Oops :-/ Yet another Python 3.5.0 regression, it's now fixed.

os.waitpid() was not tested at all on Windows. os.waitpid() is not the best option to wait for a subprocess completion. By the way, you should use the subprocess module which is more portable, is widely used, etc. IMHO os.spawn*() is more kept for backward compatibility.

> It seems to me this entire loop should be removed. The Windows C runtime doesn't set errno to EINTR. In the case of _cwait it doesn't even use an alertable wait (i.e. it can't be interrupted by a regular asynchronous procedure call).

Well, it looks like you are right: _cwait() cannot be interrupted on Windows. But I chose to only fix the if() after the loop... just in case.