Undefined define constants error when --pool=forks

[IanVS]

Describe the bug

I attempted to set --pool='forks' to debug some hanging tests that I've been having intermittently, and started to get a few test failures about global constants not being defined. I'm guessing it's a consequence of running the tests using child_process, but it surprised me a bit nonetheless.

I'm not sure if anything can be done to address this from vitest, but it might be worth at the very least adding a note in the documentation to clarify that define values should not be undefined?

Reproduction

https://github.com/IanVS/reproduction-vitest-define-pool-forks

System Info

System:
    OS: macOS 14.0
    CPU: (12) arm64 Apple M2 Max
    Memory: 1.81 GB / 64.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 18.16.0 - ~/Library/Caches/fnm_multishells/35181_1708624240853/bin/node
    Yarn: 1.22.19 - ~/Library/Caches/fnm_multishells/35181_1708624240853/bin/yarn
    npm: 9.5.1 - ~/Library/Caches/fnm_multishells/35181_1708624240853/bin/npm
    pnpm: 8.8.0 - ~/Library/Caches/fnm_multishells/35181_1708624240853/bin/pnpm
  Browsers:
    Safari: 17.0
  npmPackages:
    vite: ^5.1.4 => 5.1.4
    vitest: ^1.3.1 => 1.3.1

Used Package Manager

pnpm

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• Read the Contributing Guidelines.
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• The provided reproduction is a minimal reproducible example of the bug.

[AriPerkkio]

The difference between these two pools is that child_process uses node:v8.serialize when it's passing the configuration to worker. It seems to lose objects with undefined values - similar as JSON.stringify does. If you change the configuration to define: { __GLOBAL__: 123 } you can see that it is passed.

We already have some work-arounds when passing regexps to test runners through node:v8.serialize. I think similar approach could be used here.