test: fix race condition in watch mode tests

[mcollina]

Summary

Fixes race condition in watch mode tests that caused intermittent failures.

Problem

The watch mode tests using runInBackground() had a race condition where the stdout "Failed running" message could arrive before stderr was fully collected, causing assertions on stderr content to fail with empty strings.

Solution

When "Failed running" is detected in stdout and stderr is empty, wait for the stderr 'data' event before resolving the promise. This is an event-driven approach rather than timing-based.

Verification

• test-watch-mode-restart-esm-loading-error.mjs: 200 runs, 0 failures
• test-watch-mode.mjs: 50 runs, 0 failures

Refs: nodejs/reliability#1450

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.74%. Comparing base (27a7bbc) to head (19b861d).
⚠️ Report is 21 commits behind head on main.

Additional details and impacted files

@@ Coverage Diff @@## main #61615 +/- ##
==========================================
- Coverage 89.76% 89.74% -0.03% 
==========================================
Files 673 674 +1 Lines 203945 204193 +248 Branches 39191 39266 +75 ==========================================
+ Hits 183076 183257 +181 - Misses 13198 13244 +46 - Partials 7671 7692 +21 

see 41 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/71158/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/71163/

[mcollina]

@lpinca@MoLow I need another approval

[github-actions]

Failed to start CI

 ⚠ Commits were pushed since the last approving review:
⚠ - test: fix race condition in watch mode tests
✘ Refusing to run CI on potentially unsafe PR

https://github.com/nodejs/node/actions/runs/21602719238

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/71175/

[nodejs-github-bot]

Commit Queue failed

- Loading data for nodejs/node/pull/61615
✔ Done loading data for nodejs/node/pull/61615
----------------------------------- PR info ------------------------------------
Title test: fix race condition in watch mode tests (#61615)
Author Matteo Collina <matteo.collina@gmail.com> (@mcollina)
Branch mcollina:fix-flaky-test-watch-mode-restart-esm-loading-error -> nodejs:main
Labels test, author ready
Commits 1
- test: fix race condition in watch mode tests
Committers 1
- Matteo Collina <hello@matteocollina.com>
PR-URL: https://github.com/nodejs/node/pull/61615
Refs: https://github.com/nodejs/reliability/issues/1450
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
------------------------------ Generated metadata ------------------------------
PR-URL: https://github.com/nodejs/node/pull/61615
Refs: https://github.com/nodejs/reliability/issues/1450
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
--------------------------------------------------------------------------------
ℹ This PR was created on Sun, 01 Feb 2026 14:47:30 GMT
✔ Approvals: 2
✔ - Moshe Atlow (@MoLow): https://github.com/nodejs/node/pull/61615#pullrequestreview-3741347346
✔ - Luigi Pinca (@lpinca): https://github.com/nodejs/node/pull/61615#pullrequestreview-3738286904
✘ Last GitHub CI failed
ℹ Last Full PR CI on 2026-02-02T23:05:21Z: https://ci.nodejs.org/job/node-test-pull-request/71175/
- Querying data for job/node-test-pull-request/71175/
✔ Build data downloaded
- Querying failures of job/node-test-commit/85254/
✔ Data downloaded
✘ 1 failure(s) on the last Jenkins CI run
--------------------------------------------------------------------------------
✔ Aborted `git node land` session in /home/runner/work/node/node/.ncu

https://github.com/nodejs/node/actions/runs/21635375308

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/71180/

[nodejs-github-bot]

Landed in 232dffe