Firefox & Vitest Browser Mode Timing Problems (CI)

[muelbr]

Prerequisites

• I confirm my issue is not in the opened issues
• I confirm the Frequently Asked Questions didn't contain the answer to my issue

Environment check

• I'm using the latest msw version
• I'm using Node.js version 20 or higher

Browsers

Firefox

Reproduction repository

https://github.com/muelbr/mswjs-bug

Reproduction steps

I recently reported a bug here: #2709
Currently it seems that only Firefox has some restrictions that can lead to timing problems when run in a CI environment with perhaps not the same CPU power as locally.

Reproduction Repo
Contains: Vue / Vitest Browser Mode / msw / testing-library

• __tests__/index -> Test fixture
• mws.test.ts / mws2.test.ts -> Tests for simple REST calls
• vitest.config -> browser mode config

Test
npm run test:bug -> Test with firefox and maxWorkers: 1 (simulation slow environment)
Some errors start to appear:
SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data

npm run test:all -> Test with all major browsers locally
Everything is fine.

Remark
Just a single test file works, but as soon as there is another test file, the errors are occurring.
Seems that the timing sequence between whole tests is critical with firefox, which is perhaps more restrictive or slow to start the service worker.

Kind regards

Current behavior

2 Test files using Firefox in a slow environment are causing some JSON.parse bugs

Expected behavior

Tests are all successful.

[kettanaito]

Hi, @muelbr. Thanks for reporting this.

Oddly enough, this has the same root cause as #1105. The CLIENT_CLOSED message sent by the closing client (in this case the first, successful test) on beforeunload doesn't behave correctly because the client is gone by the time the worker script runs the check at await self.clients.get(clientId) and does an early return before actually deleting the client from the clients map.

Good news: this is already fixed in #2720 and available in #2692:

npm i https://pkg.pr.new/msw@2692

Bad news: the fix relies on fundamental changes to the worker script, which means it will be released in the next major version to prevent breaking consumers.

If you are blocked by this, I suggest staying on the edge and using the WIP 3.0 release. You can also patch-package the changes from the pull request I linked. Note that the WIP release is potentially unstable so it might be a good idea to pin it to the exact commit, if you are comfortable with doing that.

Nothing else can be done in v2.

[muelbr]

@kettanaito Thanx a lot and sorry for another issue request. It seems that msw 2.x works quite well with chromium, so I currently fine with that. It would be nice to run all 3 major browsers on CI, but I can wait to the official release of V3.
Keep going, great project...

Kind regards