Rework FileWatcher to avoid race condition upon deletion.

chrome/browser/file_watcher.h

 // Copyright (c) 2010 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.
 
 // This module provides a way to monitor a file for changes.
 
 #ifndef CHROME_BROWSER_FILE_WATCHER_H_
 #define CHROME_BROWSER_FILE_WATCHER_H_
 #pragma once
 
 #include "base/basictypes.h"
 #include "base/ref_counted.h"
 #include "chrome/browser/chrome_thread.h"
 
 class FilePath;
 
 // This class lets you register interest in changes on a file. The delegate
 // will get called whenever the file is changed, including created or deleted.
 // WARNING: To be able to get create/delete notifications and to work cross
 // platform, we actually listen for changes to the directory containing
 // the file.
 // WARNING: On OSX and Windows, the OS API doesn't tell us which file in the
 // directory changed.  We work around this by watching the file time, but this
 // can result in some extra notifications if we get other notifications within
 // 2s of the file having changed.
 class FileWatcher {
  public:
-  class Delegate {
+  // Declares the callback client code implements to receive notifications. Note
+  // that implementations of this interface should not keep a reference to the
+  // corresponding FileWatcher object to prevent a reference cycle.
+  class Delegate : public base::RefCountedThreadSafe<Delegate> {
    public:
     virtual ~Delegate() {}
     virtual void OnFileChanged(const FilePath& path) = 0;
   };
 
   FileWatcher();
   ~FileWatcher() {}
 
   // Register interest in any changes on the file |path|.
   // OnFileChanged will be called back for each change to the file.  Any
@@ skipping 15 matching lines @@
     virtual bool Watch(const FilePath& path, Delegate* delegate) = 0;
   };
 
  private:
   scoped_refptr<PlatformDelegate> impl_;
 
   DISALLOW_COPY_AND_ASSIGN(FileWatcher);
 };
 
 #endif  // CHROME_BROWSER_FILE_WATCHER_H_