Hello, I haven't find a better bug tracking the systemd problems for the edge cases when someone just accidentally or somewhat deliberately installs the systemd package (but either without systemd-sysv or without rebooting, reexecing init), because in these cases dpkg tries to use systemctl to restart services and complete package setups (postinst scripts), but it can't since systemctl can't connect to the private socket, since systemd only creates that when it runs as pid1 (at least that's my experience).
So, at least users should be warned, but working around this would be much better. Thanks, Pas
