Long-lived AbortSignals and undici cause MaxListenersExceededWarnings

[achingbrain]

Version

v20.11.0

Platform

Darwin MacBook-Pro-194.localdomain 23.4.0 Darwin Kernel Version 23.4.0: Fri Mar 15 00:10:42 PDT 2024; root:xnu-10063.101.17~1/RELEASE_ARM64_T6000 arm64

Subsystem

undici

What steps will reproduce the bug?

Making multiple fetch requests with the same AbortSignal causes MaxListenersExceededWarnings to appear in the console with a high number of listeners.

If undici is adding event listeners without removing them, this is a memory leak.

import http from 'http'

const PORT = 49823

http.createServer((req, res) => {
  res.writeHead(200, {'Content-Type': 'text/plain'})
  res.write('Hello World!')
  res.end()
}).listen(PORT)

const controller = new AbortController()

for (let i = 0; i < 200; i++) {
  // make request
  const res = await fetch(`http://localhost:${PORT}`, {
    signal: controller.signal
  })

  // drain body
  await res.text()

  // in theory any added event listeners added by undici should/could be removed
  // now as the request has finished so there's nothing left to abort.
}

There is a repro repo here: https://github.com/achingbrain/node-fetch-maxlistenersexceededwarning

How often does it reproduce? Is there a required condition?

Every time. No required condition.

What is the expected behavior? Why is that the expected behavior?

Undici should remove any added event listeners when there is nothing left to abort (e.g. the request has ended and so either errored or the body has been consumed/cancelled).

What do you see instead?

(node:37396) MaxListenersExceededWarning: Possible EventTarget memory leak detected. 101 abort listeners added to [AbortSignal]. Use events.setMaxListeners() to increase limit
(Use `node --trace-warnings ...` to show where the warning was created)
(node:37396) MaxListenersExceededWarning: Possible EventTarget memory leak detected. 102 abort listeners added to [AbortSignal]. Use events.setMaxListeners() to increase limit
(node:37396) MaxListenersExceededWarning: Possible EventTarget memory leak detected. 103 abort listeners added to [AbortSignal]. Use events.setMaxListeners() to increase limit
...etc

Additional information

A workaround appears to be to create a request-specific signal and abort it if the long lived signal aborts, taking care to remove any added listeners on the long-lived signal when we are done with the request.

See: https://github.com/achingbrain/node-fetch-maxlistenersexceededwarning/blob/main/workaround.js

This may be a duplicate of nodejs/node#52203 - please close this issue if you consider it so, but the workaround above may be of use to someone if there's no way to get undici to clean up after itself.