[reference - do not submit] Always create four pools in TaskSchedulerImpl.

base/task_scheduler/task_scheduler_impl.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_IMPL_H_
 #define BASE_TASK_SCHEDULER_TASK_SCHEDULER_IMPL_H_
 
 #include <memory>
 #include <vector>
 
 #include "base/base_export.h"
-#include "base/callback.h"
 #include "base/logging.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/synchronization/atomic_flag.h"
 #include "base/task_scheduler/scheduler_worker_pool_impl.h"
 #include "base/task_scheduler/sequence.h"
 #include "base/task_scheduler/task_scheduler.h"
 #include "base/task_scheduler/task_traits.h"
 #include "base/threading/thread.h"
 
 namespace base {
 
 class HistogramBase;
-class SchedulerWorkerPoolParams;
 
 namespace internal {
 
 class DelayedTaskManager;
 class SchedulerSingleThreadTaskRunnerManager;
 class TaskTracker;
 
 // Default TaskScheduler implementation. This class is thread-safe.
 class BASE_EXPORT TaskSchedulerImpl : public TaskScheduler {
  public:
   // Creates and returns an initialized TaskSchedulerImpl. CHECKs on failure.
-  // |worker_pool_params_vector| describes the worker pools to create.
-  // |worker_pool_index_for_traits_callback| returns the index in |worker_pools|
-  // of the worker pool in which a task with given traits should run.
+  // |name| is used to label threads and histograms. It should identify the
+  // component that creates the TaskScheduler. |init_params| is used to
+  // initialize the worker pools.
   static std::unique_ptr<TaskSchedulerImpl> Create(
-      const std::vector<SchedulerWorkerPoolParams>& worker_pool_params_vector,
-      const WorkerPoolIndexForTraitsCallback&
-          worker_pool_index_for_traits_callback);
+      const std::string& name,
+      const TaskSchedulerInitParams& init_params);
 
   ~TaskSchedulerImpl() override;
 
   // TaskScheduler:
   void PostDelayedTaskWithTraits(const tracked_objects::Location& from_here,
                                  const TaskTraits& traits,
                                  const Closure& task,
                                  TimeDelta delay) override;
   scoped_refptr<TaskRunner> CreateTaskRunnerWithTraits(
       const TaskTraits& traits) override;
   scoped_refptr<SequencedTaskRunner> CreateSequencedTaskRunnerWithTraits(
       const TaskTraits& traits) override;
   scoped_refptr<SingleThreadTaskRunner> CreateSingleThreadTaskRunnerWithTraits(
       const TaskTraits& traits) override;
   std::vector<const HistogramBase*> GetHistograms() const override;
   int GetMaxConcurrentTasksWithTraitsDeprecated(
       const TaskTraits& traits) const override;
   void Shutdown() override;
   void FlushForTesting() override;
   void JoinForTesting() override;
 
  private:
-  explicit TaskSchedulerImpl(const WorkerPoolIndexForTraitsCallback&
-                                 worker_pool_index_for_traits_callback);
+  TaskSchedulerImpl(const std::string& name);
 
-  void Initialize(
-      const std::vector<SchedulerWorkerPoolParams>& worker_pool_params_vector);
+  void Initialize(const TaskSchedulerInitParams& init_params);
 
   // Returns the worker pool that runs Tasks with |traits|.
   SchedulerWorkerPoolImpl* GetWorkerPoolForTraits(
       const TaskTraits& traits) const;
 
   // Callback invoked when a non-single-thread |sequence| isn't empty after a
   // worker pops a Task from it.
   void ReEnqueueSequenceCallback(scoped_refptr<Sequence> sequence);
 
+  const std::string name_;
   Thread service_thread_;
   std::unique_ptr<TaskTracker> task_tracker_;
   std::unique_ptr<DelayedTaskManager> delayed_task_manager_;
   std::unique_ptr<SchedulerSingleThreadTaskRunnerManager>
       single_thread_task_runner_manager_;
-  const WorkerPoolIndexForTraitsCallback worker_pool_index_for_traits_callback_;
-  std::vector<std::unique_ptr<SchedulerWorkerPoolImpl>> worker_pools_;
+  std::unique_ptr<SchedulerWorkerPoolImpl> worker_pools_[4];
 
 #if DCHECK_IS_ON()
   // Set once JoinForTesting() has returned.
   AtomicFlag join_for_testing_returned_;
 #endif
 
   DISALLOW_COPY_AND_ASSIGN(TaskSchedulerImpl);
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_TASK_SCHEDULER_IMPL_H_