support listener names for sd-listen

[flokli]

As described in https://mgdm.net/weblog/systemd-socket-activation/, it's possible to receive multiple sockets, and they can have names attached.

This can be used to have multiple .socket files (with different FileDescriptorName= values).

For example, one socket could be used for http, and another one for https connections.

[vi]

How do I test those names and multi-socket mode with systemd-socket-activate (or slimiar tool)?

[flokli]

systemd-socket-activate support multiple sockets, and fdnames.

Rememer you need to kick off one request for it to start.

As an example, my rclone patched to support socket activation:

❯ systemd-socket-activate -l 8088 -l 8089 --fdname=foo:bar -- ./rclone serve webdav :local:test/
Listening on [::]:8088 as 3.
Listening on [::]:8089 as 4.
Communication attempt on fd 3.
Execing ./rclone (./rclone serve webdav :local:test/)
2024/04/24 18:14:42 NOTICE: Local file system at /home/flokli/dev/flokli/rclone/test: WebDav Server started on [sd-listen:bar-0/ sd-listen:foo-0/]

[vi]

What user-facing API do you propose? Something like sd-listen:qqq to explicitly specify one from multiple and err if not found, keeping just sd-listen for the take-arbitrary-one behaviour? Or should plain sd-listen fail if more than one inherited FD is indicated in the environment variables?

[flokli]

I think sd-listen should keep the same behaviour (take-arbitrary-one).

I was mostly thinking about supporting parsing "sd-unix:qqq" to ListenerAddress.

[flokli]

Hmmh, I don't quite like the current sd-listen/sd-listen-unix distinction.

We shouldn't have to (let the user) decide about this, I'd feel more comfortable changing the API to return an enum which is either TcpListener or UnixListener, depending on what was actually passed.

And maybe expose another function giving you an API to enumerate the list of FDs passed, as well as getting their names, in addition to the sd-listen:idx_or_name syntax?

[vi]

How should it determine whether a file descriptor is a Tcp or Unix socket? Using additional syscall (getsockname)?

[edef1c]

That only identifies the address family. You may also want getsockopt(fd, SOL_SOCKET, SO_TYPE) to disambiguate SOCK_{STREAM,DGRAM,SEQPACKET} etc. Not every passed socket is necessarily a connection-based listener.

[vi]

tokio-listener is currently limited to stream and connection-based acceptors by API it exposes.

[vi]

Implemented, including additional feature: sd-listen:* can now accept from all the sockets simultaneously, if appropriate crate feature is enabled (but it is not enabled by default).

[flokli]

Thanks! Just tested it now, looks good. I noticed you did prepare a 0.4.0 version number, however there doesn't seem to be a tag or release published to crates.io yet.

Can you do this? Thanks!

[vi]

Published v0.4.1 with this and also better errors.

[flokli]

Thanks!