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diff --git a/Source/WebCore/storage/SQLCallbackWrapper.h b/Source/WebCore/storage/SQLCallbackWrapper.h
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+/*
+ * Copyright (C) 2011 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:
+ *
+ * 1.  Redistributions of source code must retain the above copyright
+ *     notice, this list of conditions and the following disclaimer.
+ * 2.  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.
+ * 3.  Neither the name of Apple Computer, Inc. ("Apple") 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 APPLE AND ITS 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 APPLE OR ITS 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 SQLCallbackWrapper_h
+#define SQLCallbackWrapper_h
+
+#if ENABLE(DATABASE)
+
+#include "CrossThreadTask.h"
+#include "ScriptExecutionContext.h"
+#include <wtf/ThreadingPrimitives.h>
+
+namespace WebCore {
+
+// A helper class to safely dereference the callback objects held by
+// SQLStatement and SQLTransaction on the proper thread. The 'wrapped'
+// callback is dereferenced:
+// - by destructing the enclosing wrapper - on any thread
+// - by calling clear() - on any thread
+// - by unwrapping and then dereferencing normally - on context thread only
+template<typename T> class SQLCallbackWrapper {
+public:
+    SQLCallbackWrapper(PassRefPtr<T> callback, ScriptExecutionContext* scriptExecutionContext)
+        : m_callback(callback)
+        , m_scriptExecutionContext(m_callback ? scriptExecutionContext : 0)
+    {
+        ASSERT(!m_callback || (m_scriptExecutionContext.get() && m_scriptExecutionContext->isContextThread()));
+    }
+
+    ~SQLCallbackWrapper()
+    {
+        clear();
+    }
+
+    void clear()
+    {
+        ScriptExecutionContext* context;
+        T* callback;
+        {
+            MutexLocker locker(m_mutex);
+            if (!m_callback) {
+                ASSERT(!m_scriptExecutionContext);
+                return;
+            }
+            if (m_scriptExecutionContext->isContextThread()) {
+                m_callback = 0;
+                m_scriptExecutionContext = 0;
+                return;
+            }
+            context = m_scriptExecutionContext.release().leakRef();
+            callback = m_callback.release().leakRef();
+        }
+        context->postTask(createCallbackTask(&safeRelease, callback));
+    }
+
+    PassRefPtr<T> unwrap()
+    {
+        MutexLocker locker(m_mutex);
+        ASSERT(!m_callback || m_scriptExecutionContext->isContextThread());
+        m_scriptExecutionContext = 0;
+        return m_callback.release();
+    }
+
+    // Useful for optimizations only, please test the return value of unwrap to be sure.
+    bool hasCallback() const { return m_callback; }
+
+private:
+    static void safeRelease(ScriptExecutionContext* context, T* callback)
+    {
+        ASSERT(callback && context && context->isContextThread());
+        callback->deref();
+        context->deref();
+    }
+
+    Mutex m_mutex;
+    RefPtr<T> m_callback;
+    RefPtr<ScriptExecutionContext> m_scriptExecutionContext;
+};
+
+} // namespace WebCore
+
+#endif // ENABLE(DATABASE)
+
+#endif // SQLCallbackWrapper_h