Coverage reporting not 100% accurate

[overlookmotel]

Since switched to jest-light-runner (#470), code coverage reporting is no longer working.

jest-light-runner's docs say it only supports Istanbul, but this would probably be a bad idea as that heavily instruments the code, which Livepack is also doing. Need to find a way to get V8 coverage working again.

[overlookmotel]

Actually, coverage information is not accurate.

Coveralls is showing 82% coverage on lib/serialize/arrays.js whereas previously it was 100%. I think 100% is correct.

Old coverage report
New coverage report

[overlookmotel]

Replicated on local machine. Seems to be related to multiple test files running on same thread.

Reports 100% coverage on lib/serialize/arrays.js:

npx jest --coverage test/arrays.test.js test/functions.test.js

Reports 85% coverage:

npx jest --maxWorkers 1 --coverage test/arrays.test.js test/functions.test.js

Reports 85% coverage:

npx jest --coverage test/functions.test.js

So it looks like the coverage from test/arrays.test.js is ignored when another test file runs on same thread. I think test/functions.test.js run first in these scenarios.

Maybe related to virtual fixture files. test/functions.test.js uses these, but test/symbols.test.js doesn't. When test/symbols.test.js and test/arrays.test.js are run together, get 100% coverage reported on both lib/serialize/arrays.js and lib/serialize/symbols.js - which neither test achieves on its own.

[overlookmotel]

Now each test file runs in own worker thread. This has largely fixed the problem. However, it's still not quite right. Coverage report is non-sensical in some places e.g.:

https://coveralls.io/builds/55534172/source?filename=lib%2Fshared%2Ffunctions.js#L157

This shows some lines in a function as executed, but the following lines not, even though there's no conditional or return. If the lines before execute, these lines must too.