High resolution timer fix for Windows

base/message_loop/incoming_task_queue.h

 // Copyright 2013 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_MESSAGE_LOOP_INCOMING_TASK_QUEUE_H_
 #define BASE_MESSAGE_LOOP_INCOMING_TASK_QUEUE_H_
 
 #include "base/base_export.h"
 #include "base/memory/ref_counted.h"
 #include "base/pending_task.h"
@@ skipping 20 matching lines @@
   // task is properly synchronized between different threads.
   //
   // Returns true if the task was successfully added to the queue, otherwise
   // returns false. In all cases, the ownership of |task| is transferred to the
   // called method.
   bool AddToIncomingQueue(const tracked_objects::Location& from_here,
                           const Closure& task,
                           TimeDelta delay,
                           bool nestable);
 
-  // Returns true if the message loop has high resolution timers enabled.
-  // Provided for testing.
-  bool IsHighResolutionTimerEnabledForTesting();
+  // Returns true if the queue contains tasks that require higher than default
+  // timer resolution. Currently only needed for Windows.
+  bool HasHighResolutionTasks();
 
   // Returns true if the message loop is "idle". Provided for testing.
   bool IsIdleForTesting();
 
   // Loads tasks from the |incoming_queue_| into |*work_queue|. Must be called
-  // from the thread that is running the loop.
-  void ReloadWorkQueue(TaskQueue* work_queue);
+  // from the thread that is running the loop. Returns the number of tasks that
+  // require high resolution timers.
+  int ReloadWorkQueue(TaskQueue* work_queue);
 
   // Disconnects |this| from the parent message loop.
   void WillDestroyCurrentMessageLoop();
 
  private:
   friend class RefCountedThreadSafe<IncomingTaskQueue>;
   virtual ~IncomingTaskQueue();
 
   // Calculates the time at which a PendingTask should run.
   TimeTicks CalculateDelayedRuntime(TimeDelta delay);
 
   // Adds a task to |incoming_queue_|. The caller retains ownership of
   // |pending_task|, but this function will reset the value of
   // |pending_task->task|. This is needed to ensure that the posting call stack
   // does not retain |pending_task->task| beyond this function call.
   bool PostPendingTask(PendingTask* pending_task);
 
-#if defined(OS_WIN)
-  TimeTicks high_resolution_timer_expiration_;
-#endif
+  // Number of tasks that require high resolution timing. This value is kept
+  // so that ReloadWorkQueue() completes in constant time.
+  int high_res_task_count_;
 
-  // The lock that protects access to |incoming_queue_|, |message_loop_| and
-  // |next_sequence_num_|.
+  // The lock that protects access to the members of this class.

[darin (slow to review), 2014/07/19 04:06:57]

micro-nit: How about listing the lock as the first member variable?

   base::Lock incoming_queue_lock_;
 
   // An incoming queue of tasks that are acquired under a mutex for processing
   // on this instance's thread. These tasks have not yet been been pushed to
   // |message_loop_|.
   TaskQueue incoming_queue_;
 
   // Points to the message loop that owns |this|.
   MessageLoop* message_loop_;
 
   // The next sequence number to use for delayed tasks.
   int next_sequence_num_;
 
   DISALLOW_COPY_AND_ASSIGN(IncomingTaskQueue);
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_MESSAGE_LOOP_INCOMING_TASK_QUEUE_H_