bpo-46606: os.getgroups() doesn't overallocate

[vstinner]

https://bugs.python.org/issue46606

[vstinner]

Python 3.10 already uses similar code.

@methane: Would you mind to review this change?

[methane]

n+1 can be removed.

But I think n++ should be kept to avoid n = getgropus(0, grouplist) later.
When n=0, getgroups returns number of groups without changing grouplist.
Overallocating one entry is not so bad.

[arhadthedev]

@methane It's easy to fix by surrounding:

n = getgroups(n, grouplist);
if (n == -1) {
    PyMem_Free(grouplist);
    return posix_error();
}

with: if (n > 0) { ... }.

[vstinner]

I modified my PR to not call getgroups() if n=0. I also cleaned up the code to make it easier to follow.

[vstinner]

@arhadthedev: See my updated PR. I rewrote the code more it more "flat", easier to follow for the most common path.

[vstinner]

Thanks for the review @methane! The old code was easy to misunderstand. IMO having a special code path for n == 0 makes the code simpler to review but also more reliable.