TaskScheduler: Pop a Task from its Sequence from SchedulerWorkerThread.

base/task_scheduler/scheduler_worker_thread.cc

Index: base/task_scheduler/scheduler_worker_thread.cc
diff --git a/base/task_scheduler/scheduler_worker_thread.cc b/base/task_scheduler/scheduler_worker_thread.cc
index 49dde51d6a7b036675fd47d645df60e0c0e4e242..6fcce57594a471484d9e2c1af8ae65c00c72ed26 100644
--- a/base/task_scheduler/scheduler_worker_thread.cc
+++ b/base/task_scheduler/scheduler_worker_thread.cc
@@ -74,7 +74,15 @@ void SchedulerWorkerThread::ThreadMain() {
     }
 
     task_tracker_->RunTask(sequence->PeekTask());
-    delegate_->RanTaskFromSequence(std::move(sequence));
+
+    const bool sequence_became_empty = sequence->PopTask();
+
+    // If |sequence| isn't empty immediately after the pop, enqueue it to
+    // maintain the invariant that a non-empty Sequence is always referenced by
+    // either a PriorityQueue or a SchedulerWorkerThread. If it is empty, it
+    // will be enqueued when a Task is added to it.

[robliao, 2016/04/08 17:21:39]

Nit: Technically, for the empty case...
If it is empty and there are live references to it, it will be enqueued when a
Task is added to it. Otherwise, the sequence will be destroyed at the end of
this scope.

[fdoray, 2016/04/08 17:49:12]

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_|