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diff --git a/Source/JavaScriptCore/runtime/ConcurrentJSLock.h b/Source/JavaScriptCore/runtime/ConcurrentJSLock.h
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+++ b/Source/JavaScriptCore/runtime/ConcurrentJSLock.h
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+/*
+ * Copyright (C) 2013, 2016 Apple 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.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``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 INC. 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. 
+ */
+
+#pragma once
+
+#include "DeferGC.h"
+#include <wtf/Lock.h>
+#include <wtf/NoLock.h>
+#include <wtf/Optional.h>
+
+namespace JSC {
+
+#if ENABLE(CONCURRENT_JS)
+typedef Lock ConcurrentJSLock;
+typedef LockHolder ConcurrentJSLockerImpl;
+#else
+typedef NoLock ConcurrentJSLock;
+typedef NoLockLocker ConcurrentJSLockerImpl;
+#endif
+
+class ConcurrentJSLockerBase : public AbstractLocker {
+    WTF_MAKE_NONCOPYABLE(ConcurrentJSLockerBase);
+public:
+    explicit ConcurrentJSLockerBase(ConcurrentJSLock& lockable)
+        : m_locker(&lockable)
+    {
+    }
+    explicit ConcurrentJSLockerBase(ConcurrentJSLock* lockable)
+        : m_locker(lockable)
+    {
+    }
+
+    explicit ConcurrentJSLockerBase(NoLockingNecessaryTag)
+        : m_locker(NoLockingNecessary)
+    {
+    }
+
+    ~ConcurrentJSLockerBase()
+    {
+    }
+    
+    void unlockEarly()
+    {
+        m_locker.unlockEarly();
+    }
+
+private:
+    ConcurrentJSLockerImpl m_locker;
+};
+
+class GCSafeConcurrentJSLocker : public ConcurrentJSLockerBase {
+public:
+    GCSafeConcurrentJSLocker(ConcurrentJSLock& lockable, Heap& heap)
+        : ConcurrentJSLockerBase(lockable)
+        , m_deferGC(heap)
+    {
+    }
+
+    GCSafeConcurrentJSLocker(ConcurrentJSLock* lockable, Heap& heap)
+        : ConcurrentJSLockerBase(lockable)
+        , m_deferGC(heap)
+    {
+    }
+
+    ~GCSafeConcurrentJSLocker()
+    {
+        // We have to unlock early due to the destruction order of base
+        // vs. derived classes. If we didn't, then we would destroy the 
+        // DeferGC object before unlocking the lock which could cause a GC
+        // and resulting deadlock.
+        unlockEarly();
+    }
+
+private:
+    DeferGC m_deferGC;
+};
+
+class ConcurrentJSLocker : public ConcurrentJSLockerBase {
+public:
+    ConcurrentJSLocker(ConcurrentJSLock& lockable)
+        : ConcurrentJSLockerBase(lockable)
+#if ENABLE(CONCURRENT_JS) && !defined(NDEBUG)
+        , m_disallowGC(std::in_place)
+#endif
+    {
+    }
+
+    ConcurrentJSLocker(ConcurrentJSLock* lockable)
+        : ConcurrentJSLockerBase(lockable)
+#if ENABLE(CONCURRENT_JS) && !defined(NDEBUG)
+        , m_disallowGC(std::in_place)
+#endif
+    {
+    }
+
+    ConcurrentJSLocker(NoLockingNecessaryTag)
+        : ConcurrentJSLockerBase(NoLockingNecessary)
+#if ENABLE(CONCURRENT_JS) && !defined(NDEBUG)
+        , m_disallowGC(std::nullopt)
+#endif
+    {
+    }
+    
+    ConcurrentJSLocker(int) = delete;
+
+#if ENABLE(CONCURRENT_JS) && !defined(NDEBUG)
+private:
+    std::optional<DisallowGC> m_disallowGC;
+#endif
+};
+
+} // namespace JSC