[Synchronization] Fix a crash in WaitableEventWatcher

base/synchronization/waitable_event.h

 // Copyright (c) 2012 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_SYNCHRONIZATION_WAITABLE_EVENT_H_
 #define BASE_SYNCHRONIZATION_WAITABLE_EVENT_H_
 
 #include <stddef.h>
 
 #include "base/base_export.h"
@@ skipping 137 matching lines @@
    protected:
     virtual ~Waiter() {}
   };
 
  private:
   friend class WaitableEventWatcher;
 
 #if defined(OS_WIN)
   win::ScopedHandle handle_;
 #else
-  // On Windows, one can close a HANDLE which is currently being waited on. The
-  // MSDN documentation says that the resulting behaviour is 'undefined', but
-  // it doesn't crash. However, if we were to include the following members
+  // On Windows, you must not close a HANDLE which is currently being waited on.
+  // The MSDN documentation says that the resulting behaviour is 'undefined'.
+  // To solve that issue each WaitableEventWatcher duplicates the given event
+  // handle.
+
+  // However, if we were to include the following members
   // directly then, on POSIX, one couldn't use WaitableEventWatcher to watch an
   // event which gets deleted. This mismatch has bitten us several times now,
   // so we have a kernel of the WaitableEvent, which is reference counted.
   // WaitableEventWatchers may then take a reference and thus match the Windows
   // behaviour.
   struct WaitableEventKernel :
       public RefCountedThreadSafe<WaitableEventKernel> {
    public:
     WaitableEventKernel(ResetPolicy reset_policy, InitialState initial_state);
 
@@ skipping 25 matching lines @@
 
   scoped_refptr<WaitableEventKernel> kernel_;
 #endif
 
   DISALLOW_COPY_AND_ASSIGN(WaitableEvent);
 };
 
 }  // namespace base
 
 #endif  // BASE_SYNCHRONIZATION_WAITABLE_EVENT_H_