Simplistically, put the functions in a thread also. That also allows your select loop to remain responsive to other comings and goings.

Of course, then you need to track what threads you've started on behalf of what clients. and that starts to complicate things a bit. You could have the function threads connect back to the listener to return results, but depending upon how many clients you have it could get expensive spawning threads for short processes.

A better architecture, especially with ithreads, would be to have your longer running functions run in long-running threads with loops reading from function specific queues and writing back to a shared results queue for dispatch. The select loop uses a timeout and monitors the results queue using dequeue_nb().

That's harder to describe than write, but if the tachnique interests you I could probably adapt your example to demonstrate it?