feature: support hardhat test --parallel flag

[cgewecke]

HH is supporting mocha parallelism soon. Would like to make sure the data is aggregated correctly (not sure how this works).

We have an existing recipe for knitting together reports from different jobs if that's necessary (see #662).

cf: hh 2359

[fvictorio]

There is at least one thing that doesn't work when running the coverage task and enabling parallel tests in mocha: the allowUnlimitedContractSize flag is not being set. And I think any config override just doesn't work in parallel mode.

[cgewecke]

From the HH PR referenced above:

In serial mode all the test files share the same instance of the HRE, but in parallel mode this is not always the case. Mocha uses a pool of workers to execute the tests, and each worker starts with its own instance of the HRE. This means that if one test file deploys a contract, then that deployment will exist in some of the other test files and it won't in others

@fvictorio Does each worker getting its own HRE happen automagically in Mocha or is this something you're setting up in a mocha reporter hook somewhere?

[fvictorio]

It happens because we add something like --require hardhat/register.

If you do node --register hardhat/register script.js, then the script will have a HRE initialized and available. For Hardhat tests in parallel mode, each test file is executed in a similar way. But the main test task (and the ones overriden by solidity-coverage) are in the parent process, and so any change to the HRE is not reflected in the test files.

Does that make sense?

[cgewecke]

@fvictorio Yes makes sense, thanks.

Not sure but suspect that without a provider initialization task hook to override supporting mocha parallel here might be difficult. (If that's too pessimistic lmk).

Another strategy might be:

• detect when the parallel flag is set in mocha options and unset it
• print a warning
• create docs which show recipes for generating coverage in parallel-izable CI environments. More info about that in:
  • this issue about Mocha parallel
  • this issue about Foundry

Does this sound reasonable? (Open to anything fwiw)

(register code is here)

[fvictorio]

Sorry for not responding sooner @cgewecke!

I think the best and easiest solution is to detect that parallel mode is being used and throw an error with instructions? A warning could work too, but maybe it's too easy to miss, and things are not going to work as expected more generally.

[cgewecke]

@fvictorio No worries! Your idea sgtm, will do.

[cgewecke]

With 0.8.6 an error is thrown when parallel is configured for mocha ... the error advises the user to override the option in the mocha config you can add to .solcover.js. There's also a new section in the docs about running coverage in parallel in CI.