Add tracing AutoOpenCloseEvent.

base/trace_event/auto_open_close_event.h

Index: base/trace_event/auto_open_close_event.h
diff --git a/base/trace_event/auto_open_close_event.h b/base/trace_event/auto_open_close_event.h
new file mode 100644
index 0000000000000000000000000000000000000000..38bc8d54e0b47b56e80342abdb0c56991f5f59ec
--- /dev/null
+++ b/base/trace_event/auto_open_close_event.h
@@ -0,0 +1,61 @@
+// 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_AUTO_OPEN_CLOSE_EVENT_H_
+#define BASE_AUTO_OPEN_CLOSE_EVENT_H_
+
+#include "base/macros.h"
+#include "base/memory/weak_ptr.h"
+#include "base/threading/thread_checker.h"
+#include "base/time/time.h"
+#include "base/trace_event/trace_event.h"
+
+namespace base {
+namespace trace_event {
+
+/**
+ * Class for tracing events that support "auto-opening" and "auto-closing".
+ * "auto-opening" = if the trace event is started (call Begin() before
+ * tracing is started,the trace event will be opened, with the start time
+ * being the time that the trace event was actually started.
+ * "auto-closing" = if the trace event is started but not ended by the time
+ * tracing ends, then the trace event will be automatically closed at the
+ * end of tracing
+ */
+class BASE_EXPORT AutoOpenCloseEvent
+    : public TraceLog::AsyncEnabledStateObserver {
+ public:
+  enum Type {

[oystein (OOO til 10th of July), 2016/09/16 22:22:16]

This seems unused?

[alexandermont, 2016/09/19 20:28:50]

Our plan is to eventually add additional event types to here. We wanted to set
up the API to allow that so we don't change the API when we do that.

+    ASYNC
+  };
+
+  // As in the rest of the tracing macros, the const char* arguments here
+  // must be pointers to indefinitely lived strings (e.g. hard-coded string
+  // literals are okay, but not strings created by c_str())
+  AutoOpenCloseEvent(Type type, const char* category, const char* event_name);
+  ~AutoOpenCloseEvent() override;
+
+  void Begin();
+  void End();
+
+  // AsyncEnabledStateObserver implementation
+  void OnTraceLogEnabled() override;
+  void OnTraceLogDisabled() override;
+
+  // TODO(alexandermont): Add in auto-close in OnBeforeTraceLogDisabled once
+  // the OnBeforeTraceLogDisabled CL lands.

[oystein (OOO til 10th of July), 2016/09/16 22:22:16]

Should this TODO be here, if we don't think there's an actual need for
auto-close? And I agree there shouldn't be, the trace-viewer should handle it
and if it doesn't, that sounds like a bug. The trace-viewer shouldn't silently
ignore async events that only have a begin.

[alexandermont, 2016/09/19 20:28:50]

Removed

+
+ private:
+  const char* const category_;
+  const char* const event_name_;
+  base::TimeTicks start_time_;
+  base::ThreadChecker thread_checker_;
+  WeakPtrFactory<AutoOpenCloseEvent> weak_factory_;
+  DISALLOW_COPY_AND_ASSIGN(AutoOpenCloseEvent);
+};
+
+}  // namespace trace_event
+}  // namespace base
+
+#endif  // BASE_AUTO_OPEN_CLOSE_EVENT_H_