Add TaskScheduler::JoinForTesting().

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 20 matching lines @@
 // 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,
       const Closure& task,
       TimeDelta delay) = 0;
 
@@ skipping 27 matching lines @@
   // called once.
   virtual void Shutdown() = 0;
 
   // Waits until there are no pending undelayed tasks. May be called in tests
   // to validate that a condition is met after all undelayed tasks have run.
   //
   // Does not wait for delayed tasks. Waits for undelayed tasks posted from
   // other threads during the call. Returns immediately when shutdown completes.
   virtual void FlushForTesting() = 0;
 
+  // Joins all threads. Tasks that are already running are allowed to complete
+  // their execution. This can only be called once.
+  virtual void JoinForTesting() = 0;
+
   // CreateAndSetSimpleTaskScheduler(), CreateAndSetDefaultTaskScheduler(), and
   // 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.
 
   // Creates and sets a task scheduler with one worker pool that can have up to
@@ skipping 15 matching lines @@
 
   // Retrieve the TaskScheduler set via CreateAndSetDefaultTaskScheduler() or
   // SetInstance(). This should be used very rarely; most users of TaskScheduler
   // should use the post_task.h API.
   static TaskScheduler* GetInstance();
 };
 
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_TASK_SCHEDULER_H_