Bug 1309 non-extensible objects have immutable __Proto__

src/objects.cc

 // Copyright 2010 the V8 project authors. 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
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 6848 matching lines @@
 }
 
 
 MaybeObject* JSObject::SetPrototype(Object* value,
                                     bool skip_hidden_prototypes) {
   Heap* heap = GetHeap();
   // Silently ignore the change if value is not a JSObject or null.
   // SpiderMonkey behaves this way.
   if (!value->IsJSObject() && !value->IsNull()) return value;
 
+  // From 8.6.2 Object Internal Methods
+  // ...
+  // In addition, if [[Extensible]] is false the value of the [[Class]] and
+  // [[Prototype]] internal properties of the object may not be modified.
+  // ...
+  // Implementation specific extensions that modify [[Class]], [[Prototype]]
+  // or [[Extensible]] must not violate the invariants defined in the preceding
+  // paragraph.
+  if (!this->map()->is_extensible()) {
+    HandleScope scope;
+    Handle<Object> handle(this, heap->isolate());
+    return heap->isolate()->Throw(
+        *FACTORY->NewTypeError("non_extensible_proto",
+                               HandleVector<Object>(&handle, 1)));
+  }
+
   // Before we can set the prototype we need to be sure
   // prototype cycles are prevented.
   // It is sufficient to validate that the receiver is not in the new prototype
   // chain.
   for (Object* pt = value; pt != heap->null_value(); pt = pt->GetPrototype()) {
     if (JSObject::cast(pt) == this) {
       // Cycle detected.
       HandleScope scope;
       return heap->isolate()->Throw(
           *FACTORY->NewError("cyclic_proto", HandleVector<Object>(NULL, 0)));
@@ skipping 3408 matching lines @@
   if (break_point_objects()->IsUndefined()) return 0;
   // Single beak point.
   if (!break_point_objects()->IsFixedArray()) return 1;
   // Multiple break points.
   return FixedArray::cast(break_point_objects())->length();
 }
 #endif
 
 
 } }  // namespace v8::internal