[andor]

Without support for this you can't even reboot cleanly after upgrading init or any of the libraries it uses, because init will keep old deleted executable open and make it impossible to remount root as read-only

Lennart Poettering on how this is done in systemd:

https://plus.google.com/+LennartPoetteringTheOneAndOnly/post...

[makomk]

That's quite a clever solution - except that upon reading what Lennart says in the comments, systemd-shutdown is apparently closely coupled with systemd and receives enough state from it that it cannot be used with any other init system. So I'm not sure it's safe to rely on that for any major upgrade. (Not entirely sure why it needs all that state either.)