Makes HandleWatcher block until no longer waiting on pipe

mojo/common/handle_watcher.h

 // Copyright 2013 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_COMMON_HANDLE_WATCHER_H_
 #define MOJO_COMMON_HANDLE_WATCHER_H_
 
 #include "base/basictypes.h"
 #include "base/callback_forward.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/run_loop.h"
 #include "mojo/common/mojo_common_export.h"
 #include "mojo/public/cpp/system/core.h"
 
 namespace base {
 class Thread;
 }
 
 namespace mojo {
 namespace common {
 namespace test {
 class HandleWatcherTest;
 }
 
 // HandleWatcher is used to asynchronously wait on a handle and notify a Closure
 // when the handle is ready, or the deadline has expired.
 class MOJO_COMMON_EXPORT HandleWatcher {
  public:
   HandleWatcher();
+
+  // The destructor implicitly stops listening. See Stop() for details.
   ~HandleWatcher();
 
   // Starts listening for |handle|. This implicitly invokes Stop(). In other
   // words, Start() performs one asynchronous watch at a time. It is ok to call
   // Start() multiple times, but it cancels any existing watches. |callback| is
   // notified when the handle is ready, invalid or deadline has passed and is
   // notified on the thread Start() was invoked on. If the current thread exits
   // before the handle is ready, then |callback| is invoked with a result of
   // MOJO_RESULT_ABORTED.
   void Start(const Handle& handle,
              MojoHandleSignals handle_signals,
              MojoDeadline deadline,
              const base::Callback<void(MojoResult)>& callback);
 
-  // Stops listening. Does nothing if not in the process of listening.
+  // Stops listening. Does nothing if not in the process of listening. Blocks
+  // until no longer listening on the handle.
   void Stop();
 
  private:
   class State;
 
   // If non-NULL Start() has been invoked.
   scoped_ptr<State> state_;
 
   DISALLOW_COPY_AND_ASSIGN(HandleWatcher);
 };
 
 }  // namespace common
 }  // namespace mojo
 
 #endif  // MOJO_COMMON_HANDLE_WATCHER_H_