Avoid duplication of a hidden & inherited prototype's properties.

src/runtime.cc

Index: src/runtime.cc
diff --git a/src/runtime.cc b/src/runtime.cc
index 8d84fdace2562f4f6f834940706f298bfdf481a5..eb1e173def6b9eafa95ba9941eda57d81866aa14 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -5726,6 +5726,18 @@ RUNTIME_FUNCTION(MaybeObject*, Runtime_GetPropertyNamesFast) {
 }
 
 
+// Returns true if inherited accessors have been copied down
+// to obj from proto's function template.
+static bool IsInheritedApiFunctionOf(JSObject *obj, Object *proto) {
+  Object *constructor = HeapObject::cast(proto)->map()->constructor();

[rossberg, 2013/12/20 12:54:54]

Note that .constructor is a mutable property, so I'm not sure this test is
actually correct.

+  if (!constructor->IsJSFunction()) return false;
+  JSFunction *constructor_fun = JSFunction::cast(constructor);
+  if (!constructor_fun->shared()->IsApiFunction()) return false;
+  return constructor_fun->shared()
+      ->get_api_func_data()->IsTemplateFor(obj);
+}
+
+
 // Find the length of the prototype chain that is to to handled as one. If a
 // prototype object is hidden it is to be viewed as part of the the object it
 // is prototype for.
@@ -5733,7 +5745,8 @@ static int LocalPrototypeChainLength(JSObject* obj) {
   int count = 1;
   Object* proto = obj->GetPrototype();
   while (proto->IsJSObject() &&
-         JSObject::cast(proto)->map()->is_hidden_prototype()) {
+         JSObject::cast(proto)->map()->is_hidden_prototype() &&
+         !IsInheritedApiFunctionOf(obj, proto)) {

[dcarney, 2013/12/19 19:46:12]

filtering here is problematic as there could be properties higher up the chain
that should be included or even properties set on this proto that should be
included

you need to do the full chain walk here and filter correctly in
Runtime_GetLocalPropertyNames by not inserting duplicate names into the list. 
It'll be expensive, but you only need to pay the cost for objects with hidden
prototype links, which are rare.

[sof, 2013/12/22 08:39:55]

Thanks, the initial version wasn't general enough nor complete. As you suggest,
I've switched to explicitly removing duplicates instead.

Spotting those dupes isn't without cost, but if we assume them being rare,
perhaps acceptable?

     count++;
     proto = JSObject::cast(proto)->GetPrototype();
   }