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Can Rau
Can Rau

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Signals in Deno

Yesterday I learned a few things about signals in Deno and how to use them to gracefully terminate a (sub-)subprocess including a running Oak server

Don't panic if your signal panics

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Custom { kind: Other, error: "Refusing to register signal 9" }', runtime/ops/signal.rs:185:67
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
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That's what I got trying to listen to SIGKILL, though thanks to this comment I learned that SIGKILL, SIGSTOP, SIGILL, SIGFPE, and SIGSEGV cannot be registered, as per Rust docs, which is of interest, because Deno runs on Rust πŸš€

Fortunately there's more signals we can choose from and in my case I'm using SIGQUIT which also sounds much friendlier πŸ₯°

Let's get to the actual code

So I changed the listening part of my Oak server to

const controller = new AbortController();
const sig = Deno.signal("SIGQUIT");
sig.then(controller.abort); // abort the controller when receiving the signal
const { signal } = controller;

export const server = async () => {
  await app.listen({ port, hostname, signal });
};
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And the shutDown function which I'm using in my custom CLI to stop after a set duration, to get my break and hold my promises to my partner that I'm ready in 25m πŸ˜…

function shutDown(reason?: string | void) {
  p.kill("SIGQUIT"); // `.kill()` sends the provided signal to the subprocess
  p.close(); // closes the subprocess and frees the memory
  console.log(reason ?? red(bold("Gracefully shutting down")));
  Deno.exit(); // finally terminates the CLI with everything cleaned up
}
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I wrote a much more detailed article about all this on my freshly started blog running on Deno, Oak, SSR, Go & Fly.io
Gracefully close sub-subprocess using signals in Deno πŸ¦•

Many more articles are in the pipeline, so stay tuned πŸ“»

Also please let me know if you have any feedback or question, here in the comments or on Twitter @CanRau

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