TaskScheduler: Add TaskRunnerHandle support to TaskScheduler tasks

base/task_scheduler/sequence.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_SEQUENCE_H_
 #define BASE_TASK_SCHEDULER_SEQUENCE_H_
 
 #include <stddef.h>
 
 #include <memory>
 #include <queue>
 
 #include "base/base_export.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/task_scheduler/scheduler_lock.h"
 #include "base/task_scheduler/sequence_sort_key.h"
 #include "base/task_scheduler/task.h"
 #include "base/task_scheduler/task_traits.h"
 
 namespace base {
 namespace internal {
 
 // A sequence holds tasks that must be executed in posting order.
+//
+// Note: there is a known refcounted-ownership cycle in the Scheduler
+// architecture: Sequence -> Task -> TaskRunner -> Sequence -> ...
+// This is okay so long as the other owners of Sequence (PriorityQueue and
+// SchedulerWorkerThread in alternance and
+// SchedulerThreadPoolImpl::SchedulerWorkerThreadDelegateImpl::GetWork()
+// temporarily) keep running it (and taking Tasks from it as a result). A
+// dangling reference cycle would only occur should they release their reference
+// to it while it's not empty. In other words, it is only correct for them to
+// release it after PopTask() returns false to indicate it was made empty by
+// that call (in which case the next PushTask() will return true to indicate to
+// the caller that the Sequence should be re-enqueued for execution).
+//
 // This class is thread-safe.
 class BASE_EXPORT Sequence : public RefCountedThreadSafe<Sequence> {
  public:
   Sequence();
 
   // Adds |task| at the end of the sequence's queue. Returns true if the
   // sequence was empty before this operation.
   bool PushTask(std::unique_ptr<Task> task);
 
   // Returns the task in front of the sequence's queue, if any.
@@ skipping 22 matching lines @@
   size_t num_tasks_per_priority_[static_cast<int>(TaskPriority::HIGHEST) + 1] =
       {};
 
   DISALLOW_COPY_AND_ASSIGN(Sequence);
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_SEQUENCE_H_