Separate the create and start phases in SchedulerSingleThreadTaskRunnerManager.

base/task_scheduler/scheduler_single_thread_task_runner_manager.h

Index: base/task_scheduler/scheduler_single_thread_task_runner_manager.h
diff --git a/base/task_scheduler/scheduler_single_thread_task_runner_manager.h b/base/task_scheduler/scheduler_single_thread_task_runner_manager.h
index 24d1a9a010b35661ee8235666652b16c5d0312a1..a054b5239f2b3540a3242600b990f67d8d747e53 100644
--- a/base/task_scheduler/scheduler_single_thread_task_runner_manager.h
+++ b/base/task_scheduler/scheduler_single_thread_task_runner_manager.h
@@ -37,6 +37,9 @@ class SchedulerWorkerDelegate;
 
 class BASE_EXPORT SchedulerSingleThreadTaskRunnerManager final {

[gab, 2017/04/10 18:04:56]

This class is missing a meta-comment, please work with robliao@ to add one +
mention its thread-safety properties (APIs below are thread-safe but this isn't
the default for classes in chrome)

[fdoray, 2017/04/10 19:05:00]

Done.

  public:
+  // Create a SchedulerSingleThreadTaskRunnerManager. Tasks posted to
+  // SingleThreadTaskRunners obtained from this
+  // SchedulerSingleThreadTaskRunnerManager won't run until Start() is called.
   SchedulerSingleThreadTaskRunnerManager(
       const std::vector<SchedulerWorkerPoolParams>& worker_pool_params_vector,
       const TaskScheduler::WorkerPoolIndexForTraitsCallback&
@@ -45,6 +48,10 @@ class BASE_EXPORT SchedulerSingleThreadTaskRunnerManager final {
       DelayedTaskManager* delayed_task_manager);
   ~SchedulerSingleThreadTaskRunnerManager();
 
+  // Creates threads for existing SingleThreadTaskRunners and allows threads to
+  // be created when new SingleThreadTaskRunners are created in the future.
+  void Start();
+
   scoped_refptr<SingleThreadTaskRunner> CreateSingleThreadTaskRunnerWithTraits(
       const TaskTraits& traits);
 
@@ -79,11 +86,14 @@ class BASE_EXPORT SchedulerSingleThreadTaskRunnerManager final {
   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