Note the original problem stems when you using "setup.py -j <NJOBS>".

The attached reproducer is a bit of a red herring, because indeed doing fork() + exec() in pure Python is too fragile.  That is why the standard subprocess does it in C.  The problem here is that the `distutils.spawn` module still has its hand-baked fork() + exec() pure Python code from the 1990s.

So the solution to the OP's actual problem should be to recode `distutils.spawn` using the subprocess module.

Original mailing-list discussion at:
https://mail.python.org/archives/list/python-dev@python.org/thread/PTJ3UAZNYBYBYQ4WLMSERZR2XIVWISLM/