Bug#771287: systemd: Multiple DMs can be started at the same time or none under systemd

[Didier Roche]

Package: systemd
Version: 215-7
Severity: serious


We are starting to see multiple bug reports about several display 
managers started at boot. In addition, if 
/etc/X11/default-display-manager if changed manually, we can end up in a 
situation where no display manager is started at all on system boot.

This is due to the fact that some display manager are using init scripts 
only, and the dynamic converted version of this unit is then started. 
Some systemd units as well are not checking 
/etc/X11/default-display-manager (see 
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=748668 for more details).

One solution would be to convert all dms to ship systemd services with 
the same template and /etc/X11/default-display-manager check. One 
side-effect is that if you install multiple dms, you will always end up 
with a Status: degraded in systemctl.

A less invasive solution is to create and patch systemd generators to 
dynamically (at each boot) mask in memory all sysv-init scripts-only 
services that don't match /etc/X11/default-display-manager, and redirect 
the default configured dm to the selected systemd or sysv-init unit.