Record async "task backtraces"

base/pending_task.h

 // Copyright (c) 2011 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_PENDING_TASK_H_
 #define BASE_PENDING_TASK_H_
 
 #include <queue>
 
 #include "base/base_export.h"
 #include "base/callback.h"
 #include "base/location.h"
 #include "base/time/time.h"
 #include "base/tracking_info.h"
 
 namespace base {
 
 // Contains data about a pending task. Stored in TaskQueue and DelayedTaskQueue
 // for use by classes that queue and execute tasks.
 struct BASE_EXPORT PendingTask : public TrackingInfo {
   PendingTask(const tracked_objects::Location& posted_from,
+              const PendingTask* parent_task,
               const Closure& task);
   PendingTask(const tracked_objects::Location& posted_from,
+              const PendingTask* parent_task,
               const Closure& task,
               TimeTicks delayed_run_time,
               bool nestable);
   ~PendingTask();
 
   // Used to support sorting.
   bool operator<(const PendingTask& other) const;
 
   // The task to run.
   Closure task;
 
   // The site this PendingTask was posted from.
   tracked_objects::Location posted_from;
 
+  // Task backtrace.
+  const void* task_backtrace[4];
+
   // Secondary sort key for run time.
   int sequence_num;
 
   // OK to dispatch from a nested loop.
   bool nestable;
 
   // Needs high resolution timers.
   bool is_high_res;
 };
 
 // Wrapper around std::queue specialized for PendingTask which adds a Swap
 // helper method.
 class BASE_EXPORT TaskQueue : public std::queue<PendingTask> {
  public:
   void Swap(TaskQueue* queue);
 };
 
 // PendingTasks are sorted by their |delayed_run_time| property.
 typedef std::priority_queue<base::PendingTask> DelayedTaskQueue;
 
 }  // namespace base
 
 #endif  // BASE_PENDING_TASK_H_