[reference - do not submit] Always create four pools 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 <string>
 #include <vector>
 
 #include "base/base_export.h"
 #include "base/callback_forward.h"
 #include "base/memory/ref_counted.h"
 #include "base/sequenced_task_runner.h"
 #include "base/single_thread_task_runner.h"
 #include "base/task_runner.h"
+#include "base/task_scheduler/scheduler_worker_pool_params.h"
 #include "base/task_scheduler/task_traits.h"
 #include "base/time/time.h"
+#include "build/build_config.h"
 
 namespace gin {
 class V8Platform;
 }
 
 namespace tracked_objects {
 class Location;
 }
 
 namespace base {
 
 class HistogramBase;
-class SchedulerWorkerPoolParams;
+
+struct BASE_EXPORT TaskSchedulerInitParams {
+  TaskSchedulerInitParams(
+      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);
+
+  // Returns true if this object can be used to initialize a task scheduler.
+  bool IsValid() const;
+
+  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;
+};
 
 // 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.
 class BASE_EXPORT TaskScheduler {
  public:
-  // Returns the index of the worker pool in which a task with |traits| should
-  // run. This should be coded in a future-proof way: new traits should
-  // gracefully map to a default pool.
-  using WorkerPoolIndexForTraitsCallback =
-      Callback<size_t(const TaskTraits& traits)>;
-
   // 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;
 
   // 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,
@@ skipping 47 matching lines @@
   // SetInstance() register a TaskScheduler to handle tasks posted through the
   // post_task.h API for this process. The registered TaskScheduler will only be
   // deleted when a new TaskScheduler is registered and is leaked on shutdown.
   // The methods must not be called when TaskRunners created by the previous
   // TaskScheduler are still alive. The methods are not thread-safe; proper
   // synchronization is required to use the post_task.h API after registering a
   // new TaskScheduler.
 
 #if !defined(OS_NACL)
   // Creates and sets a task scheduler using default params. |name| is used to
-  // label threads and histograms. It should identify the component that calls
-  // this. CHECKs on failure. For tests, prefer base::test::ScopedTaskScheduler
-  // (ensures isolation).
+  // label threads and histograms. It should identify the component that creates
+  // the TaskScheduler. CHECKs on failure. For tests, prefer
+  // base::test::ScopedTaskScheduler (ensures isolation).
+  //
+  // This method is not available on NaCl because there is no way to get the
+  // number of processors on that platform.
   static void CreateAndSetSimpleTaskScheduler(const std::string& name);
 #endif  // !defined(OS_NACL)
 
-  // Creates and sets a task scheduler with custom worker pools. 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. For tests,
-  // prefer base::test::ScopedTaskScheduler (ensures isolation).
+  // Creates and sets a task scheduler using custom params. |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.
+  // CHECKs on failure. For tests, prefer base::test::ScopedTaskScheduler
+  // (ensures isolation).
   static void CreateAndSetDefaultTaskScheduler(
-      const std::vector<SchedulerWorkerPoolParams>& worker_pool_params_vector,
-      const WorkerPoolIndexForTraitsCallback&
-          worker_pool_index_for_traits_callback);
+      const std::string& name,
+      const TaskSchedulerInitParams& init_params);
 
   // Registers |task_scheduler| to handle tasks posted through the post_task.h
   // API for this process. For tests, prefer base::test::ScopedTaskScheduler
   // (ensures isolation).
   static void SetInstance(std::unique_ptr<TaskScheduler> task_scheduler);
 
   // Retrieve the TaskScheduler set via SetInstance() or
   // CreateAndSet(Simple|Default)TaskScheduler(). This should be used very
   // rarely; most users of TaskScheduler should use the post_task.h API. In
   // particular, refrain from doing
@@ skipping 17 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_