Implement SequencedTaskChecker.

webrtc/base/sequenced_task_checker.h

Index: webrtc/base/sequenced_task_checker.h
diff --git a/webrtc/base/sequenced_task_checker.h b/webrtc/base/sequenced_task_checker.h
new file mode 100644
index 0000000000000000000000000000000000000000..6976797ade2f6699b78d4c26cca7156beaabb518
--- /dev/null
+++ b/webrtc/base/sequenced_task_checker.h
@@ -0,0 +1,74 @@
+/*
+ *  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef WEBRTC_BASE_SEQUENCED_TASK_CHECKER_H_
+#define WEBRTC_BASE_SEQUENCED_TASK_CHECKER_H_
+
+// Apart from debug builds, we also enable the sequence checker in
+// builds with DCHECK_ALWAYS_ON so that trybots and waterfall bots
+// with this define will get the same level of checking as debug bots.
+//
+// Note that this does not perfectly match situations where RTC_DCHECK is
+// enabled.  For example a non-official release build may have
+// DCHECK_ALWAYS_ON undefined (and therefore SequencedTaskChecker would be
+// disabled) but have RTC_DCHECKs enabled at runtime.
+#if (!defined(NDEBUG) || defined(DCHECK_ALWAYS_ON))
+#define ENABLE_SEQUENCED_TASK_CHECKER 1
+#else
+#define ENABLE_SEQUENCED_TASK_CHECKER 0
+#endif
+
+#include "webrtc/base/checks.h"
+#include "webrtc/base/constructormagic.h"
+#include "webrtc/base/thread_annotations.h"
+#include "webrtc/base/sequenced_task_checker_impl.h"
+
+namespace rtc {
+
+// Do nothing implementation, for use in release mode.
+//
+// Note: You should almost always use the SequencedTaskChecker class to get the
+// right version for your build configuration.
+class SequencedTaskCheckerDoNothing {
+ public:
+  bool IsCurrent() const { return true; }

[tommi, 2016/07/08 13:15:15]

CalledSequentially?  I'm thinking of something a little closer to the
ThreadChecker's CalledOnValidThread.

[perkj_webrtc, 2016/07/11 08:38:00]

Done.

+
+  void Detach() {}
+};
+
+// SequencedTaskChecker is a helper class used to help verify that some methods
+// of a class are called on the same task queue or thread. A
+// SequencedTaskChecker is bound to a a task queue if the object is
+// created on a task queue, or a thread otherwise.
+//
+//
+// Example:
+// class MyClass {
+//  public:
+//   void Foo() {
+//     RTC_DCHECK(sequence_checker_.IsCurrent());
+//     ... (do stuff) ...
+//   }
+//
+//  private:
+//   SequencedTaskChecker sequence_checker_;
+// }
+//
+// In Release mode, CalledOnValidThread will always return true.
+#if ENABLE_SEQUENCED_TASK_CHECKER
+class LOCKABLE SequencedTaskChecker : public SequencedTaskCheckerImpl {};
+#else
+class LOCKABLE SequencedTaskChecker : public SequencedTaskCheckerDoNothing {};
+#endif  // ENABLE_SEQUENCED_TASK_CHECKER_H_
+
+#undef ENABLE_SEQUENCED_TASK_CHECKER
+
+}  // namespace rtc
+#endif  // WEBRTC_BASE_SEQUENCED_TASK_CHECKER_H_