headersTimeout on Agent appears to be broken since 6.20.0

[Stono]

Bug Description

Using fetch with an undici agent with configuration such as new Agent({ headersTimeout: 1}) on 6.19.8 would correctly throw a UND_ERR_HEADERS_TIMEOUT error whereas on 6.20.0 and 6.20.1 the request just hangs.

This was caught by some tests we have around our http client that ensures timeouts work, and they started failing after the undici bump.

Reproducible By

As above really.

Expected Behavior

UND_ERR_HEADERS_TIMEOUT to throw on a headers timeout.

Logs & Screenshots

Environment

node v22.5.1
undici v6.20.x

Additional context

[ronag]

Du you have a full repro?

[Stono]

import express from 'express'
import should from 'should'
import { Agent, fetch } from 'undici'

describe.only('undici', function () {
  it('headers timeouts', async () => {
    const app = express()
    app.get('/delayed', (req, res) => {
      setTimeout(() => {
        res.json({ ok: true })
      }, 5_000)
    })
    app.listen(3002)

    const fetchPromise = async () => {
      const dispatcher = new Agent({
        connectTimeout: 5000,
        bodyTimeout: 300000,
        headersTimeout: 1
      })

      const result = await fetch('http://127.0.0.1:3002/delayed', {
        redirect: 'manual',
        headers: { accept: 'application/json' },
        method: 'GET',
        keepalive: true,
        dispatcher
      })
      return result.status
    }
    const timeoutPromise = new Promise((resolve) =>
      setTimeout(() => resolve('undici broken?'), 500)
    )

    const result = await Promise.race([fetchPromise(), timeoutPromise])
    should(result).not.eql('undici broken?')
  })
})

On 6.19.8:

On 6.20.1:

[metcoder95]

The behaviour is indeed changed, I couldn't reproduce it your code in isolation; but I was able to figure out that this might has to be something with the work done for the fast timers.

As, if e.g. headersTimeout is set to 1 and the server takes just a few ms to respond with the headers, undici still treats that response as valid; the difference should be notable to the timeout take effect.

For instance, my guess is that maybe mocha does something with timers that messes the fast timers implementation and causes undici to drift some the expected behaviour.

import { createServer } from 'node:http'
import { once } from 'node:events'
import { Agent, fetch } from './index.js' // main & v6.x behaves the same

const app = createServer((req, res) => {
  setTimeout(() => {
    res.writeHead(200, { 'Content-Type': 'text/plain' })
    res.end('hello world')
  }, 10) // Change to 500 or 1000 to cause a UND_ERR_HEADERS_TIMEOUT
})

app.listen(0)

await once(app, 'listening')

const dispatcher = new Agent({
  connectTimeout: 1,
  bodyTimeout: 1,
  headersTimeout: 1
})

const result = await fetch(`http://127.0.0.1:${app.address().port}/delayed`, {
  redirect: 'manual',
  headers: { accept: 'text/plain' },
  method: 'GET',
  keepalive: true,
  dispatcher
})

console.log(result.status, await result?.text())

Between v6.19 and v6.20 the fast timers were changed. v6.19.8...v6.20.1