Source code

Revision control

Other Tools

/* -*- 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/. */
#if !defined(TaskDispatcher_h_)
# define TaskDispatcher_h_
# include <queue>
# include "mozilla/AbstractThread.h"
# include "mozilla/Maybe.h"
# include "mozilla/UniquePtr.h"
# include "nsIDirectTaskDispatcher.h"
# include "nsISupportsImpl.h"
# include "nsTArray.h"
# include "nsThreadUtils.h"
namespace mozilla {
class SimpleTaskQueue {
public:
SimpleTaskQueue() = default;
virtual ~SimpleTaskQueue() = default;
void AddTask(already_AddRefed<nsIRunnable> aRunnable) {
if (!mTasks) {
mTasks.emplace();
}
mTasks->push(std::move(aRunnable));
}
void DrainTasks() {
if (!mTasks) {
return;
}
auto& queue = mTasks.ref();
while (!queue.empty()) {
nsCOMPtr<nsIRunnable> r = std::move(queue.front());
queue.pop();
r->Run();
}
}
bool HaveTasks() const { return mTasks && !mTasks->empty(); }
private:
// We use a Maybe<> because (a) when used for DirectTasks it often doesn't get
// anything put into it, and (b) the std::queue implementation in GNU
// libstdc++ does two largish heap allocations when creating a new std::queue.
Maybe<std::queue<nsCOMPtr<nsIRunnable>>> mTasks;
};
/*
* A classic approach to cross-thread communication is to dispatch asynchronous
* runnables to perform updates on other threads. This generally works well, but
* there are sometimes reasons why we might want to delay the actual dispatch of
* these tasks until a specified moment. At present, this is primarily useful to
* ensure that mirrored state gets updated atomically - but there may be other
* applications as well.
*
* TaskDispatcher is a general abstract class that accepts tasks and dispatches
* them at some later point. These groups of tasks are per-target-thread, and
* contain separate queues for several kinds of tasks (see comments below). -
* "state change tasks" (which run first, and are intended to be used to update
* the value held by mirrors), and regular tasks, which are other arbitrary
* operations that the are gated to run after all the state changes have
* completed.
*/
class TaskDispatcher {
public:
TaskDispatcher() = default;
virtual ~TaskDispatcher() = default;
// Direct tasks are run directly (rather than dispatched asynchronously) when
// the tail dispatcher fires. A direct task may cause other tasks to be added
// to the tail dispatcher.
virtual void AddDirectTask(already_AddRefed<nsIRunnable> aRunnable) = 0;
// State change tasks are dispatched asynchronously always run before regular
// tasks. They are intended to be used to update the value held by mirrors
// before any other dispatched tasks are run on the target thread.
virtual void AddStateChangeTask(AbstractThread* aThread,
already_AddRefed<nsIRunnable> aRunnable) = 0;
// Regular tasks are dispatched asynchronously, and run after state change
// tasks.
virtual nsresult AddTask(AbstractThread* aThread,
already_AddRefed<nsIRunnable> aRunnable) = 0;
virtual nsresult DispatchTasksFor(AbstractThread* aThread) = 0;
virtual bool HasTasksFor(AbstractThread* aThread) = 0;
virtual void DrainDirectTasks() = 0;
};
/*
* AutoTaskDispatcher is a stack-scoped TaskDispatcher implementation that fires
* its queued tasks when it is popped off the stack.
*/
class AutoTaskDispatcher : public TaskDispatcher {
public:
explicit AutoTaskDispatcher(nsIDirectTaskDispatcher* aDirectTaskDispatcher,
bool aIsTailDispatcher = false)
: mDirectTaskDispatcher(aDirectTaskDispatcher),
mIsTailDispatcher(aIsTailDispatcher) {}
~AutoTaskDispatcher() {
// Given that direct tasks may trigger other code that uses the tail
// dispatcher, it's better to avoid processing them in the tail dispatcher's
// destructor. So we require TailDispatchers to manually invoke
// DrainDirectTasks before the AutoTaskDispatcher gets destroyed. In truth,
// this is only necessary in the case where this AutoTaskDispatcher can be
// accessed by the direct tasks it dispatches (true for TailDispatchers, but
// potentially not true for other hypothetical AutoTaskDispatchers). Feel
// free to loosen this restriction to apply only to mIsTailDispatcher if a
// use-case requires it.
MOZ_ASSERT(!HaveDirectTasks());
for (size_t i = 0; i < mTaskGroups.Length(); ++i) {
DispatchTaskGroup(std::move(mTaskGroups[i]));
}
}
bool HaveDirectTasks() {
return mDirectTaskDispatcher && mDirectTaskDispatcher->HaveDirectTasks();
}
void DrainDirectTasks() override {
if (mDirectTaskDispatcher) {
mDirectTaskDispatcher->DrainDirectTasks();
}
}
void AddDirectTask(already_AddRefed<nsIRunnable> aRunnable) override {
MOZ_ASSERT(mDirectTaskDispatcher);
mDirectTaskDispatcher->DispatchDirectTask(std::move(aRunnable));
}
void AddStateChangeTask(AbstractThread* aThread,
already_AddRefed<nsIRunnable> aRunnable) override {
nsCOMPtr<nsIRunnable> r = aRunnable;
MOZ_RELEASE_ASSERT(r);
EnsureTaskGroup(aThread).mStateChangeTasks.AppendElement(r.forget());
}
nsresult AddTask(AbstractThread* aThread,
already_AddRefed<nsIRunnable> aRunnable) override {
nsCOMPtr<nsIRunnable> r = aRunnable;
MOZ_RELEASE_ASSERT(r);
// To preserve the event order, we need to append a new group if the last
// group is not targeted for |aThread|.
// for the details of the issue.
if (mTaskGroups.Length() == 0 ||
mTaskGroups.LastElement()->mThread != aThread) {
mTaskGroups.AppendElement(new PerThreadTaskGroup(aThread));
}
PerThreadTaskGroup& group = *mTaskGroups.LastElement();
group.mRegularTasks.AppendElement(r.forget());
return NS_OK;
}
bool HasTasksFor(AbstractThread* aThread) override {
return !!GetTaskGroup(aThread) ||
(aThread == AbstractThread::GetCurrent() && HaveDirectTasks());
}
nsresult DispatchTasksFor(AbstractThread* aThread) override {
nsresult rv = NS_OK;
// Dispatch all groups that match |aThread|.
for (size_t i = 0; i < mTaskGroups.Length(); ++i) {
if (mTaskGroups[i]->mThread == aThread) {
nsresult rv2 = DispatchTaskGroup(std::move(mTaskGroups[i]));
if (NS_WARN_IF(NS_FAILED(rv2)) && NS_SUCCEEDED(rv)) {
// We should try our best to call DispatchTaskGroup() as much as
// possible and return an error if any of DispatchTaskGroup() calls
// failed.
rv = rv2;
}
mTaskGroups.RemoveElementAt(i--);
}
}
return rv;
}
private:
struct PerThreadTaskGroup {
public:
explicit PerThreadTaskGroup(AbstractThread* aThread) : mThread(aThread) {
MOZ_COUNT_CTOR(PerThreadTaskGroup);
}
MOZ_COUNTED_DTOR(PerThreadTaskGroup)
RefPtr<AbstractThread> mThread;
nsTArray<nsCOMPtr<nsIRunnable>> mStateChangeTasks;
nsTArray<nsCOMPtr<nsIRunnable>> mRegularTasks;
};
class TaskGroupRunnable : public Runnable {
public:
explicit TaskGroupRunnable(UniquePtr<PerThreadTaskGroup>&& aTasks)
: Runnable("AutoTaskDispatcher::TaskGroupRunnable"),
mTasks(std::move(aTasks)) {}
NS_IMETHOD Run() override {
// State change tasks get run all together before any code is run, so
// that all state changes are made in an atomic unit.
for (size_t i = 0; i < mTasks->mStateChangeTasks.Length(); ++i) {
mTasks->mStateChangeTasks[i]->Run();
}
// Once the state changes have completed, drain any direct tasks
// generated by those state changes (i.e. watcher notification tasks).
// This needs to be outside the loop because we don't want to run code
// that might observe intermediate states.
MaybeDrainDirectTasks();
for (size_t i = 0; i < mTasks->mRegularTasks.Length(); ++i) {
mTasks->mRegularTasks[i]->Run();
// Scope direct tasks tightly to the task that generated them.
MaybeDrainDirectTasks();
}
return NS_OK;
}
private:
void MaybeDrainDirectTasks() {
AbstractThread* currentThread = AbstractThread::GetCurrent();
if (currentThread && currentThread->MightHaveTailTasks()) {
currentThread->TailDispatcher().DrainDirectTasks();
}
}
UniquePtr<PerThreadTaskGroup> mTasks;
};
PerThreadTaskGroup& EnsureTaskGroup(AbstractThread* aThread) {
PerThreadTaskGroup* existing = GetTaskGroup(aThread);
if (existing) {
return *existing;
}
mTaskGroups.AppendElement(new PerThreadTaskGroup(aThread));
return *mTaskGroups.LastElement();
}
PerThreadTaskGroup* GetTaskGroup(AbstractThread* aThread) {
for (size_t i = 0; i < mTaskGroups.Length(); ++i) {
if (mTaskGroups[i]->mThread == aThread) {
return mTaskGroups[i].get();
}
}
// Not found.
return nullptr;
}
nsresult DispatchTaskGroup(UniquePtr<PerThreadTaskGroup> aGroup) {
RefPtr<AbstractThread> thread = aGroup->mThread;
AbstractThread::DispatchReason reason =
mIsTailDispatcher ? AbstractThread::TailDispatch
: AbstractThread::NormalDispatch;
nsCOMPtr<nsIRunnable> r = new TaskGroupRunnable(std::move(aGroup));
return thread->Dispatch(r.forget(), reason);
}
// Task groups, organized by thread.
nsTArray<UniquePtr<PerThreadTaskGroup>> mTaskGroups;
nsCOMPtr<nsIDirectTaskDispatcher> mDirectTaskDispatcher;
// True if this TaskDispatcher represents the tail dispatcher for the thread
// upon which it runs.
const bool mIsTailDispatcher;
};
// Little utility class to allow declaring AutoTaskDispatcher as a default
// parameter for methods that take a TaskDispatcher&.
template <typename T>
class PassByRef {
public:
PassByRef() = default;
operator T&() { return mVal; }
private:
T mVal;
};
} // namespace mozilla
#endif