Add tracing AutoOpenCloseEvent.

base/trace_event/persistent_async_event.h

+// Copyright 2016 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_TRACE_EVENT_PERSISTENT_ASYNC_H_
+#define BASE_TRACE_EVENT_PERSISTENT_ASYNC_H_
+
+#include "base/macros.h"
+#include "base/time/time.h"
+#include "base/trace_event/trace_event.h"
+
+namespace base {
+
+namespace trace_event {
+
+// Note: The categories and event names must have application lifetime
+// (i.e. literals), as documented in trace_event_common.h.
+// Thus, we must make subclasses that have hard-coded categories
+// and event names, rather than passing these into the PersistentAsyncEvent.
+
+class BASE_EXPORT PersistentAsyncEvent
+    : public trace_event::TraceLog::EnabledStateObserver {
+ public:
+  PersistentAsyncEvent();
+  ~PersistentAsyncEvent() override;
+
+  void Begin();
+  void End();
+
+  void OnTraceLogEnabled() override;
+  void OnTraceLogDisabled() override;
+
+ protected:
+  virtual void RecordBeginEvent() = 0;
+  virtual void RecordBeginEventWithTimestamp(base::TimeTicks timestamp) = 0;
+  virtual void RecordEndEvent() = 0;
+  void* id() { return id_; }
+
+ private:
+  void* id_;
+  bool active_;

[fmeawad, 2016/09/02 19:43:25]

Should this be atomic, can OnTraceLogEnabled be called on a different thread
from the one the object was created on?

[benjhayden, 2016/09/09 21:54:00]

If a PersistentAsyncEvent subclass is not on the same thread as the TraceLog,
then it would need to use AsyncEnabledStateObserver instead of the
EnabledStateObserver that PAE extends.
So, yes, an AsyncPersistentAsyncEvent *would* need to make active_ atomic.

+  base::TimeTicks start_time_;
+};
+
+}  // namespace trace_event
+}  // namespace base
+
+#endif  // BASE_TRACE_EVENT_PERSISTENT_ASYNC_H_