Separate the create and start phases in TaskSchedulerImpl.

base/task_scheduler/task_scheduler.h

 // 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.
 
 #ifndef BASE_TASK_SCHEDULER_TASK_SCHEDULER_H_
 #define BASE_TASK_SCHEDULER_TASK_SCHEDULER_H_
 
 #include <memory>
 #include <vector>
 
@@ skipping 15 matching lines @@
 
 namespace tracked_objects {
 class Location;
 }
 
 namespace base {
 
 class HistogramBase;
 
 // Interface for a task scheduler and static methods to manage the instance used
-// by the post_task.h API. Note: all base/task_scheduler users should go through
-// post_task.h instead of TaskScheduler except for the one callsite per process
-// which manages the process' instance.
+// by the post_task.h API.
+//
+// The task scheduler doesn't create threads until Start() is called. Tasks can
+// be posted at any time but will not run until after Start() is called.
+//
+// The instance methods of this class are thread-safe.
+//
+// Note: All base/task_scheduler users should go through post_task.h instead of
+// TaskScheduler except for the one callsite per process which manages the
+// process's instance.
 class BASE_EXPORT TaskScheduler {
  public:
   struct BASE_EXPORT InitParams {
     InitParams(
         const SchedulerWorkerPoolParams& background_worker_pool_params_in,
         const SchedulerWorkerPoolParams&
             background_blocking_worker_pool_params_in,
         const SchedulerWorkerPoolParams& foreground_worker_pool_params_in,
         const SchedulerWorkerPoolParams&
             foreground_blocking_worker_pool_params_in);
     ~InitParams();
 
     const SchedulerWorkerPoolParams background_worker_pool_params;
     const SchedulerWorkerPoolParams background_blocking_worker_pool_params;
     const SchedulerWorkerPoolParams foreground_worker_pool_params;
     const SchedulerWorkerPoolParams foreground_blocking_worker_pool_params;
   };
 
   // Destroying a TaskScheduler is not allowed in production; it is always
   // leaked. In tests, it should only be destroyed after JoinForTesting() has
   // returned.
   virtual ~TaskScheduler() = default;
 
+  // Allows the task scheduler to create threads and run tasks following the
+  // |init_params| specification. CHECKs on failure.
+  virtual void Start(const InitParams& init_params) = 0;
+
   // Posts |task| with a |delay| and specific |traits|. |delay| can be zero.
   // For one off tasks that don't require a TaskRunner.
   virtual void PostDelayedTaskWithTraits(
       const tracked_objects::Location& from_here,
       const TaskTraits& traits,
       OnceClosure task,
       TimeDelta delay) = 0;
 
   // Returns a TaskRunner whose PostTask invocations result in scheduling tasks
   // using |traits|. Tasks may run in any order and in parallel.
@@ skipping 105 matching lines @@
   // each process n/GetMaxConcurrentTasksWithTraitsDeprecated() items.
   //
   // TODO(fdoray): Remove this method. https://crbug.com/687264
   virtual int GetMaxConcurrentTasksWithTraitsDeprecated(
       const TaskTraits& traits) const = 0;
 };
 
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_TASK_SCHEDULER_H_