TaskScheduler: Add SequenceToken and ScopedSetSequenceTokenForCurrentThread

base/sequence_checker.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.
 
 #ifndef BASE_SEQUENCE_CHECKER_H_
 #define BASE_SEQUENCE_CHECKER_H_
 
 // See comments for the similar block in thread_checker.h.
 #if (!defined(NDEBUG) || defined(DCHECK_ALWAYS_ON))
 #define ENABLE_SEQUENCE_CHECKER 1
 #else
 #define ENABLE_SEQUENCE_CHECKER 0
 #endif
 
 #include "base/sequence_checker_impl.h"
 
 namespace base {
 
 // Do nothing implementation, for use in release mode.
 //
 // Note: You should almost always use the SequenceChecker class to get
 // the right version for your build configuration.
 class SequenceCheckerDoNothing {
  public:
   bool CalledOnValidSequencedThread() const {

[gab, 2016/07/28 15:16:41]

Just had a post-commit thought when looking at other code:

CalledOnValidSequencedThread() is the wrong name for this method after this
change, CalledOnValidSequence() would be more appropriate (the old name works
when you verify you're on any thread in a pool, but now that we closely verify
tokens it goes beyond that -- I guess it did verify SWP tokens before but eh
this CL increases this gap).

Plus CalledOnValidSequence() is a shorter and nicer name anyways :-).

WDYT?

[fdoray, 2016/07/28 18:10:04]

I agree! Working on a CL that makes this change.

     return true;
   }
 
   void DetachFromSequence() {}
 };
 
-// SequenceChecker is a helper class used to help verify that some
-// methods of a class are called in sequence -- that is, called from
-// the same SequencedTaskRunner. It is a generalization of
-// ThreadChecker; see comments in sequence_checker_impl.h for details.
+// SequenceChecker is a helper class to verify that calls to some methods of a
+// class are sequenced. Calls are sequenced when they are issued:
+// - From tasks posted to SequencedTaskRunners or SingleThreadTaskRunners bound
+//   to the same sequence, or,
+// - From a single thread outside of any task.

[gab, 2016/07/26 20:49:48]

I find this last statement confusing, MessageLoop as SingleThreadTaskRunners but
they don't have a "sequence" per se (i.e. no SequenceToken), which category are
they in?

[fdoray, 2016/07/28 18:10:04]

I think we don't plan to support child single-threaded TaskRunners in
base/task_scheduler? If that's true, then this comment could become:

[...] Calls are sequenced when they are issued:
 * From tasks posted to SequencedTaskRunners bound to the same sequence.
(task_scheduler and SequencedWorkerPool can have different SequencedTaskRunners
bound to the same sequence)
 * From tasks posted to the same SingleThreadTaskRunner.
 * From a single thread, outside of any task.

[gab, 2016/07/29 13:31:37]

 On 2016/07/28 18:10:04, fdoray wrote:
I don't think this (..) precision is required.
Otherwise yes sgtm (and we can adjust the comment if we decide to allow
SingleThreadTaskRunners to share a sequence later).

 //
 // Example:
 // class MyClass {
 //  public:
 //   void Foo() {
 //     DCHECK(sequence_checker_.CalledOnValidSequencedThread());
 //     ... (do stuff) ...
 //   }
 //
 //  private:
 //   SequenceChecker sequence_checker_;
 // }
 //
 // In Release mode, CalledOnValidSequencedThread() will always return true.
 #if ENABLE_SEQUENCE_CHECKER
 class SequenceChecker : public SequenceCheckerImpl {
 };
 #else
 class SequenceChecker : public SequenceCheckerDoNothing {
 };
 #endif  // ENABLE_SEQUENCE_CHECKER
 
 #undef ENABLE_SEQUENCE_CHECKER
 
 }  // namespace base
 
 #endif  // BASE_SEQUENCE_CHECKER_H_