Control: retitle -2 start-stop-daemon: please implement service readiness protocol
The only supported way in udev to signal readiness is the sd-notify API.
My gut feeling is, that having an sd-notify wrapper for non-systemd
systems (e.g. directly built into start-stop-daemon via say an
--wait-for-sd-notify switch) would be the cleanest and most robust way.
This might even benefit other daemons besides udevd.
Indeed, it seems to me that start-stop-daemon supporting the same service readiness protocol systemd implements[1] would make things easier for udev and other services.
Short description for those not familiar: the service manager (or ssd in this case) sets up a AF_UNIX socket, and passes on the address to the daemon via the NOTIFY_SOCKET environment variable. When the daemon sends a message containing READY=1, then ssd should consider the service up and it can exit. More details at the linked manpage.