TaskScheduler: Pop a Task from its Sequence from SchedulerWorkerThread.

base/task_scheduler/scheduler_worker_thread.cc

 // 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.
 
 #include "base/task_scheduler/scheduler_worker_thread.h"
 
 #include <stddef.h>
 
 #include <utility>
 
@@ skipping 56 matching lines @@
   while (!task_tracker_->shutdown_completed() && !ShouldExitForTesting()) {
     // Get the sequence containing the next task to execute.
     scoped_refptr<Sequence> sequence = delegate_->GetWork(this);
 
     if (!sequence) {
       wake_up_event_.Wait();
       continue;
     }
 
     task_tracker_->RunTask(sequence->PeekTask());
-    delegate_->RanTaskFromSequence(std::move(sequence));
+
+    const bool sequence_became_empty = sequence->PopTask();

[gab, 2016/04/07 21:37:33]

In other places you have comments explaining why this atomic operation always
leads to the correct state (i.e. either we made it atomically empty and the next
Add() will be in charge of re-inserting or there was a task behind our pop and
we're in charge of re-inserting).
Think it's worth re-iterating the algorithmic guarantees relied on here?

[fdoray, 2016/04/08 15:59:12]

Done.

+    if (!sequence_became_empty)
+      delegate_->EnqueueSequence(std::move(sequence));
 
     // Calling WakeUp() guarantees that this SchedulerWorkerThread will run
     // Tasks from Sequences returned by the GetWork() method of |delegate_|
     // until it returns nullptr. Resetting |wake_up_event_| here doesn't break
     // this invariant and avoids a useless loop iteration before going to sleep
     // if WakeUp() is called while this SchedulerWorkerThread is awake.
     wake_up_event_.Reset();
   }
 
   delegate_->OnMainExit();
 }
 
 bool SchedulerWorkerThread::ShouldExitForTesting() const {
   AutoSchedulerLock auto_lock(should_exit_for_testing_lock_);
   return should_exit_for_testing_;
 }
 
 }  // namespace internal
 }  // namespace base