Catch illegal hash table modifications when they happen.

third_party/WebKit/Source/wtf/LinkedHashSet.h

Index: third_party/WebKit/Source/wtf/LinkedHashSet.h
diff --git a/third_party/WebKit/Source/wtf/LinkedHashSet.h b/third_party/WebKit/Source/wtf/LinkedHashSet.h
index d4bb0fbf02cdee762844acae5c7963d1475dbfc7..ba05506ad9001697cc84fe13f7fa6e5f9d5a3000 100644
--- a/third_party/WebKit/Source/wtf/LinkedHashSet.h
+++ b/third_party/WebKit/Source/wtf/LinkedHashSet.h
@@ -279,8 +279,10 @@ public:
     template<typename VisitorDispatcher>
     void trace(VisitorDispatcher visitor) { m_impl.trace(visitor); }
 
-    int64_t modifications() const { return m_impl.modifications(); }
-    void checkModifications(int64_t mods) const { m_impl.checkModifications(mods); }
+#if ENABLE(ASSERT)
+    void enterModificationForbiddenScope() { m_impl.enterModificationForbiddenScope(); }
+    void leaveModificationForbiddenScope() { m_impl.leaveModificationForbiddenScope(); }
+#endif
 
 private:
     Node* anchor() { return reinterpret_cast<Node*>(&m_anchor); }
@@ -416,8 +418,7 @@ protected:
     LinkedHashSetConstIterator(const LinkedHashSetNodeBase* position, const LinkedHashSetType* container)
         : m_position(position)
 #if ENABLE(ASSERT)
-        , m_container(container)
-        , m_containerModifications(container->modifications())
+        , m_modificationForbiddenScope(position ? container : nullptr)
 #endif
     {
     }
@@ -425,7 +426,6 @@ protected:
 public:
     PointerType get() const
     {
-        checkModifications();
         return &static_cast<const Node*>(m_position)->m_value;
     }
     ReferenceType operator*() const { return *get(); }
@@ -434,16 +434,28 @@ public:
     LinkedHashSetConstIterator& operator++()
     {
         ASSERT(m_position);
-        checkModifications();
         m_position = m_position->m_next;
+#if ENABLE(ASSERT)
+        // If now at the end, leave the forbidden scope. Waiting until
+        // the iterator goes out of (stack) scope disallows perfectly valid
+        // code idioms.
+        if (!m_position)
+            m_modificationForbiddenScope.leaveScope();
+#endif
         return *this;
     }
 
     LinkedHashSetConstIterator& operator--()
     {
         ASSERT(m_position);
-        checkModifications();
         m_position = m_position->m_prev;
+#if ENABLE(ASSERT)
+        // If now at the end, leave the forbidden scope. Waiting until
+        // the iterator goes out of (stack) scope disallows perfectly valid
+        // code idioms.
+        if (!m_position)
+            m_modificationForbiddenScope.leaveScope();
+#endif
         return *this;
     }
 
@@ -460,16 +472,13 @@ public:
     }
 
 private:
+    template<typename T, typename U, typename V, typename W> friend class LinkedHashSet;
+    friend class LinkedHashSetIterator<LinkedHashSetType>;
+
     const LinkedHashSetNodeBase* m_position;
 #if ENABLE(ASSERT)
-    void checkModifications() const { m_container->checkModifications(m_containerModifications); }
-    const LinkedHashSetType* m_container;
-    int64_t m_containerModifications;
-#else
-    void checkModifications() const { }
+    HashTableModificationForbiddenScope<LinkedHashSetType> m_modificationForbiddenScope;
 #endif
-    template<typename T, typename U, typename V, typename W> friend class LinkedHashSet;
-    friend class LinkedHashSetIterator<LinkedHashSetType>;
 };
 
 template<typename LinkedHashSetType>