Separate the create and start phases in SchedulerSingleThreadTaskRunnerManager.

base/task_scheduler/scheduler_single_thread_task_runner_manager.h

 // Copyright 2017 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.
 
 #ifndef BASE_TASK_SCHEDULER_SCHEDULER_SINGLE_THREAD_TASK_RUNNER_MANAGER_H_
 #define BASE_TASK_SCHEDULER_SCHEDULER_SINGLE_THREAD_TASK_RUNNER_MANAGER_H_
 
 #include <memory>
 #include <vector>
 
@@ skipping 17 matching lines @@
 class DelayedTaskManager;
 class SchedulerWorker;
 class TaskTracker;
 
 namespace {
 
 class SchedulerWorkerDelegate;
 
 }  // namespace
 
+// Manages a pool of threads which are each associated with one
+// SingleThreadTaskRunner.
+//
+// No threads are created (and hence no tasks can run) before Start() is called.
+//
+// This class is thread-safe.
 class BASE_EXPORT SchedulerSingleThreadTaskRunnerManager final {
  public:
   SchedulerSingleThreadTaskRunnerManager(
       const std::vector<SchedulerWorkerPoolParams>& worker_pool_params_vector,
       const TaskScheduler::WorkerPoolIndexForTraitsCallback&
           worker_pool_index_for_traits_callback,
       TaskTracker* task_tracker,
       DelayedTaskManager* delayed_task_manager);
   ~SchedulerSingleThreadTaskRunnerManager();
 
+  // Starts threads for existing SingleThreadTaskRunners and allows threads to
+  // be started when SingleThreadTaskRunners are created in the future.
+  void Start();
+
   scoped_refptr<SingleThreadTaskRunner> CreateSingleThreadTaskRunnerWithTraits(
       const TaskTraits& traits);
 
 #if defined(OS_WIN)
   scoped_refptr<SingleThreadTaskRunner> CreateCOMSTATaskRunnerWithTraits(
       const TaskTraits& traits);
 #endif  // defined(OS_WIN)
 
   void JoinForTesting();
 
@@ skipping 14 matching lines @@
       const SchedulerWorkerPoolParams& params);
 
   void UnregisterSchedulerWorker(SchedulerWorker* worker);
 
   const std::vector<SchedulerWorkerPoolParams> worker_pool_params_vector_;
   const TaskScheduler::WorkerPoolIndexForTraitsCallback
       worker_pool_index_for_traits_callback_;
   TaskTracker* const task_tracker_;
   DelayedTaskManager* const delayed_task_manager_;
 
-  // Synchronizes access to |workers_| and |worker_id_|.
-  SchedulerLock workers_lock_;
+  // Synchronizes access to |workers_|, |next_worker_id_| and |started_|.
+  SchedulerLock lock_;
   std::vector<scoped_refptr<SchedulerWorker>> workers_;
   int next_worker_id_ = 0;
 
+  // Set to true when Start() is called.
+  bool started_ = false;
+
 #if DCHECK_IS_ON()
   subtle::Atomic32 workers_unregistered_during_join_ = 0;
 #endif
 
   DISALLOW_COPY_AND_ASSIGN(SchedulerSingleThreadTaskRunnerManager);
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_SCHEDULER_SINGLE_THREAD_TASK_RUNNER_MANAGER_H_