TaskScheduler: Add SequenceToken and ScopedSetSequenceTokenForCurrentThread

base/threading/thread_checker_impl.h

 // Copyright (c) 2011 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_IMPL_H_
 #define BASE_THREADING_THREAD_CHECKER_IMPL_H_
 
 #include "base/base_export.h"
 #include "base/compiler_specific.h"
+#include "base/sequence_token.h"
 #include "base/synchronization/lock.h"
 #include "base/threading/platform_thread.h"
 
 namespace base {
 
-// Real implementation of ThreadChecker, for use in debug mode, or
-// for temporary use in release mode (e.g. to CHECK on a threading issue
-// seen only in the wild).
+// Real implementation of ThreadChecker, for use in debug mode, or for temporary
+// use in release mode (e.g. to CHECK on a threading issue seen only in the
+// wild).
 //
-// Note: You should almost always use the ThreadChecker class to get the
-// right version for your build configuration.
+// Note: You should almost always use the ThreadChecker class to get the right
+// version for your build configuration.
 class BASE_EXPORT ThreadCheckerImpl {
  public:
   ThreadCheckerImpl();
   ~ThreadCheckerImpl();
 
   bool CalledOnValidThread() const WARN_UNUSED_RESULT;
 
   // Changes the thread that is checked for in CalledOnValidThread.  This may
   // be useful when an object may be created on one thread and then used
   // exclusively on another thread.
   void DetachFromThread();
 
  private:
-  void EnsureThreadIdAssigned() const;
+  void EnsureAssigned() const;
 
+  // Members are mutable so that CalledOnValidThread() can set them.
+
+  // Synchronizes access to all members.
   mutable base::Lock lock_;
-  // This is mutable so that CalledOnValidThread can set it.
-  // It's guarded by |lock_|.
-  mutable PlatformThreadRef valid_thread_id_;
+
+  // Thread on which CalledOnValidThread() may return true.
+  mutable PlatformThreadRef thread_id_;
+
+  // SequenceToken for which CalledOnValidThread() may return true. Used to
+  // ensure that CalledOnValidThread() doesn't return true for TaskScheduler
+  // tasks that happen to run on the same thread but weren't posted to the same
+  // SingleThreadTaskRunner.
+  mutable internal::SequenceToken sequence_token_;
 };
 
 }  // namespace base
 
 #endif  // BASE_THREADING_THREAD_CHECKER_IMPL_H_