TaskScheduler: Don't delete Tasks in the scope of a Sequence's lock.

base/task_scheduler/sequence.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/sequence.h"
 
 #include <utility>
 
 #include "base/logging.h"
 #include "base/time/time.h"
@@ skipping 18 matching lines @@
 const Task* Sequence::PeekTask() const {
   AutoSchedulerLock auto_lock(lock_);
 
   if (queue_.empty())
     return nullptr;
 
   return queue_.front().get();
 }
 
 bool Sequence::PopTask() {
+  // Delete the popped task outside the scope of |lock_|. That prevents a double

[gab, 2016/07/20 20:34:51]

s/That/This/

[fdoray, 2016/07/21 13:36:03]

Done.

+  // acquisition of |lock_| if the task's destructor tries to post a task to
+  // this Sequence and reduces contention.
+  std::unique_ptr<Task> delete_outside_lock_scope;
+
   AutoSchedulerLock auto_lock(lock_);

[robliao, 2016/07/20 22:39:00]

This would be more robust and clearer if AutoSchedulerLock and associated
statements were in its own block. You'd need to store the value of
queue_.empty() in a temporary. 

[fdoray, 2016/07/21 13:36:03]

Done.

   DCHECK(!queue_.empty());
 
   const int priority_index =
       static_cast<int>(queue_.front()->traits.priority());
   DCHECK_GT(num_tasks_per_priority_[priority_index], 0U);
   --num_tasks_per_priority_[priority_index];
 
+  delete_outside_lock_scope = std::move(queue_.front());
   queue_.pop();
   return queue_.empty();
 }
 
 SequenceSortKey Sequence::GetSortKey() const {
   TaskPriority priority = TaskPriority::LOWEST;
   base::TimeTicks next_task_sequenced_time;
 
   {
     AutoSchedulerLock auto_lock(lock_);
@@ skipping 13 matching lines @@
     next_task_sequenced_time = queue_.front()->sequenced_time;
   }
 
   return SequenceSortKey(priority, next_task_sequenced_time);
 }
 
 Sequence::~Sequence() = default;
 
 }  // namespace internal
 }  // namespace base