Coverage not calculated correctly if coverage report is uploaded without files being present

[undsoft]

We have recently updated our CI workflow to use sharding in Jest to speed up the tests. The flow is something like this:

• Job 1: Checkout, install and build.
• Then job 2 and 3 in parallel: checkout, install and run tests. Each job runs 50% of the tests. Coverage then gets uploaded as an artifact via actions/upload-artifact.
• Job 4: download coverage with actions/download-artifact, merge into a single json and report to Codecov via codecov/codecov-action.

The problem was that job 4 didn't checkout the repo (because it didn't really need those files).
When codecov-action was used without the repo being present, even though the coverage file was uploaded, it always said All modified and coverable lines are covered by tests ✅ and all changes were reported as indirect changes.

File paths were incorrect (extra webui) and clicking on the file resulted in an error:

2024-11-29.10.45.32.mov

Example of a bad job:
https://github.com/truenas/webui/actions/runs/12073223216/job/33669795147?pr=11114

After I added repo checkout as a step, everything started to work correctly:
https://github.com/truenas/webui/actions/runs/12082374244/job/33694460349?pr=11114

You can see my previous CI configuration and fixes in the associated PR. There were no important changes in how coverage file is generated.

While this is a configuration error, I feel like there could be something to improve on Codecov's end:

• It could have given me an error on upload if it really needed files to be present.
• We have patch coverage target set up. It would have been better if it started failing on all jobs. Instead it was always green for a month, before we spotted that something was wrong.