Disable externalization of sliced/cons strings in old pointer space.

src/api.cc

 // 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 5646 matching lines @@
     isolate->heap()->external_string_table()->AddString(*external);
   }
   return result;
 }
 
 
 bool v8::String::CanMakeExternal() {
   if (!internal::FLAG_clever_optimizations) return false;
   i::Handle<i::String> obj = Utils::OpenHandle(this);
   i::Isolate* isolate = obj->GetIsolate();
+
+  // TODO(yangguo): Externalizing sliced/cons strings allocates.
+  // This rule can be removed when all code that can
+  // trigger an access check is handlified and therefore GC safe.
+  if (isolate->heap()->old_pointer_space()->Contains(*obj)) return false;
+
   if (isolate->string_tracker()->IsFreshUnusedString(obj)) return false;
   int size = obj->Size();  // Byte size of the original string.
   if (size < i::ExternalString::kShortSize) return false;
   i::StringShape shape(*obj);
   return !shape.IsExternal();
 }
 
 
 Local<v8::Object> v8::Object::New() {
   i::Isolate* isolate = i::Isolate::Current();
@@ skipping 1889 matching lines @@
   Isolate* isolate = reinterpret_cast<Isolate*>(info.GetIsolate());
   Address callback_address =
       reinterpret_cast<Address>(reinterpret_cast<intptr_t>(callback));
   VMState<EXTERNAL> state(isolate);
   ExternalCallbackScope call_scope(isolate, callback_address);
   callback(info);
 }
 
 
 } }  // namespace v8::internal