TaskScheduler: Add SequenceToken and ScopedSetSequenceTokenForCurrentThread

base/threading/thread_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_THREADING_THREAD_CHECKER_H_
 #define BASE_THREADING_THREAD_CHECKER_H_
 
 #include "base/logging.h"
 #include "base/threading/thread_checker_impl.h"
 
@@ skipping 45 matching lines @@
 //  public:
 //   void Foo() {
 //     DCHECK(thread_checker_.CalledOnValidThread());
 //     ... (do stuff) ...
 //   }
 //
 //  private:
 //   ThreadChecker thread_checker_;
 // }
 //
+// Note that, when enabled, CalledOnValidThread() returns false when called from
+// tasks posted to SingleThreadTaskRunners bound to different sequences, even if
+// the tasks happen to run on the same thread (e.g. two independent TaskRunners
+// with ExecutionMode::SINGLE_THREADED on the TaskScheduler that happen to share
+// a thread). Also, CalledOnValidThread() returns false when called from a non-
+// single-threaded task which is associated with a SequenceToken (e.g. a task
+// posted with ExecutionMode::PARALLEL or ExecutionMode::SEQUENCED to the

[gab, 2016/07/21 21:14:24]

PARALLEL won't have a SequenceToken -- keeping the example about SEQUENCED is
fine IMO

[fdoray, 2016/07/25 13:24:28]

keeping the example about SEQUENCED to avoid confusion sgtm. But PARALLEL tasks
are in a single-task sequence and hence have a SequenceToken.

[gab, 2016/07/25 14:09:31]

Ah right, but IMO that's an implementation detail of the scheduler (because the
queued type internally happens to be sequences). Don't think we should make API
guarantees about that though.

+// TaskScheduler).
+//
 // In Release mode, CalledOnValidThread will always return true.
 #if ENABLE_THREAD_CHECKER
 class ThreadChecker : public ThreadCheckerImpl {
 };
 #else
 class ThreadChecker : public ThreadCheckerDoNothing {
 };
 #endif  // ENABLE_THREAD_CHECKER
 
 #undef ENABLE_THREAD_CHECKER
 
 }  // namespace base
 
 #endif  // BASE_THREADING_THREAD_CHECKER_H_