gh-126316: Use mutexes in the grp module

[vstinner]

• Issue: grp is not thread safe #126316

[ZeroIntensity]

Actually, we'll need a separate fix for subinterpreters on 3.12, because there's no PyMutex there.

[vstinner]

Actually, we'll need a separate fix for subinterpreters on 3.12, because there's no PyMutex there.

The regular build (not Free Threaded build) is not affected by #126316 since function calls are serialized by the GIL.

[ZeroIntensity]

Yeah, but a subinterpreter can have its own GIL, so if these functions aren't thread safe on a system level, then it won't be thread safe across multiple interpreters. I think we just need to add a Py_mod_multiple_interpreters module slot with Py_MOD_MULTIPLE_INTERPRETERS_SUPPORTED, to indicate that subinterpreters need to use a shared GIL to use the module.

[vstinner]

Yeah, but a subinterpreter can have its own GIL, so if these functions aren't thread safe on a system level, then it won't be thread safe across multiple interpreters.

My change only fix free-threaded build. Should I also change the regular build for this use case?

[ZeroIntensity]

On second thought, I think we should address subinterpreters in another PR for backporting reasons. The fix is what I said with the module slot above.

[vstinner]

#127055 was merged instead. I close this issue.