> If Emacs hangs because the file notification system runs out of
> resources (file descriptors or processes), it should detect this by
> itself and revert to other, safer, methods (e.g. the timer-based
> system).

A general support for polling in filenotify.el is not foreseen, too much
effort. But I believe, in the auto-revert case Emacs works already this
way. When file-notify-add-watch returns nil, auto-revert falls back to
polling (to be tested, I'll I add a test).