[Synchronization] Fix a crash in WaitableEventWatcher

base/win/object_watcher.h

 // Copyright (c) 2011 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_WIN_OBJECT_WATCHER_H_
 #define BASE_WIN_OBJECT_WATCHER_H_
 
 #include <windows.h>
 
 #include "base/base_export.h"
 #include "base/callback.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
 #include "base/sequenced_task_runner.h"
 
 namespace base {
 namespace win {
 
 // A class that provides a means to asynchronously wait for a Windows object to
 // become signaled.  It is an abstraction around RegisterWaitForSingleObject
 // that provides a notification callback, OnObjectSignaled, that runs back on
 // the origin sequence (i.e., the sequence that called StartWatching).
 //
 // This class acts like a smart pointer such that when it goes out-of-scope,
 // UnregisterWaitEx is automatically called, and any in-flight notification is
 // suppressed.
 //
+// The waiting handle MUST NOT be closed while watching is in progress. If this
+// handle is closed while the wait is still pending, the behavior is undefined
+// (see MSDN:RegisterWaitForSingleObject).
+//
 // Typical usage:
 //
 //   class MyClass : public base::win::ObjectWatcher::Delegate {
 //    public:
 //     void DoStuffWhenSignaled(HANDLE object) {
 //       watcher_.StartWatchingOnce(object, this);
 //     }
 //     void OnObjectSignaled(HANDLE object) override {
 //       // OK, time to do stuff!
 //     }
@@ skipping 79 matching lines @@
 
   WeakPtrFactory<ObjectWatcher> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ObjectWatcher);
 };
 
 }  // namespace win
 }  // namespace base
 
 #endif  // BASE_WIN_OBJECT_WATCHER_H_