support for systemd readiness notification mechanism

[q66]

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.

[capezotte]

Would this feature be restricted to a subset of the platforms, or should we try to make it for all POSIX OSes?

Since (afaik) only Linux attaches credentials to SOCK_DGRAM messages, ¹ on other platforms this would imply multiple sockets (possibly with different permissions each if we want to prevent services from impersonating one another).

Besides, the "readiness notification" mechanism has already expanded the scope to fd-holding too, and there's no way to negotiate the supported feature set or to return errors for these calls other than to close the socket since it's one way from the service to the init system.

If we take the "close the socket after getting READY=1" route, we might as well implement something like dbus-wait-for/s6-notifyoncheck that binds a self-cleaning (abstract) socket, receives and authenticates the message, and exits.

¹See https://github.com/InitWare/InitWare/blob/bcb4aa2d894101b49f142aca0d7941930b5841fa/lib/basic/socket-util.h#L36

[netbsduser]

Since (afaik) only Linux attaches credentials to SOCK_DGRAM messages, on other platforms this would imply multiple sockets (possibly with different permissions each if we want to prevent services from impersonating one another).

Credential passing over datagram sockets is also available on NetBSD, FreeBSD, and DragonFly BSD. OpenBSD does not though some patches, not accepted, seem to have been sent to them to implement it. I am not aware that macOS or Solaris support it, I don't think they do.