Object.oberve: assertions to narrow down flaky crashes with array length mutation.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index d61d3da8c501cb33cc9f84d0bb9d9a9d4ac82159..50323d71c5748c0332c549f8a9855c7ece081d2e 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -1417,6 +1417,12 @@ void JSObject::initialize_elements() {
 
 MaybeObject* JSObject::ResetElements() {
   Object* obj;
+  Map* old_map = map();
+  bool is_observed = old_map->is_observed();
+  // Temporarily disable observation bit, so that invariant forbidding
+  // observation on fast elements isn't violated when transitioning below.
+  // If it was set, we first normalize and then reenable below.
+  if (is_observed) old_map->set_is_observed(false);

[Michael Starzinger, 2012/12/14 11:47:51]

This modifies the old_map before a new one is allocated. If the allocation fails
it will kick you out of this function and on re-entry observation bit is gone.
In short: This function is no longer idempotent.

[rossberg, 2012/12/14 12:18:14]

Indeed. This was way too ugly anyway.

Removed use of NormalizeElements and instead construct an empty dictionary
directly. While this duplicates the special-casing of arguments objects, it
ultimately is simpler than before.

   ElementsKind elements_kind = GetInitialFastElementsKind();
   if (!FLAG_smi_only_arrays) {
     elements_kind = FastSmiToObjectElementsKind(elements_kind);
@@ -1426,12 +1432,14 @@ MaybeObject* JSObject::ResetElements() {
   if (!maybe_obj->ToObject(&obj)) return maybe_obj;
   set_map(Map::cast(obj));
   initialize_elements();
-  if (FLAG_harmony_observation && map()->is_observed()) {
+  if (is_observed) {
     // Maintain invariant that observed elements are always in dictionary mode.
     // For this to work on arrays, we have to make sure to reset length first.
     if (IsJSArray()) JSArray::cast(this)->set_length(Smi::FromInt(0));
     maybe_obj = NormalizeElements();
     if (maybe_obj->IsFailure()) return maybe_obj;
+    old_map->set_is_observed(true);
+    map()->set_is_observed(true);
   }
   return this;
 }
@@ -3362,6 +3370,9 @@ bool Map::owns_descriptors() {
 
 
 void Map::set_is_observed(bool is_observed) {
+  ASSERT(instance_type() < FIRST_JS_OBJECT_TYPE ||
+         instance_type() > LAST_JS_OBJECT_TYPE ||
+         has_slow_elements_kind() || has_external_array_elements());
   set_bit_field3(IsObserved::update(bit_field3(), is_observed));
 }