PHPC-512 quickly addressed the issues of connecting to an IPv6 address (from DNS resolution) when MongoDB was not actually listening on an IPv6 interface by connecting to the server in blocking mode. Restoring async stream initialization will require PHPC to do its own resolution up front and either prefer IPv4 addresses outright or check for a valid connection (instead of accepting an EINPROGRESS response on an IPv6 connection that may later fail during topology checking).