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
@@ skipping 11 matching lines @@
 // the right version for your build configuration.
 class SequenceCheckerDoNothing {
  public:
   bool CalledOnValidSequencedThread() const {
     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 bound to the same sequence or from a single

[danakj, 2016/07/26 19:34:37]

or *to* a single thread, right?

[fdoray, 2016/07/26 20:35:45]

No. I rephrased this comment.

CalledOnValidSequencedThread() will return false if called from 
SingleThreadTaskRunners bound to different sequences, even if they happen to run
on the same thread.

+// thread with no associated sequence token.
 //
 // 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_