add support for socket activation

[flokli]

If nsncd is started using socket activation, use the passed FD instead of manually binding on SOCKET_PATH.

This doesn't change any current behaviour in the currently documented startup mode, but makes nsncd fit to run in a socket-activated environment.

[blinsay]

Looks good, some comments in line. Have you tested it locally with socket activation yet?

[blinsay]

I think it makes slightly more sense to log an error and exit here but I don't feel strongly about it.

[blinsay]

let's throw this in a fn

[flokli]

Looks good, some comments in line. Have you tested it locally with socket activation yet?

I did check it actually starting up, via systemd-socket-activate:

❯ systemd-socket-activate -l /tmp/foobar.sock target/debug/nsncd
Listening on /tmp/foobar.sock as 3.
Communication attempt on fd 3.
Execing target/debug/nsncd (target/debug/nsncd)

I wrote some spam to it, pointing curl to it, and (successfully) got back an error about the wrong protocol version:

May 08 11:43:02.917 DEBG accepted connection, stream: UnixStream { fd: FileDesc(OwnedFd { fd: 4 }), local: "/tmp/foobar.sock" (pathname), peer: (unnamed) }, thread: worker_2
May 08 11:43:02.917 DEBG parsing request, err: wrong protocol version 542393671, thread: worker_2

I have some systemd config to wire it up there, but I ran into some unrelated other issues and didn't get to fully test it yet. I will, happy to mark it as draft until then.

[flokli]

Shelving these efforts for now, let's revisit this once systemd/systemd#30360 has landed in a release, as it'd probably be revisited after that anyways.