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redirecting >11 times causes "MaxListenersExceededWarning" #1711
Description
Bug Description
Each redirect adds one terminated listener to fetchParams and it's never removed.
Reproducible By
import { once } from 'events'
import { createServer } from 'http'
import { fetch } from 'undici'
let redirects = 0
const server = createServer((req, res) => {
if (redirects === 15) {
res.end('Okay goodbye')
return
}
res.writeHead(302, {
Location: `/${redirects++}`
})
res.end()
}).listen(0)
await once(server, 'listening')
const url = `http://localhost:${server.address().port}`
fetch(url, { redirect: 'follow' })Expected Behavior
Logs & Screenshots
MaxListenersExceededWarning: Possible EventEmitter memory leak detected. 11 terminated listeners added to [Fetch]. Use emitter.setMaxListeners() to increase limit
Activity
jonseymour commentedon Feb 23, 2024
Right, but if you have to lift the warning so high that it masks actual leaks you cease to get any benefit from the warning since if there is an actual leak in one of these event sources the warning never fires or if does fires once due a spurious condition then, when an actual leak occurs later, the code fails to emit a second warning because it has already emitted a spurious warning.
Maybe the most pragmatic thing would be to place a rate limit on the number of warnings, but ensure they are issued periodically along with the current number of listeners. That way, an actual leak could be observed (by observing a steady increase in the watermark) without necessarily polluting the logs with too many spurious warnings.
edit: of course, this would likely an upstream change.
Uzlopak commentedon Feb 23, 2024
The problem is solved by @KhafraDev. You can open an issue in node core and talk there about more strategies to improve this.
From the undici perspective the bug will be gone when #2823 is merged.
jonseymour commentedon Feb 23, 2024
Ok, good to see the root cause is being addressed now, thanks.
daveyarwood commentedon Apr 15, 2024
I've been running into what I think is the same or similar issue to what was reported in this issue - lots of MaxListenersExceededWarnings being logged, and my service eventually runs out of memory and dies. When I run with
--trace-warnings, the stacktrace points to Undici.It looks like the fix (#2823) was released in Undici 6.7.0, and I was hopeful that upgrading the dependency in my project would fix my issue. But after updating Undici to 6.13.0, I'm still getting a lot of MaxListenersExceededWarnings:
daveyarwood commentedon Apr 15, 2024
Another note: I did try increasing the default max event listeners to a high number:
And I'm still seeing the warnings come up. Granted, my service is doing a lot of work in parallel (many promises being fired off), so I'm not sure if the problem might be user error. Are there best practices about using this library in many promises concurrently?
daveyarwood commentedon Apr 15, 2024
For context, I am using undici transitively via Elasticsearch.
KhafraDev commentedon Apr 15, 2024
It doesn't look like elasticsearch is using fetch, which is what #2823 fixed the issue for. I would recommend opening up a new issue for this and, if possible, provide a minimal reproducible example.