TaskScheduler [12] Support SINGLE_THREADED in SchedulerThreadPool DO NOT SUBMIT

base/task_scheduler/scheduler_thread_pool.h

Index: base/task_scheduler/scheduler_thread_pool.h
diff --git a/base/task_scheduler/scheduler_thread_pool.h b/base/task_scheduler/scheduler_thread_pool.h
index 75c8a3290cfbc2cb9c2a60a6a40ef8510a841b13..b7094e78ee9863bd302d1ee5ab0260a53a876269 100644
--- a/base/task_scheduler/scheduler_thread_pool.h
+++ b/base/task_scheduler/scheduler_thread_pool.h
@@ -19,6 +19,7 @@
 #include "base/task_runner.h"
 #include "base/task_scheduler/priority_queue.h"
 #include "base/task_scheduler/scheduler_lock.h"
+#include "base/task_scheduler/scheduler_stack.h"
 #include "base/task_scheduler/scheduler_task_executor.h"
 #include "base/task_scheduler/scheduler_worker_thread.h"
 #include "base/task_scheduler/sequence.h"
@@ -97,9 +98,8 @@ class BASE_EXPORT SchedulerThreadPool : public SchedulerTaskExecutor {
   // Adds |worker_thread| to |idle_worker_threads_stack_|.
   void AddToIdleWorkerThreadsStack(SchedulerWorkerThread* worker_thread);
 
-  // Pops one idle worker thread from |idle_worker_thread_stack_| and returns
-  // it. Returns nullptr if |idle_worker_thread_stack_| is empty.
-  SchedulerWorkerThread* PopOneIdleWorkerThread();
+  // Removes |worker_thread| from |idle_worker_threads_stack_|.
+  void RemoveFromIdleWorkerThreadsStack(SchedulerWorkerThread* worker_thread);
 
   // PriorityQueue from which all threads of this thread pool get work.
   PriorityQueue shared_priority_queue_;
@@ -108,6 +108,9 @@ class BASE_EXPORT SchedulerThreadPool : public SchedulerTaskExecutor {
   // initialization of the thread pool.
   std::vector<std::unique_ptr<SchedulerWorkerThread>> worker_threads_;
 
+  // Synchronizes the creation of SINGLE_THREADED TaskRunners.
+  SchedulerLock single_thread_task_runner_creation_lock_;
+
   // Synchronizes access to |idle_worker_threads_stack_| and
   // |idle_worker_threads_stack_cv_for_testing_|. Has |shared_priority_queue_|'s
   // lock as its predecessor so that a thread can be pushed to
@@ -115,8 +118,8 @@ class BASE_EXPORT SchedulerThreadPool : public SchedulerTaskExecutor {
   // details in GetWork()).
   SchedulerLock idle_worker_threads_stack_lock_;
 
-  // Stack of idle worker threads.
-  std::stack<SchedulerWorkerThread*> idle_worker_threads_stack_;
+  // Stack of idle worker threads. Values are indexes in |worker_threads_|.
+  SchedulerStack idle_worker_threads_stack_;
 
   // Signaled when all worker threads become idle.
   std::unique_ptr<ConditionVariable> idle_worker_threads_stack_cv_for_testing_;