Added ability to lock strings to prevent their representation or encoding from changing.

src/handles.h

Index: src/handles.h
diff --git a/src/handles.h b/src/handles.h
index 3f4e2293a4e167b3667202caedc81a0f3222316d..8dcca3eab8ab0523901d2614bd4ec208f84abe9a 100644
--- a/src/handles.h
+++ b/src/handles.h
@@ -163,9 +163,11 @@ class HandleScope {
 
 // ----------------------------------------------------------------------------
 // Handle operations.
-// They might invoke garbage collection. The result is an handle to
-// an object of expected type, or the handle is an error if running out
-// of space or encountering an internal error.
+// They might invoke garbage collection. The result is an handle to an
+// object of expected type, or the handle is a null handle if encountering
+// an internal error. Will not return a null handle due to out-of-memory
+// unless otherwise stated, but will instead try to do GC and die fatally
+// if that doesn't help.
 
 void NormalizeProperties(Handle<JSObject> object,
                          PropertyNormalizationMode mode,
@@ -183,7 +185,7 @@ MUST_USE_RESULT Handle<NumberDictionary> NumberDictionarySet(
 void FlattenString(Handle<String> str);
 
 // Flattens a string and returns the underlying external or sequential
-// string.
+// string. Never returns a null handle.
 Handle<String> FlattenGetString(Handle<String> str);
 
 Handle<Object> SetProperty(Handle<JSReceiver> object,
@@ -379,6 +381,22 @@ class NoHandleAllocation BASE_EMBEDDED {
 #endif
 };
 
+
+// Prevents a (non-cons, non-slice) string from having its representation
+// changed. This is just a Handle based wrapper around Heap::LockString.
+// Use Heap::UnlockString to unlock (that one can't cause allocation, so
+// it doesn't need a Handle wrapper).
+void LockString(Handle<String> string);
+
+// Scoped lock on a string.
+class StringLock {
+ public:
+  explicit StringLock(Handle<String> string);
+  ~StringLock();
+ private:
+  Handle<String> string_;
+};
+
 } }  // namespace v8::internal
 
 #endif  // V8_HANDLES_H_