I'm well aware that it does not work in all cases. However it works good enough in most cases. Right now the fileno argument must be considered broken because it leads to wrong results. It is a problem and possible security issue for a couple of use cases, e.g. passing of sockets through AF_UNIX AUX data or systemd socket activation.

On Windows it is less problematic because socket(filno) works correctly with WSAPROTOCOL_INFO. It's only broken for integer fd.

I have considered to set type, family and proto to 0 (unspec) when the getsockopt and getsockname fail.

I have a differnt ticket for the protocol issue, #27816.