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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/. */
+
+#if !defined(TaskDispatcher_h_)
+# define TaskDispatcher_h_
+
+# include <queue>
+
+# include "mozilla/AbstractThread.h"
+# include "mozilla/Maybe.h"
+# include "mozilla/ProfilerRunnable.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();
+ AUTO_PROFILE_FOLLOWING_RUNNABLE(r);
+ 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|.
+ // See https://bugzilla.mozilla.org/show_bug.cgi?id=1318226&mark=0-3#c0
+ // 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) {
+ AUTO_PROFILE_FOLLOWING_RUNNABLE(mTasks->mRegularTasks[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
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