Here's a patch with a unittest that reproduces the problem with fixes to stop using any end_fds.  The max fd is only ever used in the absolute fallback situation where no way to get a list of open fd's is available. In that case it is obtained from sysconf() at the time it is needed rather than module load time as sysconf() is async-signal-safe.

I'm not worried about calling close() an additional time on EINTR in the single threaded child process prior to exec().  The most that will happen is one extra call with a different error if the fd is in a bad state from the previous one.  That is better than any chance of one being left open.