The select function does not work for file descriptors with number >= 1024. It has FD_SETSIZE limit (see https://stackoverflow.com/questions/7976388/increasing-limit-of-fd-setsize-and-select#7977082).
So, serve_forever won't work if more than 1024 file descriptors are opened in the process (ValueError: filedescriptor out of range in select() is raised).
Moreover, the select function is considered inefficient, hard to scale and old-fashioned nowadays. Refer to https://stackoverflow.com/questions/970979/what-are-the-differences-between-poll-and-select#3951845.
Better alternative will be to use poll or epoll even.

Better alternative will be to use poll or epoll even.

epoll requires mulitple system calls, whereas the selector seems to only be used only once in most cases, no?

Anyway, maybe selectors.DefaultSelector can be used here?

About poll(): be aware of macOS issues with poll() :-/

I recommend to use selectors.PollSelector and fallback on selectors.SelectSelector where not available (Windows) in order to use 1 syscall only. That's what we did already in socket.py, subprocess.py and others.

I recommend to use selectors.PollSelector and fallback on selectors.SelectSelector where not available (Windows) in order to use 1 syscall only. That's what we did already in socket.py, subprocess.py and others.

It would be nice to have an API in selectors to get a "lightweight" selector, for selector and poll.

This API would be appropriate to check on macOS if poll() works or not: bpo-28087.