Does not delete unix sockets on shutdown

[Nemo157]

> systemfd -s unix::$XDG_RUNTIME_DIR/ssh-agent.socket -- cargo run -- daemon -s
~> socket /run/user/1000/ssh-agent.socket (unix listener)
   Compiling sshagmux v0.1.0 (/home/nemo157/sources/sshagmux)
    Finished dev [unoptimized + debuginfo] target(s) in 3.02s
     Running `target/debug/sshagmux daemon -s`
2023-10-10T13:56:43.652739Z  INFO sshagmux::app: getting systemd socket
2023-10-10T13:56:43.652898Z  INFO sshagmux::app: bound to /run/user/1000/ssh-agent.socket
...
^C2023-10-10T14:12:13.742422Z  INFO sshagmux: initial SIGINT, shutdown requested

> systemfd -s unix::$XDG_RUNTIME_DIR/ssh-agent.socket -- cargo run -- daemon -s
error: EADDRINUSE: Address already in use

From man systemd.socket:

A daemon listening on an AF_UNIX socket may, but does not need to, call close(2) on the received socket before exiting. However, it must not unlink the socket from a file system.

When you systemctl stop foobar.socket it then unlinks the socket from the file system, so it similarly should be systemfd's responsibility to delete the socket during shutdown so the service can be started again.

[mitsuhiko]

I wonder if it should unlink on startup. Since we exec there is not really a good way to shut down unless I keep a separate process around.

[Nemo157]

Hmmm, I would like if it errored if something else was using the socket (like in the above example I was replacing a systemd managed socket for testing, so if I forgot to systemctl stop it); but if there's not a nice way to do that I guess force overwriting it would be fine. Maybe with a --force flag to make it explicit?

[mitsuhiko]

I think the most common way to do this would be to delete before bind. Maybe it should only be done if nobody uses it.