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diff --git a/chromium/net/disk_cache/blockfile/file_lock.h b/chromium/net/disk_cache/blockfile/file_lock.h
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+++ b/chromium/net/disk_cache/blockfile/file_lock.h
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+// 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.
+
+// See net/disk_cache/disk_cache.h for the public interface of the cache.
+
+#ifndef NET_DISK_CACHE_BLOCKFILE_FILE_LOCK_H_
+#define NET_DISK_CACHE_BLOCKFILE_FILE_LOCK_H_
+
+#include "net/base/net_export.h"
+#include "net/disk_cache/blockfile/disk_format_base.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 NET_EXPORT_PRIVATE FileLock {
+ public:
+  explicit FileLock(BlockFileHeader* header);
+  virtual ~FileLock();
+
+  // 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_BLOCKFILE_FILE_LOCK_H_