Currently, we calculate the diff between the data parameter and m_data, call dataReceived(), then set m_data to data. If something inside dataReceived() tries to access m_data via CachedResource::resourceBuffer(), it will see the old data instead of the new data, which seems inconsistent. To the best of my knowledge, this isn't needed currently, but it's necessary for caching main resources, as much more work will be done on main resource dataReceived() callbacks.