We have support for s6-style readiness notification with the pipe/file descriptor. It would be nice to additionally have support for systemd-style readiness notification, at very least for its READY state and perhaps a few others that fit (ignoring the rest). This could be accomplished with something like ready-notification = socket or similar.

The mechanism is fairly simple and generic, and would allow seamless support for everything that already implements systemd readiness notification. It's also somewhat more flexible in terms of what can do the notification; since you only need a socket address (carried in through the NOTIFY_SOCKET environment variable), it can be easily passed down to children and the likes, without having to worry about keeping track of an open file descriptor.