Introduce TaskRunnerHandle.

base/threading/task_runner_handle.cc

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "base/threading/task_runner_handle.h"
+
+#include <utility>
+
+#include "base/lazy_instance.h"
+#include "base/logging.h"
+#include "base/threading/sequenced_worker_pool.h"
+#include "base/threading/thread_local.h"
+
+namespace base {
+
+namespace {
+
+// TaskScopeState for the current thread stored in TLS.
+LazyInstance<ThreadLocalPointer<internal::TaskScopeState>>::Leaky
+    g_task_scope_state = LAZY_INSTANCE_INITIALIZER;
+
+}  // namespace
+
+scoped_refptr<TaskRunner> TaskRunnerHandle::Get() {
+  DCHECK(HasTaskScope());
+
+  const internal::TaskScopeState* state = g_task_scope_state.Get().Get();
+  if (state)
+    return state->task_runners.task_runner;
+
+  // Explicitly support the SequencedWorkerPool here while it still exists
+  // (supporting TaskRunnerHandles from it would be non-trivial and it will soon
+  // be deprecated in favor of TaskScheduler).
+  return SequencedWorkerPool::GetWorkerPoolForCurrentThread();
+}
+
+scoped_refptr<SequencedTaskRunner> TaskRunnerHandle::GetSequenced() {
+  DCHECK(HasSequencedTaskScope());
+
+  const internal::TaskScopeState* state = g_task_scope_state.Get().Get();
+  if (state)
+    return state->task_runners.sequenced_task_runner;
+
+  // Assuming there is a sequenced scope and it isn't in our TLS, the current
+  // context is the SequencedWorkerPool: get its SequencedTaskRunner. Note: this
+  // special case will go away when SequencedWorkerPool is deprecated in favor
+  // of TaskScheduler.
+  scoped_refptr<SequencedTaskRunner> sequenced_worker_pool_runner =
+      SequencedWorkerPool::GetSequencedTaskRunnerForCurrentThread();
+  return sequenced_worker_pool_runner;
+}
+
+scoped_refptr<SingleThreadTaskRunner> TaskRunnerHandle::GetSingleThreaded() {
+  DCHECK(HasSingleThreadTaskScope());
+  return g_task_scope_state.Get().Get()->task_runners.single_thread_task_runner;
+}
+
+bool TaskRunnerHandle::HasTaskScope() {
+  return g_task_scope_state.Get().Get() ||
+         SequencedWorkerPool::GetWorkerPoolForCurrentThread();
+}
+
+bool TaskRunnerHandle::HasSequencedTaskScope() {
+  // TODO(gab): Replace GetWorkerPoolForCurrentThread() below with
+  // GetSequenceTokenForCurrentThread() once returning a SequencedTaskRunner
+  // from an unsequenced task is deprecated.
+  const internal::TaskScopeState* state = g_task_scope_state.Get().Get();
+  return state ? state->type >= internal::TaskScopeState::SEQUENCED
+               : !!SequencedWorkerPool::GetWorkerPoolForCurrentThread();
+}
+
+bool TaskRunnerHandle::HasSingleThreadTaskScope() {
+  const internal::TaskScopeState* state = g_task_scope_state.Get().Get();
+  return (state && state->type >= internal::TaskScopeState::SINGLE_THREADED);
+}
+
+TaskRunnerHandle::TaskScope::TaskScope(scoped_refptr<TaskRunner> task_runner)
+    : state_(std::move(task_runner)) {}
+
+TaskRunnerHandle::SequencedTaskScope::SequencedTaskScope(
+    scoped_refptr<SequencedTaskRunner> sequenced_task_runner)
+    : state_(std::move(sequenced_task_runner)) {}
+
+TaskRunnerHandle::SingleThreadTaskScope::SingleThreadTaskScope(
+    scoped_refptr<SingleThreadTaskRunner> single_thread_task_runner)
+    : state_(std::move(single_thread_task_runner)) {}
+
+namespace internal {
+
+TaskScopeState::TaskScopeState(scoped_refptr<TaskRunner> task_runner)
+    : type(PARALLEL), task_runners(std::move(task_runner)) {
+  DCHECK(!TaskRunnerHandle::HasTaskScope());
+  g_task_scope_state.Get().Set(this);
+}
+
+TaskScopeState::TaskScopeState(
+    scoped_refptr<SequencedTaskRunner> sequenced_task_runner)
+    : type(SEQUENCED), task_runners(std::move(sequenced_task_runner)) {
+  DCHECK(!TaskRunnerHandle::HasTaskScope());
+  g_task_scope_state.Get().Set(this);
+}
+
+TaskScopeState::TaskScopeState(
+    scoped_refptr<SingleThreadTaskRunner> single_thread_task_runner)
+    : type(SINGLE_THREADED),
+      task_runners(std::move(single_thread_task_runner)) {
+  DCHECK(!TaskRunnerHandle::HasTaskScope());
+  g_task_scope_state.Get().Set(this);
+}
+
+TaskScopeState::~TaskScopeState() {
+  DCHECK(TaskRunnerHandle::HasTaskScope());
+  DCHECK(TaskRunnerHandle::HasSequencedTaskScope() || type < SEQUENCED);
+  DCHECK(TaskRunnerHandle::HasSingleThreadTaskScope() ||
+         type < SINGLE_THREADED);
+
+  DCHECK(TaskRunnerHandle::Get()->RunsTasksOnCurrentThread());
+
+  g_task_scope_state.Get().Set(nullptr);
+  DCHECK(!TaskRunnerHandle::HasTaskScope());
+}
+
+TaskScopeState::TaskRunnerUnion::TaskRunnerUnion(
+    scoped_refptr<TaskRunner> task_runner_in)
+    : task_runner(std::move(task_runner_in)) {
+  DCHECK(task_runner->RunsTasksOnCurrentThread());
+}
+
+TaskScopeState::TaskRunnerUnion::TaskRunnerUnion(
+    scoped_refptr<SequencedTaskRunner> sequenced_task_runner_in)
+    : sequenced_task_runner(std::move(sequenced_task_runner_in)) {
+  DCHECK(sequenced_task_runner->RunsTasksOnCurrentThread());
+}
+
+TaskScopeState::TaskRunnerUnion::TaskRunnerUnion(
+    scoped_refptr<SingleThreadTaskRunner> single_thread_task_runner_in)
+    : single_thread_task_runner(std::move(single_thread_task_runner_in)) {
+  DCHECK(single_thread_task_runner->BelongsToCurrentThread());
+}
+
+TaskScopeState::TaskRunnerUnion::~TaskRunnerUnion() {
+  // Unions with complex members require explicit destruction. All types in the
+  // union derive from base::TaskRunner, as such it is ok to always use that
+  // type for deletion.
+  if (task_runner)
+    task_runner.get()->Release();

[fdoray, 2016/06/17 14:29:02]

I'm not strongly against the union, but with a cast, no deep knowledge of C++
would be required to understand how TaskScopeState works.

http://www.stroustrup.com/C++11FAQ.html#unions

[gab, 2016/06/20 19:55:36]

Right, done (based on what I read in this I think the current impl freed the
TaskRunner but not the scoped_refptr itself -- which was probably okay if it
only used stack memory but skipping its destructor is not very kosher).

+}
+
+}  // namespace internal
+
+}  // namespace base