Invalidate cross-thread persistents on heap termination.

Source/platform/heap/PersistentNode.h

Index: Source/platform/heap/PersistentNode.h
diff --git a/Source/platform/heap/PersistentNode.h b/Source/platform/heap/PersistentNode.h
index fa18fa32cff10145a0f7fe1bd0dece9d240dd449..1fae6119f9893007943df7fe7571dd238db99bfb 100644
--- a/Source/platform/heap/PersistentNode.h
+++ b/Source/platform/heap/PersistentNode.h
@@ -44,6 +44,7 @@ public:
     // TraceMethodDelegate in Visitor.h for how this is done.
     void tracePersistentNode(Visitor* visitor)
     {
+        ASSERT(isValid());
         ASSERT(!isUnused());
         ASSERT(m_trace);
         m_trace(visitor, m_self);
@@ -77,6 +78,21 @@ public:
         return !m_trace;
     }
 
+    bool isValid() const
+    {
+        return !(reinterpret_cast<uintptr_t>(m_self) & 0x1);
+    }
+
+    void invalidate()
+    {
+        m_self = reinterpret_cast<Address>(reinterpret_cast<uintptr_t>(m_self) | 0x1);

[haraken, 2015/08/03 00:45:25]

Or I'd prefer just clearing the pointer. The semantics will be:

- CrossThreadPersistent can outlive the thread that has allocated the pointing
object. The outlived CrossThreadPersistent is cleared out when the owner thread
gets detached.

[sof, 2015/08/03 08:30:04]

Can do that instead, that is, clear out the underlying raw pointer of the m_self
here.

+    }
+
+    void* self() const
+    {
+        return m_self;
+    }
+
 private:
     // If this PersistentNode is in use:
     //   - m_self points to the corresponding Persistent handle.
@@ -94,6 +110,7 @@ private:
     PersistentNodeSlots* m_next;
     PersistentNode m_slot[slotCount];
     friend class PersistentRegion;
+    friend class CrossThreadPersistentRegion;
 };
 
 // PersistentRegion provides a region of PersistentNodes. PersistentRegion
@@ -139,6 +156,8 @@ public:
     int numberOfPersistents();
 
 private:
+    friend CrossThreadPersistentRegion;
+
     void ensurePersistentNodeSlots(void*, TraceCallback);
 
     PersistentNode* m_freeListHead;
@@ -170,6 +189,8 @@ public:
         m_persistentRegion->tracePersistentNodes(visitor);
     }
 
+    void prepareForThreadStateTermination(ThreadState*);
+
 private:
     // We don't make CrossThreadPersistentRegion inherit from PersistentRegion
     // because we don't want to virtualize performance-sensitive methods