Access SO_PASSCRED credentials in handler

[Natureshadow]

While writing a backend for systemd-userdbd, which uses Varlink, I need to authorize user access to the Varlink service depending on the credentials passed over the Unix socket using SO_PASSCRED.

I am not sure how to best do that. Two options that come to mind are:

• Pass the socket to the handler method – in that case, I could try to get the SO_PASSCRED message directly
• Support SO_PASSCRED (and SO_PEERCRED) natively in the Varlink crate, and provide an interface to determine client credentials

Some advice or thoughts on how it could be done would be welcome.