public nsRunnable
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{
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public:
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explicit TaskGroupRunnable(UniquePtr<PerThreadTaskGroup>&& aTasks) : mTasks(Move(aTasks)) {}
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NS_IMETHODIMP Run()
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{
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// State change tasks get run all together before any code is run, so
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// that all state changes are made in an atomic unit.
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for (size_t i = 0; i < mTasks->mStateChangeTasks.Length(); ++i) {
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mTasks->mStateChangeTasks[i]->Run();
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}
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// Once the state changes have completed, drain any direct tasks
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// generated by those state changes (i.e. watcher notification tasks).
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// This needs to be outside the loop because we don't want to run code
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// that might observe intermediate states.
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MaybeDrainDirectTasks();
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for (size_t i = 0; i < mTasks->mRegularTasks.Length(); ++i) {
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mTasks->mRegularTasks[i]->Run();
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// Scope direct tasks tightly to the task that generated them.
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MaybeDrainDirectTasks();
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}
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return NS_OK;
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}
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private:
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void MaybeDrainDirectTasks()
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{
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AbstractThread* currentThread = AbstractThread::GetCurrent();
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if (currentThread) {
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currentThread->TailDispatcher().DrainDirectTasks();
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}
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}
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UniquePtr<PerThreadTaskGroup> mTasks;
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};
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PerThreadTaskGroup& EnsureTaskGroup(AbstractThread* aThread)
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{
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PerThreadTaskGroup* existing = GetTaskGroup(aThread);
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if (existing) {
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return *existing;
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}
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mTaskGroups.AppendElement(new PerThreadTaskGroup(aThread));
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return *mTaskGroups.LastElement();
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}
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PerThreadTaskGroup* GetTaskGroup(AbstractThread* aThread)
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{
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for (size_t i = 0; i < mTaskGroups.Length(); ++i) {
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if (mTaskGroups[i]->mThread == aThread) {
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return mTaskGroups[i].get();
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}
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}
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// Not found.
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return nullptr;
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}
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void DispatchTaskGroup(UniquePtr<PerThreadTaskGroup> aGroup)
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{
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RefPtr<AbstractThread> thread = aGroup->mThread;
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AbstractThread::DispatchFailureHandling failureHandling = aGroup->mFailureHandling;
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AbstractThread::DispatchReason reason = mIsTailDispatcher ? AbstractThread::TailDispatch
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: AbstractThread::NormalDispatch;
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nsCOMPtr<nsIRunnable> r = new TaskGroupRunnable(Move(aGroup));
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thread->Dispatch(r.forget(), failureHandling, reason);
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}
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// Direct tasks.
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std::queue<nsCOMPtr<nsIRunnable>> mDirectTasks;
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// Task groups, organized by thread.
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nsTArray<UniquePtr<PerThreadTaskGroup>> mTaskGroups;
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// True if this TaskDispatcher represents the tail dispatcher for the thread
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// upon which it runs.
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const bool mIsTailDispatcher;
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};
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// Little utility class to allow declaring AutoTaskDispatcher as a default
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// parameter for methods that take a TaskDispatcher&.
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template<typename T>
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class PassByRef
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{
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public:
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PassByRef() {}
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operator T&() { return mVal; }
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private:
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T mVal;
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};
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} // namespace mozilla
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#endif
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