Provide listen() variant that accepts listener

[Natureshadow]

In order to use systemd socket activation, it would be good to provide a variant of listen that, instead of an address string, accepts a Listener.

[Natureshadow]

Hmm, reading the code, it seems that passed sockets are silently listened on.

[Gunni]

It works fine, just listen on unix:///proc/self/fd/3.

You can test it using something like: systemd-socket-activate -l /tmp/test.sock -- cargo run bonus points by using idle timeout to exit when not needed 😁

This issue can be closed.

[Natureshadow]

It works fine, just listen on `unix:///proc/self/fd/3`.

And to find out that the fd is 3, I use a dice ;)?

[Gunni]

And to find out that the fd is 3, I use a dice ;)?

It's part of systemd socket activation https://www.freedesktop.org/software/systemd/man/latest/systemd-socket-activate.html

Otherwise, standard input and output will be inherited, and sockets will be passed through file descriptors 3 and higher.

[ppentchev]

I think there is one single piece missing before this issue can be closed. The current varlink crate goes halfway towards the socket activation - it parses the LISTEN_FDS and LISTEN_FDNAMES environment variables; however, it then still attempts to invoke accept() on the varlink socket, which IMHO is incorrect: I think the socket is supposed to be an already-accepted connection, or at least that's what systemd's varlinkctl program does when invoking a program to communicate with.

I have submitted a pull request, #105, to not invoke accept() in that case.

Thanks for having implemented the parsing part already, though!