Crate listenfd

listenfd is a crate that provides support for working with externally managed and passed file descriptors. This lets you work with systems that support socket activation or similar.

Currently this supports a slightly modified systemd protocl on unix and a custom protocol on Windows. If you want to use this for development you can use the systemfd utility which implements both those protocols.

The systemd extension is that if the LISTEN_PID variable is not set or empty, the check for the pid is removed. This is useful when binaries are proxied in between like cargo-watch. For the windows protocol have a look at the systemfd documentation.

Example

This example shows how to use this crate with an actix-web server:

use listenfd::ListenFd;

let mut listenfd = ListenFd::from_env();
let mut server = make_a_server();

// if we are given a tcp listener on listen fd 0, we use that one
server = if let Some(listener) = listenfd.take_tcp_listener(0)? {
    server.listen(listener)
// otherwise fall back to local listening
} else {
    server.bind("127.0.0.1:3000")?
};

You can then use this with cargo watch and systemfd:

$ cargo install systemfd cargo-watch
systemfd --no-pid -s 3000 -- cargo watch -x run

Now systemfd will open the socket and keep it open. cargo watch will recompile the code on demand and the server will pick up the socket that systemfd opened. No more connection resets.

The --no-pid flag is necessary to ensure that the LISTEN_PID environment variable is not set or the socket passing will be prevented by the pid check.

Structs

ListenFd

A helper object that gives access to raw file descriptors.