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_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 scoped_refptr<SequencedTaskRunner>(
+        static_cast<SequencedTaskRunner*>(state->task_runner.get()));
+  }
+
+  // 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 scoped_refptr<SingleThreadTaskRunner>(
+      static_cast<SingleThreadTaskRunner*>(
+          g_task_scope_state.Get().Get()->task_runner.get()));
+}
+
+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_in)
+    : type(PARALLEL), task_runner(std::move(task_runner_in)) {
+  DCHECK(task_runner->RunsTasksOnCurrentThread());
+
+  DCHECK(!TaskRunnerHandle::HasTaskScope());
+  g_task_scope_state.Get().Set(this);
+  DCHECK(TaskRunnerHandle::HasTaskScope());
+}
+
+TaskScopeState::TaskScopeState(
+    scoped_refptr<SequencedTaskRunner> sequenced_task_runner_in)
+    : type(SEQUENCED), task_runner(std::move(sequenced_task_runner_in)) {
+  DCHECK(task_runner->RunsTasksOnCurrentThread());
+
+  DCHECK(!TaskRunnerHandle::HasTaskScope());
+  g_task_scope_state.Get().Set(this);
+  DCHECK(TaskRunnerHandle::HasSequencedTaskScope());
+}
+
+TaskScopeState::TaskScopeState(
+    scoped_refptr<SingleThreadTaskRunner> single_thread_task_runner_in)
+    : type(SINGLE_THREADED),
+      task_runner(std::move(single_thread_task_runner_in)) {
+  DCHECK(task_runner->RunsTasksOnCurrentThread());
+
+  DCHECK(!TaskRunnerHandle::HasTaskScope());
+  g_task_scope_state.Get().Set(this);
+  DCHECK(TaskRunnerHandle::HasSingleThreadTaskScope());
+}
+
+TaskScopeState::~TaskScopeState() {
+  DCHECK(task_runner->RunsTasksOnCurrentThread());
+
+  DCHECK_EQ(task_runner, TaskRunnerHandle::Get());
+
+  DCHECK(TaskRunnerHandle::HasTaskScope());
+  DCHECK(TaskRunnerHandle::HasSequencedTaskScope() || type < SEQUENCED);
+  DCHECK(TaskRunnerHandle::HasSingleThreadTaskScope() ||
+         type < SINGLE_THREADED);
+  g_task_scope_state.Get().Set(nullptr);
+  DCHECK(!TaskRunnerHandle::HasTaskScope());
+}
+
+}  // namespace internal
+
+}  // namespace base