Basically, Windows EINVAL here looks like UNIX EPIPE. I agree to always
ignore it (remove the check on poll()).

I added the poll() check when I added the EINVAL test because I don't know
well Windows and I didn't know that write () can also fail with EINVAL if
the process is still running.

Your example makes it perfectly clear.

In short, Python must behave the same on Windows and UNIX on your example:
ignore errors on stdin.write().