Select-based, sends periodic keep-alives to keep the connection open, outputs payloads in a way that's friendly to pipe into xargs. (Also the bare bones of a notify-based scheduler).
Without any kind of access controls on NOTIFY channels, nor any kind of payload validation, i just don't feel comfortable using the payload at all. Besides, the payload is hardly necessary given that there's a database on which you can scribble the payload :) It suffices that you receive a notification, and you can then check if there's anything to do.
My version of this doesn't have connection keepalives, but that's ok because that can be added in the form of.... notifications, and the consumer of pqasyncnotifier can implement timeouts. But i agree that timeouts and keepalives would be nice, and even invoking a given SQL function would be nice.
But the question i have is: how to get such functionality integrated into PostgreSQL? Is a standalone program (plus manpage plus Makefile changes) enough, or would a psql \wait command be better?
