TaskScheduler: Change Sequence::PeekTask to Sequence::TakeTask.

base/task_scheduler/sequence.cc

Index: base/task_scheduler/sequence.cc
diff --git a/base/task_scheduler/sequence.cc b/base/task_scheduler/sequence.cc
index 86e99f0a4c390907f101f99cf6a08d29b86f08a7..59621364b13fba17432009184a441739cf96e7f1 100644
--- a/base/task_scheduler/sequence.cc
+++ b/base/task_scheduler/sequence.cc
@@ -26,37 +26,33 @@ bool Sequence::PushTask(std::unique_ptr<Task> task) {
   return queue_.size() == 1;
 }
 
-const Task* Sequence::PeekTask() const {
+std::unique_ptr<Task> Sequence::TakeTask() {
   AutoSchedulerLock auto_lock(lock_);
 
-  if (queue_.empty())
+  if (queue_.empty() || !queue_.front())

[gab, 2016/10/07 19:51:08]

I don't think we should allow calling TakeTask() when front slot is empty, that
seems like misuse. i.e. When TakeTask() is called there should only be a single
owner and they shouldn't call TakeTask twice without Pop().

[fdoray, 2016/10/07 20:30:30]

Done.

     return nullptr;
 
-  return queue_.front().get();
-}
-
-bool Sequence::PopTask() {
-  // Delete the popped task outside the scope of |lock_|. This prevents a double
-  // 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;
-  bool sequence_empty_after_pop = false;
-
-  {
-    AutoSchedulerLock auto_lock(lock_);
-    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];
 
-    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];
+  return std::move(queue_.front());
+}
 
-    delete_outside_lock_scope = std::move(queue_.front());
-    queue_.pop();
-    sequence_empty_after_pop = queue_.empty();
-  }
+TaskTraits Sequence::GetFrontTaskTraits() const {
+  AutoSchedulerLock auto_lock(lock_);
+  DCHECK(!queue_.empty());
+  DCHECK(queue_.front());
+  return queue_.front()->traits;
+}
 
-  return sequence_empty_after_pop;
+bool Sequence::RemoveFrontSlot() {
+  AutoSchedulerLock auto_lock(lock_);
+  DCHECK(!queue_.empty());
+  DCHECK(!queue_.front());
+  queue_.pop();
+  return queue_.empty();
 }
 
 SequenceSortKey Sequence::GetSortKey() const {