Use SequenceChecker to allow Timer to run in SequencedWorkerPool

base/timer/timer.h

Index: base/timer/timer.h
diff --git a/base/timer/timer.h b/base/timer/timer.h
index c5bd9ced5ae277f614fc53b49b83441458ec5cdc..3f2f639e7af04fb455c07d91b7bac9639fbb5c51 100644
--- a/base/timer/timer.h
+++ b/base/timer/timer.h
@@ -39,7 +39,8 @@
 // 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_
@@ -55,17 +56,17 @@
 #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:
@@ -90,8 +91,8 @@ class BASE_EXPORT Timer {
 
   // 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|.
@@ -135,9 +136,9 @@ class BASE_EXPORT Timer {
   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.
@@ -157,8 +158,8 @@ class BASE_EXPORT Timer {
   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_;
@@ -181,8 +182,11 @@ 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_;
+  // 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.
@@ -242,8 +246,7 @@ 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 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