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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.
+
+#ifndef BASE_LOCK_IMPL_H__
+#define BASE_LOCK_IMPL_H__
+
+#include "base/basictypes.h"
+
+#if defined(WIN32)
+#include <Windows.h>
+#elif defined(__APPLE__)
+#include <libkern/OSAtomic.h>
+#endif
+
+// This class implements the underlying platform-specific spin-lock mechanism
+// used for the Lock class.  Most users should not use LockImpl directly, but
+// should instead use Lock.
+class LockImpl {
+ public:
+  LockImpl();
+  ~LockImpl();
+
+  // If the lock is not held, take it and return true.  If the lock is already
+  // held by something else, immediately return false.
+  bool Try();
+
+  // Take the lock, blocking until it is available if necessary.
+  void Lock();
+
+  // Release the lock.  This must only be called by the lock's holder: after
+  // a successful call to Try, or a call to Lock.
+  void Unlock();
+
+ private:
+#if defined(WIN32)
+  CRITICAL_SECTION critical_section_;
+#elif defined(__APPLE__)
+  OSSpinLock spin_lock_;
+#endif
+
+  DISALLOW_EVIL_CONSTRUCTORS(LockImpl);
+};
+
+class AutoLockImpl {
+ public:
+  AutoLockImpl(LockImpl* lock_impl)
+      : lock_impl_(lock_impl) {
+    lock_impl_->Lock();
+  }
+
+  ~AutoLockImpl() {
+    lock_impl_->Unlock();
+  }
+
+ private:
+  DISALLOW_EVIL_CONSTRUCTORS(AutoLockImpl);
+
+  LockImpl* lock_impl_;
+};
+
+#endif  // BASE_LOCK_IMPL_H__