Support async cacheResults in custom testSequencer

[yairhaimo]

Summary

PR #8642 makes it so that the sort method of a testSequencer can be async in order to support use cases where you fetch test data from an async endpoint. This PR aims to complement it by adding the possibility to store the cache in an async fashion as well.

The entire PR comes down to adding the await keyword to the cacheResults method call.

Test plan

I am opening this PR as a draft PR because I want to consult with the community on how I should test this feature.
At first I added more tests to the customTestSequencers.test.ts suite. Those tests are e2e tests that spawn a process which will wait for the async operation to end before exiting, making the async process work eventually, thus making the test useless.

Instead, I thought about adding unit tests but currently there are no tests for customSequencers other than the aforementioned e2e. The unit tests for the test_sequencer.test.js test the default implementation and I think adding tests for a custom one will make a mess of it all.

Where and how should I test this?
Thanks

[SimenB]

I'm not sure how to best test this... Since it's fire and forget it's not clean indeed. What happens if you return a rejected promise (which was previously ignored) - that should propagate properly now which it didn't before. Maybe that's enough?

[jeysal]

This'll look a bit weird, but if you have a "only works if a cache is there" sequencer with a cacheResults impl of

new Promise(resolve => {
  setTimeout(() => {
    writeFileSomewhere();
    resolve();
  }), 1000).unref()
})

and a sort impl of

if (fileWrittenIsNotThere()) throw new Error();

and runJest twice and assert that the second run passes, that should check that cacheResults was awaited properly 😄

[SimenB]

@yairhaimo ping 🙂

[yairhaimo]

Hi. Closing this for now as I am working on other stuff right now. I'll reopen if its relevant again. Thanks for all your work!

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