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

src/objects-inl.h

 // Copyright 2012 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 1399 matching lines @@
 void JSObject::initialize_elements() {
   ASSERT(map()->has_fast_smi_or_object_elements() ||
          map()->has_fast_double_elements());
   ASSERT(!GetHeap()->InNewSpace(GetHeap()->empty_fixed_array()));
   WRITE_FIELD(this, kElementsOffset, GetHeap()->empty_fixed_array());
 }
 
 
 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);
   }
   MaybeObject* maybe_obj = GetElementsTransitionMap(GetIsolate(),
                                                     elements_kind);
   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;
 }
 
 
 MaybeObject* JSObject::AddFastPropertyUsingMap(Map* map) {
   ASSERT(this->map()->NumberOfOwnDescriptors() + 1 ==
          map->NumberOfOwnDescriptors());
   if (this->map()->unused_property_fields() == 0) {
     int new_size = properties()->length() + map->unused_property_fields() + 1;
@@ skipping 1910 matching lines @@
   set_bit_field3(OwnsDescriptors::update(bit_field3(), is_shared));
 }
 
 
 bool Map::owns_descriptors() {
   return OwnsDescriptors::decode(bit_field3());
 }
 
 
 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));
 }
 
 
 bool Map::is_observed() {
   return IsObserved::decode(bit_field3());
 }
 
 
 void Code::set_flags(Code::Flags flags) {
@@ skipping 2370 matching lines @@
 #undef WRITE_UINT32_FIELD
 #undef READ_SHORT_FIELD
 #undef WRITE_SHORT_FIELD
 #undef READ_BYTE_FIELD
 #undef WRITE_BYTE_FIELD
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_INL_H_