lib/python/pygrass: fix Python 3.14 pickling error in grid tests

[SyedAhad01]

Fixes PicklingError failures when running tests with Python 3.14.

Python 3.14 cannot pickle local functions defined inside other functions
when using multiprocessing. This fix moves all run_grid_module inner
functions to module-level helper functions and uses functools.partial
to pass parameters.

Related to #6104

[echoix]

Read our contribution guide, you'll see that most of the back and forth of precious shared CI time can be avoided by using precommit (or prek) to have most simple formatting checks already made.

And I assume you're using an AI or agent or something like that, and I saw you were in VS Code in another PR, so you can reread and stage each line to make sure you're only committing what is relevant to the PR. Touching unrelated lines only makes reviewing harder: the last PR together, it started with many many changes, whilst it ended up being less than 10 lines, two parts. Way easier when concise like that.

[SyedAhad01]

Thank you for the feedback and the guidance. I will read the contribution guide and set up pre-commit before pushing further changes. I will also make sure to carefully review each line and only commit what is directly relevant to the fix

[echoix]

Don't worry, your general approach is fine. There's already some examples in the repo of the pattern needed, that I've fixed as a proof of concept a while ago. And it looked a bit like that.

[SyedAhad01]

After removing xfail_mp_spawn, all tests now fail with RuntimeError: Subprocess failed with exit code 1 on both macOS and Windows. It seems the issue is inside GridModule itself where the internal worker functions are not picklable under spawn. Could you point me to the example in the repo that shows the correct pattern for fixing this? I want to make sure I apply the right fix

[SyedAhad01]

I investigated the failures further and it looks like the issue is not in the test wrapper but inside GridModule itself, likely due to multiprocessing spawn requiring top-level worker functions.

I’ll explore the GridModule implementation to identify non-picklable functions and propose a fix.

Please let me know if there is an existing pattern in the repo I should follow.

[echoix]

For the tests, it’s about right. For the other part, I’m not sure there’s a pattern yet, and that is the core of the work, to investigate what’s happening under the hood. Debugging subprocesses is maybe a bit harder, but you probably did a little bit.

It will be easier for you if you manage to do that locally. That’s not the kind of task that you can do by iterating in a PR.

[SyedAhad01]

Thank you for the feedback. I understand now I should debug locally using spawn mode instead of iterating in the PR. I will reproduce the issue locally on Windows with multiprocessing.set_start_method('spawn', force=True), find the root cause, and then submit a clean fix. I will convert back to "Ready for review" once the local debugging is done.

[echoix]

I just had an idea. In some python code recently-ish (last two years), we ended up needing to pass around env/session around in order to be able to properly initialize c-based tools. Is it possible that one of the functions here need to have that too? Like the failures would’ve been because the environment variables are not defined in the new subrosse

[SyedAhad01]

Thank you for the suggestion! I implemented the fix by passing the environment variables explicitly through the work tuple to cmd_exe. However, the tests are still failing with the same RuntimeError: Subprocess failed with exit code 1 on both macOS and Windows. I am continuing to investigate the issue locally to find the root cause.