TaskScheduler: Make the worker pools of TaskSchedulerImpl configurable

base/task_scheduler/task_scheduler_impl.h

Index: base/task_scheduler/task_scheduler_impl.h
diff --git a/base/task_scheduler/task_scheduler_impl.h b/base/task_scheduler/task_scheduler_impl.h
index f9d789ea4c550daec7c001f591ce2068f672e936..6a5797fdd510c8bf25c01efadc1a69f70ee9821a 100644
--- a/base/task_scheduler/task_scheduler_impl.h
+++ b/base/task_scheduler/task_scheduler_impl.h
@@ -5,7 +5,11 @@
 #ifndef BASE_TASK_SCHEDULER_TASK_SCHEDULER_IMPL_H_
 #define BASE_TASK_SCHEDULER_TASK_SCHEDULER_IMPL_H_
 
+#include <stddef.h>
+
 #include <memory>
+#include <string>
+#include <vector>
 
 #include "base/base_export.h"
 #include "base/logging.h"
@@ -14,10 +18,12 @@
 #include "base/synchronization/waitable_event.h"
 #include "base/task_runner.h"
 #include "base/task_scheduler/delayed_task_manager.h"
+#include "base/task_scheduler/scheduler_thread_pool_impl.h"
 #include "base/task_scheduler/sequence.h"
 #include "base/task_scheduler/task_scheduler.h"
 #include "base/task_scheduler/task_tracker.h"
 #include "base/task_scheduler/task_traits.h"
+#include "base/threading/platform_thread.h"
 
 namespace base {
 namespace internal {
@@ -28,9 +34,40 @@ class SchedulerThreadPoolImpl;
 // Default TaskScheduler implementation. This class is thread-safe.
 class BASE_EXPORT TaskSchedulerImpl : public TaskScheduler {
  public:
-  // Creates and returns an initialized TaskSchedulerImpl. CHECKs on failure to
-  // do so (never returns null).
-  static std::unique_ptr<TaskSchedulerImpl> Create();
+  // Delegate interface for TaskSchedulerImpl.
+  class BASE_EXPORT Delegate {
+   public:
+    struct ThreadPoolCreationArgs {
+      // Name of the pool. Use to label the pool's threads.
+      std::string name;
+
+      // Priority of the pool's threads.
+      ThreadPriority thread_priority;
+
+      // Whether IO is allowed in the pool.
+      SchedulerThreadPoolImpl::IORestriction io_restriction;
+
+      // Maximum number of threads in the pool.
+      size_t max_threads;
+    };
+
+    virtual ~Delegate() = default;
+
+    // Returns the number of pools to create.
+    virtual size_t GetNumThreadPools() = 0;
+
+    // Returns creation arguments for the pool at index |pool_index|.
+    virtual ThreadPoolCreationArgs GetCreationArgsForThreadPool(
+        size_t pool_index) = 0;
+
+    // Returns the index of the pool in which a task with |traits| should run.
+    virtual size_t GetThreadPoolIndexForTraits(const TaskTraits& traits) = 0;
+  };
+
+  // Creates and returns an initialized TaskSchedulerImpl. CHECKs on failure
+  // (never returns null).
+  static std::unique_ptr<TaskSchedulerImpl> Create(
+      std::unique_ptr<Delegate> delegate);
 
   // Destroying a TaskSchedulerImpl is not allowed in production; it is always
   // leaked. In tests, it can only be destroyed after JoinForTesting() has
@@ -51,7 +88,7 @@ class BASE_EXPORT TaskSchedulerImpl : public TaskScheduler {
   void JoinForTesting();
 
  private:
-  TaskSchedulerImpl();
+  TaskSchedulerImpl(std::unique_ptr<Delegate> delegate);
 
   void Initialize();
 
@@ -66,21 +103,10 @@ class BASE_EXPORT TaskSchedulerImpl : public TaskScheduler {
   // DelayedTaskManager.
   void OnDelayedRunTimeUpdated();
 
+  std::unique_ptr<Delegate> delegate_;
   TaskTracker task_tracker_;
   DelayedTaskManager delayed_task_manager_;
-
-  // Thread pool for BACKGROUND Tasks without file I/O.
-  std::unique_ptr<SchedulerThreadPoolImpl> background_thread_pool_;
-
-  // Thread pool for BACKGROUND Tasks with file I/O.
-  std::unique_ptr<SchedulerThreadPoolImpl> background_file_io_thread_pool_;
-
-  // Thread pool for USER_VISIBLE and USER_BLOCKING Tasks without file I/O.
-  std::unique_ptr<SchedulerThreadPoolImpl> normal_thread_pool_;
-
-  // Thread pool for USER_VISIBLE and USER_BLOCKING Tasks with file I/O.
-  std::unique_ptr<SchedulerThreadPoolImpl> normal_file_io_thread_pool_;
-
+  std::vector<std::unique_ptr<SchedulerThreadPoolImpl>> thread_pools_;
   std::unique_ptr<SchedulerServiceThread> service_thread_;
 
 #if DCHECK_IS_ON()