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/sequenced_task_runner.h"
+#include "base/single_thread_task_runner.h"
+#include "base/task_runner.h"
+#include "base/threading/sequenced_worker_pool.h"
+#include "base/threading/thread_local.h"
+
+namespace base {
+
+namespace {
+
+// Thread-local TaskScopes, at most one of these should be set at a time on any
+// given thread.
+LazyInstance<ThreadLocalPointer<TaskRunnerHandle::TaskScope>>::Leaky
+    task_scope_tls = LAZY_INSTANCE_INITIALIZER;
+LazyInstance<ThreadLocalPointer<TaskRunnerHandle::SequencedTaskScope>>::Leaky
+    sequenced_task_scope_tls = LAZY_INSTANCE_INITIALIZER;
+LazyInstance<ThreadLocalPointer<TaskRunnerHandle::SingleThreadTaskScope>>::Leaky
+    single_thread_task_scope_tls = LAZY_INSTANCE_INITIALIZER;
+
+}  // namespace
+
+scoped_refptr<TaskRunner> TaskRunnerHandle::Get() {
+  DCHECK(HasTaskScope());
+
+  if (single_thread_task_scope_tls.Pointer()->Get()) {

[fdoray, 2016/06/08 18:10:44]

if (HasSingleThreadTaskScope())
  return GetSingleThreaded()
...

[gab, 2016/06/08 20:29:19]

That feels inconsistent (because it wouldn't be optimal to use "if
(HasSequencedTaskScope())" in the next if).

+    return single_thread_task_scope_tls.Pointer()
+        ->Get()
+        ->single_thread_task_runner_;
+  } else if (sequenced_task_scope_tls.Pointer()->Get()) {
+    return sequenced_task_scope_tls.Pointer()->Get()->sequenced_task_runner_;
+  }
+  return task_scope_tls.Pointer()->Get()->task_runner_;
+}
+
+scoped_refptr<SequencedTaskRunner> TaskRunnerHandle::GetSequenced() {
+  DCHECK(HasSequencedTaskScope());
+
+  if (single_thread_task_scope_tls.Pointer()->Get()) {
+    return single_thread_task_scope_tls.Pointer()
+        ->Get()
+        ->single_thread_task_runner_;
+  } else if (sequenced_task_scope_tls.Pointer()->Get()) {
+    return sequenced_task_scope_tls.Pointer()->Get()->sequenced_task_runner_;
+  }
+
+  // As a last resort, attempt to return the SequencedWorkerPool's
+  // SequencedTaskRunner, if that's the current context.
+  // TODO(gab): Remove this when SequencedWorkerPool is deprecated in favor of
+  // TaskScheduler.
+  scoped_refptr<base::SequencedTaskRunner> task_runner =
+      SequencedWorkerPool::GetSequencedTaskRunnerForCurrentThread();
+  return task_runner;
+}
+
+scoped_refptr<SingleThreadTaskRunner> TaskRunnerHandle::GetSingleThreaded() {
+  DCHECK(HasSingleThreadTaskScope());
+  return single_thread_task_scope_tls.Pointer()
+      ->Get()
+      ->single_thread_task_runner_;
+}
+
+bool TaskRunnerHandle::HasTaskScope() {
+  return task_scope_tls.Pointer()->Get() || HasSequencedTaskScope();
+}
+
+bool TaskRunnerHandle::HasSequencedTaskScope() {
+  return sequenced_task_scope_tls.Pointer()->Get() ||
+         SequencedWorkerPool::GetWorkerPoolForCurrentThread() ||
+         HasSingleThreadTaskScope();
+}
+
+bool TaskRunnerHandle::HasSingleThreadTaskScope() {
+  return !!single_thread_task_scope_tls.Pointer()->Get();
+}
+
+TaskRunnerHandle::TaskScope::TaskScope(scoped_refptr<TaskRunner> task_runner)
+    : task_runner_(std::move(task_runner)) {
+  DCHECK(task_runner_->RunsTasksOnCurrentThread());
+  DCHECK(!HasTaskScope());
+  task_scope_tls.Pointer()->Set(this);
+}
+
+TaskRunnerHandle::TaskScope::~TaskScope() {
+  DCHECK(HasTaskScope());
+  DCHECK(Get()->RunsTasksOnCurrentThread());
+  task_scope_tls.Pointer()->Set(nullptr);
+  DCHECK(!HasTaskScope());
+}
+
+TaskRunnerHandle::SequencedTaskScope::SequencedTaskScope(
+    scoped_refptr<SequencedTaskRunner> sequenced_task_runner)
+    : sequenced_task_runner_(std::move(sequenced_task_runner)) {
+  DCHECK(sequenced_task_runner_->RunsTasksOnCurrentThread());
+  DCHECK(!HasTaskScope());
+  sequenced_task_scope_tls.Pointer()->Set(this);
+}
+
+TaskRunnerHandle::SequencedTaskScope::~SequencedTaskScope() {
+  DCHECK(HasSequencedTaskScope());
+  DCHECK(GetSequenced()->RunsTasksOnCurrentThread());
+  sequenced_task_scope_tls.Pointer()->Set(nullptr);
+  DCHECK(!HasTaskScope());
+}
+
+TaskRunnerHandle::SingleThreadTaskScope::SingleThreadTaskScope(
+    scoped_refptr<SingleThreadTaskRunner> single_thread_task_runner)
+    : single_thread_task_runner_(std::move(single_thread_task_runner)) {
+  DCHECK(single_thread_task_runner_->BelongsToCurrentThread());
+  DCHECK(!HasTaskScope());
+  single_thread_task_scope_tls.Pointer()->Set(this);
+}
+
+TaskRunnerHandle::SingleThreadTaskScope::~SingleThreadTaskScope() {
+  DCHECK(HasSingleThreadTaskScope());
+  DCHECK(GetSingleThreaded()->BelongsToCurrentThread());
+  single_thread_task_scope_tls.Pointer()->Set(nullptr);
+  DCHECK(!HasTaskScope());
+}
+
+}  // namespace base