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diff --git a/Source/JavaScriptCore/heap/MutatorScheduler.h b/Source/JavaScriptCore/heap/MutatorScheduler.h
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+++ b/Source/JavaScriptCore/heap/MutatorScheduler.h
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+/*
+ * Copyright (C) 2017 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 <wtf/FastMalloc.h>
+#include <wtf/MonotonicTime.h>
+#include <wtf/Noncopyable.h>
+
+namespace JSC {
+
+class MutatorScheduler {
+    WTF_MAKE_NONCOPYABLE(MutatorScheduler);
+    WTF_MAKE_FAST_ALLOCATED;
+public:
+    enum State {
+        Normal, // Not collecting.
+        Stopped,
+        Resumed
+    };
+    
+    MutatorScheduler();
+    virtual ~MutatorScheduler();
+    
+    virtual State state() const = 0;
+    
+    virtual void beginCollection() = 0;
+    
+    virtual void didStop();
+    virtual void willResume();
+    
+    // At the top of an iteration, the GC will may call didReachTermination.
+    virtual void didReachTermination();
+    
+    // If it called didReachTermination, it will then later call didExecuteConstraints.
+    virtual void didExecuteConstraints();
+    
+    // After doing that, it will do synchronous draining. When this stalls - either due to timeout or
+    // just 'cause, it will call this.
+    virtual void synchronousDrainingDidStall();
+    
+    virtual MonotonicTime timeToStop() = 0; // Call while resumed, to ask when to stop.
+    virtual MonotonicTime timeToResume() = 0; // Call while stopped, to ask when to resume.
+    
+    virtual void log();
+    
+    bool shouldStop(); // Call while resumed, to ask if we should stop now.
+    bool shouldResume(); // Call while stopped, to ask if we should resume now.
+    
+    virtual void endCollection() = 0;
+};
+
+} // namespace JSC
+