It's maybe a little heavy-handed, but is there any technical reason not to just pick an arbitrary FD, like say 3, and just always use that for a particular daemon?
For a given daemon, no, there's no reason not to pick an arbitrary fd and stick to it. That's the intended usage. For a supervisor, the flexibility is a good thing, because daemons may already be using arbitrary fds so it's better not to hardcode anything. -- Laurent