port-file

Facilities for reading and writing port files.

Motivation

In many situations, services don’t have a fixed TCP or UDP port assigned to them. In that case, these services typically bind to port 0. All operating systems use port 0 as a signal to find an available ephemeral port for the service to bind to.

But, if a supervisor process (such as a test) wants to find out the actual port the service was bound to, there must be a protocol to report that information from the service to the supervisor. A port file is one such very simple protocol, using a text file to do this reporting.

A port file is most useful when the service lives in another process from the supervisor. Sometimes, the service is spun up in-process—in that case, it’s likely easier for the supervisor to query the service through some kind of in-memory function call. (But a port file can still be used for external synchronization.)

Terminology

As of this writing, port file is not standard terminology, but it is the term that the author believes communicates the intent best.

Some other terms that have been used for this concept:

• Listening URL file, which usually indicates an entire URL; this crate isn’t necessarily tied to a port (SocketAddr) specifically, and can be used just as well for such a file

• Connection file, as used by Jupyter

• Rendezvous file, which is a term that comes from distributed systems. At Oxide, we have an analogous concept called rendezvous tables (see RFD 541 if you have access).

Usage

We’re going to assume a simple scenario where a parent process asks a child process to bind a single service to TCP port 0.

In the child process, expose the bind address and the port file location as command-line options:

use camino::Utf8PathBuf;
use clap::Parser;
use std::net::{SocketAddr, TcpListener};

/// A service that publishes its bound address to a port file.
#[derive(Parser)]
struct Args {
    /// The address to bind to (use port 0 for an ephemeral port).
    #[arg(long)]
    bind_address: SocketAddr,

    /// If set, the file to publish the bound address to.
    #[arg(long)]
    port_file: Option<Utf8PathBuf>,
}

let args = Args::parse();
let listener = TcpListener::bind(args.bind_address)?;

// Publish the address that was actually bound.
if let Some(path) = &args.port_file {
    port_file::write(path, listener.local_addr()?)?;
}

// Now, serve connections on `listener`.

In the parent process:

use port_file::{PollInterval, Timeout};
use std::{net::SocketAddr, process::Command, time::Duration};

// Pick a location for the port file. A fresh temporary directory keeps
// concurrent runs isolated from each other.
let dir = camino_tempfile::tempdir()?;
let path = dir.path().join("service.port");

// Spawn the child, telling it where to bind and where to write the
// port file.
let mut child = Command::new("my-service")
    .args(["--bind-address", "[::1]:0"])
    .args(["--port-file", path.as_str()])
    .spawn()?;

// Wait for the child to publish its address. Passing `child.try_wait()`
// means a child that dies before binding fails fast rather than hanging
// until the timeout.
let addr: SocketAddr = match port_file::wait_for_blocking(
    &path,
    || child.try_wait(),
    PollInterval(Duration::from_millis(25)),
    Timeout(Duration::from_secs(30)),
) {
    Ok(addr) => addr,
    Err(error) => {
        // The child is not stopped automatically on failure, so remember
        // to kill and reap it so it doesn't outlive the supervisor.
        //
        // In this example, we ignore cleanup errors, since the error
        // produced by wait_for_blocking is the one worth reporting.
        let _ = child.kill();
        let _ = child.wait();
        return Err(error.into());
    }
};

// The service is now reachable at `addr`.
println!("service is listening at {addr}");

// Now you can make requests to addr.

Notes

The port file must not already exist. A write to an existing file will fail with a WriteStage::Persist error. It is strongly recommended that a fresh temporary directory is used to store the port file in.

If you’re using Tokio, use wait_for instead, which is the async equivalent of wait_for_blocking.

Extensions

The port file logic is not tied to SocketAddr–rather, it is generic over any type that implements both FromStr and Display. To communicate information more complicated than a single IP-port pair, use your own type that implements these traits with roundtrip serialization. (This can even be something like a JSON blob.)

Alternatives

Roll it yourself

This crate is quite minimal and straightforward. You’re welcome to write your own version of this. There are a few details this crate gets right that you may want to copy. In particular:

• The write side uses atomic writes and refuses to overwrite existing files.
• The read side has careful handling for permanent errors to avoid waiting out the entire timeout.

In-process services

If your service can run within the same process as the supervisor, you may simply be able to make an in-memory function call to the service to get the bound port.

Query the kernel

You can use tools like ss or lsof on Linux, or equivalents on other operating systems, to see which sockets a child process has active. But this is a heavyweight approach for a purely user-level concern. This approach has also been known to cause process crashes in some situations.

Unix domain sockets

Instead of a TCP or UDP port, you can use a Unix domain socket (UDS), also known as a local socket. If possible, this is the preferred way to bind to ephemeral ports. But this has a few limitations:

• While UDS (despite the name) are available on Windows, as of Rust 1.96 they’re not part of the stable Rust standard library yet. Many ecosystem tools might only support UDS on Unix-like platforms.
• Many tools do not support UDS. For example, it is uncommon for HTTP servers and clients to support UDS as their transport (though there are crates like hyperlocal for this, and reqwest 0.13 has a unix_socket method).
• If UDS are only used in tests, this would introduce a divergence between test and production code. Bugs can often be specific to a particular transport layer.

License

This project is available under the terms of either the Apache 2.0 license or the MIT license.