Use Mojo pipes to signal sync IPC events

mojo/public/cpp/system/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 MOJO_PUBLIC_CPP_SYSTEM_EVENT_H_
+#define MOJO_PUBLIC_CPP_SYSTEM_EVENT_H_
+
+#include "base/macros.h"
+#include "mojo/public/cpp/system/message_pipe.h"
+
+namespace mojo {
+
+// An Event is a simple wrapper around a Mojo message pipe which supports common
+// WaitableEvent-like methods of Signal() and Reset(). All public methods on
+// this object are safe to call from any thread as long as the object is still
+// alive.
+class Event {
+ public:
+  Event();

[yzshen1, 2016/06/07 17:14:51]

Please comment on the initial state, whether the signal is auto-reset, etc.

[Ken Rockot(use gerrit already), 2016/06/07 18:02:25]

Done

+  ~Event();
+
+  // Gets a Handle that can be waited on for this Event. Waiting may be done
+  // synchronously via MojoWait (or indirectly via a wait set), or
+  // asynchronously using a Watcher. When the Event is signaled, this handle
+  // will have |MOJO_HANDLE_SIGNAL_READABLE| satisfied.
+  const Handle& GetHandle() const { return wait_handle_.get(); }
+
+  // Ensures that the Handle returned by |GetHandle()| has the
+  // |MOJO_HANDLE_SIGNAL_READABLE| signal satisfied.
+  void Signal();
+
+  // Ensures that the Handle returned by |GetHandle()| does NOT have the
+  // |MOJO_HANDLE_SIGNAL_READABLE| signal satisfied.
+  void Reset();
+
+ private:
+  ScopedMessagePipeHandle signal_handle_;
+  ScopedMessagePipeHandle wait_handle_;
+
+  DISALLOW_COPY_AND_ASSIGN(Event);
+};
+
+}  // namespace mojo
+
+#endif  // MOJO_PUBLIC_CPP_SYSTEM_EVENT_H_