[test] Unflake the enabled-features-trace test suite (#96175)

<img width="428" height="179" alt="Screenshot 2026-07-24 at 22 42 04"
src="https://github.com/user-attachments/assets/38478940-d23e-4faa-bd88-5758190be158"
/>

[Flakiness
metrics](https://app.datadoghq.com/ci/test/runs?query=test_level%3Atest%20%40git.repository.id%3A%22github.com%2Fvercel%2Fnext.js%22%20%40test.name%3A%22enabled%20features%20in%20trace%20should%20denormalize%20inherited%20enabled%20features%20during%20upload%22%20%40test.type%3A%22nextjs%22%20%40test.status%3A%28%22fail%22%20OR%20pass%29&agg_m=count&agg_m_source=base&agg_t=count&fromUser=true&index=citest&start=1784320894753&end=1784925694753&paused=false)

The `render-path` span is recorded when a request's response closes,
which is too late for any flush other than the one the dev server
performs while shutting down. The parent `next dev` process escalates to
SIGKILL 100ms after signalling the child, and on a machine running eight
test files at once the child does not reliably get scheduled to run its
cleanup within that window, so the span never reached the trace file and
the upload assertions failed. This change raises the budget for the test
through `NEXT_EXIT_TIMEOUT_MS`, which was added alongside that timeout
in #67165 so that it can be increased when the child's exit work matters
more than a fast exit. The same approach is already used in
`test/e2e/filesystem-cache/warm-restart-task-stats.test.ts`, where the
timeout would otherwise cut off a Rust `on_exit` handler before it
writes its task statistics.

Both test cases previously guarded their request with a check for the
existence of the trace file, which the dev server creates on its own
once the first compile finishes. When that happened before the first
test body ran, neither case issued a request and the trace file
contained no `compile-path` or `render-path` span at all. The request
and the shutdown now happen once in `beforeAll`, and the fixed 500ms
sleep that followed the shutdown is replaced by a `retry` that waits for
the spans the assertions depend on.


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test/development/app-dir/enabled-features-trace/enabled-features-trace.test.ts

@@ -1,14 +1,27 @@
 import { nextTestSetup } from 'e2e-utils'
 import { join } from 'path'
-import { existsSync, readFileSync } from 'fs'
+import { readFileSync } from 'fs'
 import { createServer } from 'http'
 import { spawn } from 'child_process'
+import { retry } from 'next-test-utils'
 import { parseTraceFile } from '../../../lib/parse-trace-file'
 
 describe('enabled features in trace', () => {
   const { next, isNextDev } = nextTestSetup({
     files: __dirname,
     startArgs: ['--no-server-fast-refresh'],
+    env: {
+      // Trace events are buffered in memory, and `render-path` is recorded when
+      // the response closes, too late for any flush other than the one the dev
+      // server performs while shutting down. The parent `next dev` process
+      // escalates to SIGKILL 100ms after signalling the child, and on a busy
+      // machine the child does not reliably get scheduled to run its cleanup
+      // within that window, so the span never reaches the trace file.
+      // `NEXT_EXIT_TIMEOUT_MS` raises that budget for cases like this one,
+      // where the flushed output matters more than how quickly the server
+      // exits.
+      NEXT_EXIT_TIMEOUT_MS: '30000',
+    },
   })
 
   if (!isNextDev) {
@@ -16,42 +29,46 @@ describe('enabled features in trace', () => {
     return
   }
 
-  it('should record enabled features on root span', async () => {
-    const tracePath = join(next.testDir, '.next/dev/trace')
+  let tracePath: string
 
-    // Trigger page request to generate traces
-    if (!existsSync(tracePath)) {
-      const $ = await next.render$('/')
-      expect($('p').text()).toBe('hello world')
-      await next.stop('SIGTERM')
-      await new Promise((resolve) => setTimeout(resolve, 500))
+  beforeAll(async () => {
+    tracePath = join(next.testDir, '.next/dev/trace')
+
+    // Request a page so that the spans under test get recorded. The dev server
+    // creates the trace file on its own as soon as the first compile finishes,
+    // so its existence says nothing about whether this request has happened.
+    const $ = await next.render$('/')
+    const pageText = $('p').text()
+    if (pageText !== 'hello world') {
+      throw new Error(`Unexpected content rendered for "/": ${pageText}`)
     }
 
-    const traceStructure = parseTraceFile(tracePath)
+    // Shutting the server down flushes the buffered events to the trace file.
+    await next.stop('SIGTERM')
+
+    await retry(async () => {
+      const { eventsByName } = parseTraceFile(tracePath)
+      for (const name of ['start-dev-server', 'compile-path', 'render-path']) {
+        if (!eventsByName.has(name)) {
+          throw new Error(`The trace file has no "${name}" span`)
+        }
+      }
+    }, 5000)
+  })
+
+  it('should record enabled features on root span', async () => {
+    const { eventsByName } = parseTraceFile(tracePath)
 
     // Verify start-dev-server span has feature tags
-    const startDevServerEvents =
-      traceStructure.eventsByName.get('start-dev-server')
-    expect(startDevServerEvents).toBeDefined()
-    expect(startDevServerEvents!.length).toBeGreaterThan(0)
-
-    const startDevServerEvent = startDevServerEvents![0]
-    expect(startDevServerEvent.tags).toBeDefined()
-    expect(startDevServerEvent.tags!['feature.serverFastRefreshDisabled']).toBe(
-      true
-    )
+    const [startDevServerEvent] = eventsByName.get('start-dev-server') ?? []
+    expect(startDevServerEvent).toBeDefined()
+    expect(startDevServerEvent?.tags).toBeDefined()
+    expect(
+      startDevServerEvent?.tags?.['feature.serverFastRefreshDisabled']
+    ).toBe(true)
   })
 
   it('should denormalize inherited enabled features during upload', async () => {
-    const tracePath = join(next.testDir, '.next/dev/trace')
-
-    if (!existsSync(tracePath)) {
-      const $ = await next.render$('/')
-      expect($('p').text()).toBe('hello world')
-      await next.stop('SIGTERM')
-      await new Promise((resolve) => setTimeout(resolve, 500))
-    }
-
     const fakeServer = await createTestTraceUploadServer()
 
     // Get trace ID from the trace file
@@ -59,6 +76,7 @@ describe('enabled features in trace', () => {
     const firstLine = traceContent.trim().split('\n')[0]
     const firstEvents = JSON.parse(firstLine)
     const traceId = firstEvents[0]?.traceId
+    expect(traceId).toBeDefined()
 
     const uploaderPath = join(
       __dirname,