Use SequenceChecker to allow Timer to run in SequencedWorkerPool

base/timer/timer.h

 // Copyright (c) 2012 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.
 
 // OneShotTimer and RepeatingTimer provide a simple timer API.  As the names
 // suggest, OneShotTimer calls you back once after a time delay expires.
 // RepeatingTimer on the other hand calls you back periodically with the
 // prescribed time interval.
 //
 // OneShotTimer and RepeatingTimer both cancel the timer when they go out of
@@ skipping 21 matching lines @@
 //     base::RepeatingTimer timer_;
 //   };
 //
 // Both OneShotTimer and RepeatingTimer also support a Reset method, which
 // allows you to easily defer the timer event until the timer delay passes once
 // again.  So, in the above example, if 0.5 seconds have already passed,
 // calling Reset on timer_ would postpone DoStuff by another 1 second.  In
 // other words, Reset is shorthand for calling Stop and then Start again with
 // the same arguments.
 //
-// NOTE: These APIs are not thread safe. Always call from the same thread.
+// NOTE: These APIs are not thread safe. Always call from the same sequence
+// task runner.

[danakj, 2015/11/16 22:46:42]

can you still mention same-thread here somehow too?

[gab, 2015/11/17 15:23:58]

I think the new comment is actually correct. i.e. "not thread safe" doesn't mean
"thread affine" it just means "needs to be sequenced (+memory barriers between
thread hops)" which SequencedTaskRunner provide.

 
 #ifndef BASE_TIMER_TIMER_H_
 #define BASE_TIMER_TIMER_H_
 
 // IMPORTANT: If you change timer code, make sure that all tests (including
 // disabled ones) from timer_unittests.cc pass locally. Some are disabled
 // because they're flaky on the buildbot, but when you run them locally you
 // should be able to tell the difference.
 
 #include "base/base_export.h"
 #include "base/basictypes.h"
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/callback.h"
 #include "base/location.h"
+#include "base/sequence_checker.h"
 #include "base/time/time.h"
 
 namespace base {
 
 class BaseTimerTaskInternal;
-class SingleThreadTaskRunner;
 
 //-----------------------------------------------------------------------------
-// This class wraps MessageLoop::PostDelayedTask to manage delayed and repeating
-// tasks. It must be destructed on the same thread that starts tasks. There are
-// DCHECKs in place to verify this.
+// This class wraps TaskRunner::PostDelayedTask to manage delayed and
+// repeating tasks. It must be destructed on the same task runner that starts
+// tasks. There are DCHECKs in place to verify this.
 //
 class BASE_EXPORT Timer {
  public:
   // Construct a timer in repeating or one-shot mode. Start or SetTaskInfo must
   // be called later to set task info. |retain_user_task| determines whether the
   // user_task is retained or reset when it runs or stops.
   Timer(bool retain_user_task, bool is_repeating);
 
   // Construct a timer with retained task info.
   Timer(const tracked_objects::Location& posted_from,
         TimeDelta delay,
         const base::Closure& user_task,
         bool is_repeating);
 
   virtual ~Timer();
 
   // Returns true if the timer is running (i.e., not stopped).
   virtual bool IsRunning() const;
 
   // Returns the current delay for this timer.
   virtual TimeDelta GetCurrentDelay() const;
 
   // Set the task runner on which the task should be scheduled. This method can
   // only be called before any tasks have been scheduled. The task runner must

[gab, 2015/11/17 15:23:57]

s/The task runner/|task_runner|

[jsbell, 2015/12/01 00:23:10]

Done.

-  // run tasks on the same thread the timer is used on.
-  virtual void SetTaskRunner(scoped_refptr<SingleThreadTaskRunner> task_runner);
+  // run tasks on the sequenced runner the timer is used on.

[gab, 2015/11/17 15:23:58]

s/sequence runner/same sequence/ ?

[jsbell, 2015/12/01 00:23:10]

Done.

+  virtual void SetTaskRunner(scoped_refptr<SequencedTaskRunner> task_runner);
 
   // Start the timer to run at the given |delay| from now. If the timer is
   // already running, it will be replaced to call the given |user_task|.
   virtual void Start(const tracked_objects::Location& posted_from,
                      TimeDelta delay,
                      const base::Closure& user_task);
 
   // Call this method to stop and cancel the timer.  It is a no-op if the timer
   // is not running.
   virtual void Stop();
@@ skipping 23 matching lines @@
 
  private:
   friend class BaseTimerTaskInternal;
 
   // Allocates a new scheduled_task_ and posts it on the current MessageLoop
   // with the given |delay|. scheduled_task_ must be NULL. scheduled_run_time_
   // and desired_run_time_ are reset to Now() + delay.
   void PostNewScheduledTask(TimeDelta delay);
 
   // Returns the task runner on which the task should be scheduled. If the
-  // corresponding task_runner_ field is null, the task runner for the current
-  // thread is returned.
-  scoped_refptr<SingleThreadTaskRunner> GetTaskRunner();
+  // corresponding task_runner_ field is null, the current task runner is

[gab, 2015/11/17 15:23:57]

s/task_runner_/|task_runner_|

[gab, 2015/11/17 15:23:58]

s/current task runner/SequencedTaskRunner for the current thread/

[jsbell, 2015/12/01 00:23:10]

Done.

+  // returned.
+  scoped_refptr<SequencedTaskRunner> GetTaskRunner();
 
   // Disable scheduled_task_ and abandon it so that it no longer refers back to
   // this object.
   void AbandonScheduledTask();
 
   // Called by BaseTimerTaskInternal when the MessageLoop runs it.
   void RunScheduledTask();
 
   // Stop running task (if any) and abandon scheduled task (if any).
   void StopAndAbandon() {
     Stop();
     AbandonScheduledTask();
   }
 
   // When non-NULL, the scheduled_task_ is waiting in the MessageLoop to call
   // RunScheduledTask() at scheduled_run_time_.
   BaseTimerTaskInternal* scheduled_task_;
 
   // The task runner on which the task should be scheduled. If it is null, the
-  // task runner for the current thread should be used.
-  scoped_refptr<SingleThreadTaskRunner> task_runner_;
+  // current sequenced task runner should be used.
+  scoped_refptr<SequencedTaskRunner> task_runner_;
 
   // Location in user code.
   tracked_objects::Location posted_from_;
   // Delay requested by user.
   TimeDelta delay_;
   // user_task_ is what the user wants to be run at desired_run_time_.
   base::Closure user_task_;
 
   // The estimated time that the MessageLoop will run the scheduled_task_ that
   // will call RunScheduledTask(). This time can be a "zero" TimeTicks if the
   // task must be run immediately.
   TimeTicks scheduled_run_time_;
 
   // The desired run time of user_task_. The user may update this at any time,
   // even if their previous request has not run yet. If desired_run_time_ is
   // greater than scheduled_run_time_, a continuation task will be posted to
   // wait for the remaining time. This allows us to reuse the pending task so as
   // not to flood the MessageLoop with orphaned tasks when the user code
   // excessively Stops and Starts the timer. This time can be a "zero" TimeTicks
   // if the task must be run immediately.
   TimeTicks desired_run_time_;
 
-  // Thread ID of current MessageLoop for verifying single-threaded usage.
-  int thread_id_;
+  // Verify usage with a single SequencedTaskRunner.

[gab, 2015/11/17 15:23:57]

This comment in redundant in specifying what a SequenceChecker is IMO.

Instead I think it should mention what needs to be on the same sequence and why.

[jsbell, 2015/12/01 00:23:10]

Done.

+  SequenceChecker sequence_checker_;
+
+  // Once the timer has been scheduled, the task runner may not be changed.
+  bool was_scheduled_;
 
   // Repeating timers automatically post the task again before calling the task
   // callback.
   const bool is_repeating_;
 
   // If true, hold on to the user_task_ closure object for reuse.
   const bool retain_user_task_;
 
   // If true, user_task_ is scheduled to run sometime in the future.
   bool is_running_;
@@ skipping 39 matching lines @@
 
 //-----------------------------------------------------------------------------
 // A simple, repeating timer.  See usage notes at the top of the file.
 class RepeatingTimer : public BaseTimerMethodPointer {
  public:
   RepeatingTimer() : BaseTimerMethodPointer(REPEATING) {}
 };
 
 //-----------------------------------------------------------------------------
 // A Delay timer is like The Button from Lost. Once started, you have to keep
-// calling Reset otherwise it will call the given method in the MessageLoop
-// thread.
+// calling Reset otherwise it will call the given method in task runner.
 //
 // Once created, it is inactive until Reset is called. Once |delay| seconds have
 // passed since the last call to Reset, the callback is made. Once the callback
 // has been made, it's inactive until Reset is called again.
 //
 // If destroyed, the timeout is canceled and will not occur even if already
 // inflight.
 class DelayTimer : protected Timer {
  public:
   template <class Receiver>
@@ skipping 13 matching lines @@
 // to link in MSVC. But clang-plugin does not allow inline definitions of
 // virtual methods, so the inline definition lives in the header file here
 // to satisfy both.
 inline void DelayTimer::Reset() {
   Timer::Reset();
 }
 
 }  // namespace base
 
 #endif  // BASE_TIMER_TIMER_H_