Bug#764607: systemd: systemctl does not re-create display-manager.service symlink

[Andrei POPESCU]

On Vi, 10 oct 14, 00:10:18, Andrei POPESCU wrote:
> On Jo, 09 oct 14, 17:55:18, Andrei POPESCU wrote:
> > 
> > I have to run 'dpkg-reconfigure lightdm' to have lightdm's postinst 
> > recreate the display-manager.service symlink, which is not obvious.
> 
> Actually that wasn't enough to re-enable lightdm, in the end I had to 
> resorte to 'dpkg --purge --force-depends' and then install again.

Given also the recent lxdm bug I've been giving this issue more thought 
and here's what I came up with:

1. Keep the current ExecStartPre= hack until systemd has better support 
for this use case[*].

2a. Each display manager must add a WantedBy=graphical.target

2b. Each display manager must add a Conflicts=[all other DMs]

If I'm not mistaken this will ensure that at least one display manager 
is started (because of the WantedBy=), and only one can run at any given 
time (because of the Conflicts=). Which one would actually start would 
still be controlled via /etc/X11/default-display-manager, same as 
before.

systemctl/update-rc.d enable/disable should then Just Work(tm).

If you think the above makes sense I can start testing it with all 
display managers currently handling the display-manager.service symlink.

[*] See #770633 for a possible implementation of this use case

Kind regards,
Andrei
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