Prevent GCs when constructing GC mixin objects.

Source/platform/heap/ThreadState.h

 /*
  * Copyright (C) 2013 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
@@ skipping 620 matching lines @@
     void cleanup();
     void cleanupPages();
 
     void unregisterPreFinalizerInternal(void*);
     void invokePreFinalizers(Visitor&);
 
 #if ENABLE(GC_PROFILING)
     void snapshotFreeList();
 #endif
 
+    // By entering a gc-forbidden scope, conservative GCs will not
+    // be allowed while handling an out-of-line allocation request.
+    // Intended used when constructing subclasses of GC mixins, where
+    // the object being constructed cannot be safely traced & marked
+    // fully should a GC be allowed while its subclasses are being
+    // constructed.
+    template<typename U, typename V> friend class AllocateObjectTrait;
+    void enterGCForbiddenScope() { m_gcForbiddenCount++; }
+    void leaveGCForbiddenScope()
+    {
+        ASSERT(m_gcForbiddenCount > 0);
+        m_gcForbiddenCount--;
+    }
+
     static WTF::ThreadSpecific<ThreadState*>* s_threadSpecific;
     static uintptr_t s_mainThreadStackStart;
     static uintptr_t s_mainThreadUnderestimatedStackSize;
     static SafePointBarrier* s_safePointBarrier;
 
     // We can't create a static member of type ThreadState here
     // because it will introduce global constructor and destructor.
     // We would like to manage lifetime of the ThreadState attached
     // to the main thread explicitly instead and still use normal
     // constructor and destructor for the ThreadState class.
     // For this we reserve static storage for the main ThreadState
     // and lazily construct ThreadState in it using placement new.
     static uint8_t s_mainThreadStateStorage[];
 
     ThreadIdentifier m_thread;
     OwnPtr<PersistentNode> m_persistents;
     StackState m_stackState;
     intptr_t* m_startOfStack;
     intptr_t* m_endOfStack;
     void* m_safePointScopeMarker;
     Vector<Address> m_safePointStackCopy;
     bool m_atSafePoint;
     Vector<Interruptor*> m_interruptors;
     bool m_hasPendingIdleTask;
     bool m_didV8GCAfterLastGC;
     bool m_sweepForbidden;
     size_t m_noAllocationCount;
+    size_t m_gcForbiddenCount;
     size_t m_allocatedObjectSizeBeforeGC;
     BaseHeap* m_heaps[NumberOfHeaps];
 
     Vector<OwnPtr<CleanupTask>> m_cleanupTasks;
     bool m_isTerminating;
 
     bool m_shouldFlushHeapDoesNotContainCache;
     double m_collectionRate;
     GCState m_gcState;
 
@@ skipping 82 matching lines @@
         m_locked = false;
     }
 
     MutexBase& m_mutex;
     bool m_locked;
 };
 
 } // namespace blink
 
 #endif // ThreadState_h