Support binding to file descriptors passed to the process

[drewwells]

Systemd can open root level ports and pass a socket to another process. This is helpful because the program can run in user-level permission but appear to be listening to port 80.

See golang port for an example of the idea: https://github.com/librespeed/speedtest-go/pull/35/files#diff-17bd1293d66c18344655b66a75ac56c5798e7407a1670834166b12ee3585ad2b

Rust similar code to activate this

use listenfd::ListenFd;
use std::net::TcpListener;

fn main() {
    let mut listenfd = ListenFd::from_env();
    
    // Try to get socket from systemd first
    let listener = match listenfd.take_tcp_listener(0).unwrap() {
        // Got socket from systemd
        Some(l) => l,
        // No systemd socket, create our own
        None => TcpListener::bind("0.0.0.0:80").unwrap()
    };
}

[SudoDios]

Yes, good idea 👍