test: resolve flaky timeout in issue-3356 (#5188)

Assisted-by: claude:opus-4.6

Signed-off-by: Kamat, Trivikram <16024985+trivikr@users.noreply.github.com>

Author: trivikr

test/issue-3356.js

@@ -10,16 +10,17 @@ const { fetch, Agent, RetryAgent } = require('..')
 test('https://github.com/nodejs/undici/issues/3356', { skip: process.env.CITGM }, async (t) => {
   t = tspl(t, { plan: 3 })
 
-  let shouldRetry = true
+  let requestCount = 0
   const server = createServer({ joinDuplicateHeaders: true })
   server.on('request', (req, res) => {
+    requestCount++
     res.writeHead(200, { 'content-type': 'text/plain' })
-    if (shouldRetry) {
-      shouldRetry = false
-
+    if (requestCount === 1) {
+      // First request: send headers and partial body, then delay the rest
+      // long enough for the bodyTimeout to fire via fast timers
       res.flushHeaders()
       res.write('h')
-      setTimeout(() => { res.end('ello world!') }, 100)
+      setTimeout(() => { res.end('ello world!') }, 3000)
     } else {
       res.end('hello world!')
     }
@@ -29,8 +30,10 @@ test('https://github.com/nodejs/undici/issues/3356', { skip: process.env.CITGM }
 
   await once(server, 'listening')
 
-  const agent = new RetryAgent(new Agent({ bodyTimeout: 50 }), {
-    errorCodes: ['UND_ERR_BODY_TIMEOUT']
+  const agent = new RetryAgent(new Agent({ bodyTimeout: 1500 }), {
+    errorCodes: ['UND_ERR_BODY_TIMEOUT'],
+    minTimeout: 10,
+    maxTimeout: 100
   })
 
   after(async () => {
@@ -44,11 +47,14 @@ test('https://github.com/nodejs/undici/issues/3356', { skip: process.env.CITGM }
     dispatcher: agent
   })
 
-  fastTimersTick()
+  // Advance fast timers to trigger the body timeout.
+  // The fast timer resolution is ~1s, so we need to tick past the 1500ms bodyTimeout.
+  fastTimersTick(2000)
 
   try {
     t.equal(response.status, 200)
-    // consume response
+    // consume response - this should throw because the retry mechanism
+    // cannot transparently retry after headers have already been forwarded
     await response.text()
   } catch (err) {
     t.equal(err.name, 'TypeError')