Make base::Timer sequence-friendly.

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.
+// These APIs are not thread safe. Always call from the same sequenced task
+// runner - thread or worker pool. By default the scheduled tasks will be run
+// on the same sequence that the APIs are called from, but this can be changed
+// prior to any tasks being scheduled using SetTaskRunner().
 
 #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 <memory>
 
 #include "base/base_export.h"
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/callback.h"
 #include "base/location.h"
 #include "base/macros.h"
+#include "base/memory/weak_ptr.h"
+#include "base/sequence_checker.h"
 #include "base/time/time.h"
 
 namespace base {
 
 class BaseTimerTaskInternal;
-class SingleThreadTaskRunner;
+class SequencedTaskRunner;
 class TickClock;
 
 //-----------------------------------------------------------------------------
-// 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. A Timer is bound to the sequence on which it is started. To destroy it
+// on its initial sequence (shall it differ) it must first be stopped on the
+// sequence it's bound to. There are DCHECKs in place to verify this. Note:
+// Timer is sequence-affine, but may become thread-affine if coupled with a
+// thread-affine TickClock.
 //
 class BASE_EXPORT Timer {
  public:
-  // Construct a timer in repeating or one-shot mode. Start or SetTaskInfo must
+  // Construct a timer in repeating or one-shot mode. Start 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. If |tick_clock| is
   // provided, it is used instead of TimeTicks::Now() to get TimeTicks when
   // scheduling tasks.
   Timer(bool retain_user_task, bool is_repeating);
   Timer(bool retain_user_task, bool is_repeating, TickClock* tick_clock);
 
   // Construct a timer with retained task info. If |tick_clock| is provided, it
   // is used instead of TimeTicks::Now() to get TimeTicks when scheduling tasks.
   Timer(const tracked_objects::Location& posted_from,
         TimeDelta delay,
         const base::Closure& user_task,
         bool is_repeating);
   Timer(const tracked_objects::Location& posted_from,
         TimeDelta delay,
         const base::Closure& user_task,
         bool is_repeating,
         TickClock* tick_clock);
 
   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
-  // run tasks on the same thread the timer is used on.
-  virtual void SetTaskRunner(scoped_refptr<SingleThreadTaskRunner> task_runner);
+  // only be called while this Timer is stopped. If a TickClock is set, it must
+  // support being used on both the "origin sequence" for this Timer and
+  // |task_runner|.
+  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();
 
   // Call this method to reset the timer delay. The user_task_ must be set. If
   // the timer is not running, this will start it by posting a task.
   virtual void Reset();
 
   const base::Closure& user_task() const { return user_task_; }
   const TimeTicks& desired_run_time() const { return desired_run_time_; }
 
  protected:
   // Returns the current tick count.
   TimeTicks Now() const;
 
-  // Used to initiate a new delayed task.  This has the side-effect of disabling
-  // scheduled_task_ if it is non-null.
-  void SetTaskInfo(const tracked_objects::Location& posted_from,
-                   TimeDelta delay,
-                   const base::Closure& user_task);
-
   void set_user_task(const Closure& task) { user_task_ = task; }
   void set_desired_run_time(TimeTicks desired) { desired_run_time_ = desired; }
   void set_is_running(bool running) { is_running_ = running; }
 
   const tracked_objects::Location& posted_from() const { return posted_from_; }
   bool retain_user_task() const { return retain_user_task_; }
   bool is_repeating() const { return is_repeating_; }
   bool is_running() const { return is_running_; }
 
  private:
-  friend class BaseTimerTaskInternal;
+  // Used to initiate a new delayed task.  This has the side-effect of disabling
+  // scheduled_task_ if it is non-null.
+  void SetTaskInfo(const tracked_objects::Location& posted_from,
+                   TimeDelta delay,
+                   const base::Closure& user_task);
 
   // 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.
+  // with the given |delay|. This Timer must not be running when this is called.
   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();
-
   // 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_;
+  // This pointer must only be used to post Run()/Abandon() tasks through
+  // |scheduled_task_ref_->AsWeakPtr()| to the BaseTimerTaskInternal itself, it
+  // must not be dereferenced nor even used to check for null on the origin task
+  // runner (|is_running_| is used instead to know whether there is a pending
+  // task). Note: for performance benefits it *can* be dereferenced inline iff
+  // GetTaskRunner()->RunsTasksOnCurrentThread().
+  WeakPtr<BaseTimerTaskInternal> scheduled_task_weak_ref_;
 
   // 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_;
+  // task runner for the current sequence is used.
+  scoped_refptr<SequencedTaskRunner> destination_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_;
+  // Calls to the timer are not thread safe, so a checker is used detect
+  // incorrect usage in debug builds. Uses SequenceCheckerImpl to allow usage in
+  // non-DCHECK logic in ~Timer(). Note that this only verifies interactions
+  // with this Timer's API; tasks themselves may be scheduled by on a different
+  // sequence if SetTaskRunner() is used.
+  SequenceCheckerImpl origin_sequence_checker_;
 
   // 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_;
 
   // The tick clock used to calculate the run time for scheduled tasks.
   TickClock* const tick_clock_;
 
   // If true, user_task_ is scheduled to run sometime in the future.
   bool is_running_;
 
+  WeakPtrFactory<Timer> weak_ptr_factory_;
+
   DISALLOW_COPY_AND_ASSIGN(Timer);
 };
 
 //-----------------------------------------------------------------------------
 // This class is an implementation detail of OneShotTimer and RepeatingTimer.
 // Please do not use this class directly.
 class BaseTimerMethodPointer : public Timer {
  public:
   // This is here to work around the fact that Timer::Start is "hidden" by the
   // Start definition below, rather than being overloaded.
@@ skipping 32 matching lines @@
 // A simple, repeating timer.  See usage notes at the top of the file.
 class RepeatingTimer : public BaseTimerMethodPointer {
  public:
   RepeatingTimer() : RepeatingTimer(nullptr) {}
   explicit RepeatingTimer(TickClock* tick_clock)
       : BaseTimerMethodPointer(REPEATING, tick_clock) {}
 };
 
 //-----------------------------------------------------------------------------
 // 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 on the 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 22 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_