Custom Solidity errors throw Uncaught TypeError with Mocha --parallel

[davidtperk]

In Solidity ^0.8.0, reverting with a custom Solidity error while testing in parallel using Mocha throws the following error:

1) Uncaught error outside test suite:
     Uncaught TypeError: Converting circular structure to JSON
    --> starting at object with constructor 'SourceFile'
    |     property 'contracts' -> object with constructor 'Array'
    |     index 0 -> object with constructor 'Contract'
    |     property 'location' -> object with constructor 'SourceLocation'
    --- property 'file' closes the circle
      at stringify (<anonymous>)
      at writeChannelMessage (internal/child_process/serialization.js:120:20)
      at process.target._send (internal/child_process.js:812:17)
      at process.target.send (internal/child_process.js:710:19)
      at /Users/davidperkins/Desktop/hardhat-examples/parallel-tests/node_modules/workerpool/src/worker.js:131:22

Removing the --parallel flag from mocha throws the expected error: Error: VM Exception while processing transaction: reverted with custom error 'MyCustomError()'.

To reproduce, clone my fork of hardhat-examples. In the parallel-tests folder, run yarn followed by yarn test:parallel, once with --parallel in the npm script and once without.

[feuGeneA]

See #2331

[davidtperk]

@fvictorio's parallel-tests already runs successfully with mocha --parallel and "mocha": "^8.3.2", just not with custom Solidity errors. Is --parallel just not "officially" supported?

[fvictorio]

@davidtperk: notice that that example uses a somewhat old version of Hardhat. Are you using the latest one? If so, can you please create a minimal reproducible example?

[davidtperk]

@fvictorio yes, using "hardhat": "^2.8.3". Here's a reproducible example: https://github.com/davidtperk/hardhat-examples/tree/master/parallel-tests. Run yarn test:parallel, once with --parallel in the npm script and once without.

[fvictorio]

Thank you! I'm not sure if we should consider this a Hardhat bug or a bug in my example, but I'll re-open this just in case.

[davidtperk]

Should be a Hardhat bug because I'm having the same issue in my own project.

[fvictorio]

Thanks a lot for finding and reporting this @davidtperk. We were about to release parallel tests support in core, but this is definitely a blocker.

The underlying reason is this issue. The tl;dr is that our errors have circular references, and parallel mode serializes test errors in a way that doesn't support circular references.

[alcuadrado]

This has been fixed in v2.9.0.