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diff --git a/net/disk_cache/file_lock.h b/net/disk_cache/file_lock.h
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+// Copyright 2008, Google Inc.
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//    * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//    * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//    * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+// See net/disk_cache/disk_cache.h for the public interface of the cache.
+
+#ifndef NET_DISK_CACHE_FILE_LOCK_H__
+#define NET_DISK_CACHE_FILE_LOCK_H__
+
+#include "net/disk_cache/disk_format.h"
+
+namespace disk_cache {
+
+// This class implements a file lock that lives on the header of a memory mapped
+// file. This is NOT a thread related lock, it is a lock to detect corruption
+// of the file when the process crashes in the middle of an update.
+// The lock is acquired on the constructor and released on the destructor.
+// The typical use of the class is:
+//    {
+//      BlockFileHeader* header = GetFileHeader();
+//      FileLock lock(header);
+//      header->max_entries = num_entries;
+//      // At this point the destructor is going to release the lock.
+//    }
+// It is important to perform Lock() and Unlock() operations in the right order,
+// because otherwise the desired effect of the "lock" will not be achieved. If
+// the operations are inlined / optimized, the "locked" operations can happen
+// outside the lock.
+class FileLock {
+ public:
+  explicit FileLock(BlockFileHeader* header);
+  ~FileLock() {
+    Unlock();
+  }
+  // Virtual to make sure the compiler never inlines the calls.
+  virtual void Lock();
+  virtual void Unlock();
+ private:
+  bool acquired_;
+  volatile int32* updating_;
+};
+
+}  // namespace disk_cache
+
+#endif  // NET_DISK_CACHE_FILE_LOCK_H__