In all PyMUTEX_* and most PyCOND_* functions, all the underlying APIs are void. So there's no point for the functions to return anything other than as convention.

A comment at the start of condvar.h says: "APIs generally return 0 on success and non-zero on error".
In Python\ceval_gil.h:57, there are boilerplace checker macros for them
(which will never fire).

So I guess this is done for uniformity.
Regardless, PyMUTEX_UNLOCK and PyMUTEX_LOCK at least are called both with and without checking the return value.


So I would drop the pretense and make these fns void.

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That said, the reported error still persists after the fix.
The code confusing about which value LeaveNonRecursiveMutex should return on success. Pushed another commit, confirmed to work now.