Bug#784604: systemd: can't remove systemd unless it is correctly running

[Brian May]

Package: systemd
Version: 215-17
Severity: normal

As discussed here:
https://lists.debian.org/debian-devel/2015/05/msg00013.html

root at scrooge:/# apt-get remove systemd-sysv
Reading package lists... Done
Building dependency tree      
Reading state information... Done
The following extra packages will be installed:
  cgmanager libcgmanager0 libnih-dbus1 libnih1 systemd-shim
  sysvinit-core
Suggested packages:
  pm-utils
The following packages will be REMOVED:
  systemd-sysv
The following NEW packages will be installed:
  cgmanager libcgmanager0 libnih-dbus1 libnih1 systemd-shim
  sysvinit-core
0 upgraded, 6 newly installed, 1 to remove and 1 not upgraded.
Need to get 493 kB of archives.
After this operation, 1074 kB of additional disk space will be used.
Do you want to continue? [Y/n] y
Adding 'diversion of /usr/share/dbus-1/system-services/org.freedesktop.systemd1.service to /usr/share/dbus-1/system-services/org.freedesktop.systemd1.service.systemd by systemd-shim'
Unpacking systemd-shim (9-1) ...
Processing triggers for systemd (215-17) ...
Failed to get D-Bus connection: No such file or directory
dpkg: error processing package systemd (--unpack):
 subprocess installed post-installation script returned error exit status 1
Processing triggers for man-db (2.7.0.2-5) ...
Processing triggers for dbus (1.8.16-1) ...
Errors were encountered while processing:
 systemd
E: Sub-process /usr/bin/dpkg returned an error code (1)

I think other packages that call systemctl unconditionally in prerm or postrm
scripts will have exactly the same problem.

This doesn't bother me personally, however it could affect other people so am
filing this bug report.