gh-127081: lock non-re-entrant *pwent calls

[duaneg]

The libc setpwent, getpwent, and endpwent functions are not thread-safe. Protect them with mutexs in free-threading builds.

• Issue: Thread-unsafe libc functions #127081

[ZeroIntensity]

We need a critical section here. Py_CLEAR and Py_DECREF are re-entrant (as in, they can invoke the eval loop), so a mutex is prone to lock-ordering or re-entrancy deadlocks.

[ZeroIntensity]

Actually, sorry, we need to fix subinterpreter thread-safety here too. We can't do that with a critical section. Could you adjust the mutex to go over solely the libc calls?

[duaneg]

Py_CLEAR and Py_DECREF are re-entrant...Could you adjust the mutex to go over solely the libc calls?

Hmm, yeah, good point. I guess we can just defer the decrefs until exit.