Make base::Timer sequence-friendly.

base/timer/timer.h

Index: base/timer/timer.h
diff --git a/base/timer/timer.h b/base/timer/timer.h
index 50aedbd4cec28e97c408b2acd8b896c70526732c..f6117336e54b8aa5a591787d82acfc249f30f0a0 100644
--- a/base/timer/timer.h
+++ b/base/timer/timer.h
@@ -39,7 +39,10 @@
 // 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_
@@ -57,26 +60,30 @@
 #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
-  // 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.
+  // 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);
 
@@ -101,9 +108,10 @@ class BASE_EXPORT 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|.
@@ -123,15 +131,6 @@ class BASE_EXPORT Timer {
   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; }
@@ -142,18 +141,10 @@ class BASE_EXPORT Timer {
   bool is_running() const { return is_running_; }
 
  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.
+  // 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();
@@ -161,19 +152,17 @@ class BASE_EXPORT Timer {
   // 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_;
+  // A weak pointer to the BaseTimerTaskInternal associated with this Timer
+  // while it is running. It must only be used to post tasks to that
+  // BaseTimerTaskInternal and 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_;
@@ -196,8 +185,12 @@ class BASE_EXPORT Timer {
   // 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 to run on a
+  // different sequence through SetTaskRunner().
+  SequenceCheckerImpl origin_sequence_checker_;
 
   // Repeating timers automatically post the task again before calling the task
   // callback.
@@ -212,6 +205,8 @@ class BASE_EXPORT Timer {
   // 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);
 };
 
@@ -264,8 +259,7 @@ class RepeatingTimer : public BaseTimerMethodPointer {
 
 //-----------------------------------------------------------------------------
 // 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