Bug#770404: systemd: breaks lightdm, does not start anymore

[Simon McVittie]

On Fri, 21 Nov 2014 at 09:27:57 +0900, Norbert Preining wrote:
> Aehm, since yesterday or so ligthdm does not start anymore, only
> updates related were systemd, lightdm hasn't changed.

To be clear about the situation, you have lightdm, lxdm and xdm installed,
of which you intend to use lightdm; and you have no display managers
other than those three. Is that correct?

Which versions of those packages?

Have you installed or upgraded a display manager since the
last reboot where they worked the way you intended? (see /var/log/apt)

What's in your /etc/X11/default-display-manager?

/etc/systemd/system/display-manager.service should be a symbolic link.
What is its target?

If /etc/X11/default-display-manager and
/etc/systemd/system/display-manager.service have got out of sync,
which I suspect they have, then that's what is causing your bug.

> ==> /var/lib/systemd/deb-systemd-helper-enabled/lxdm.service.dsh-also <==
> /etc/systemd/system/display-manager.service

This looks wrong. I think it might be caused by this in lxdm.service:

    [Install]
    Alias=display-manager.service

Neither gdm3 nor lightdm have that, which suggests that it isn't
meant to be necessary.

I think what's happening is that when you install lxdm, that Alias directive
causes the debhelper snippets in its postinst[1] to break the mechanism
that is meant to arbitrate who owns display-manager.service: the part of
its postinst headed "# set default-display-manager systemd service link"
is correct, but then the #DEBHELPER# snippet runs "systemctl enable lxdm"
which sees the Alias, obeys it, and overwrites the display-manager.service
symlink with an incorrect target.

[1] http://git.lxde.org/gitweb/?p=debian/lxdm.git;a=blob;f=debian/lxdm.postinst;h=b5ee873fab5a8400965329f649029ffbe2fb1aa7;hb=HEAD

systemd maintainers: do you agree with my reasoning, and that that
bit of lxdm.service is wrong? If so, please reassign this to lxdm
(which is conveniently not a blocker for jessie, since it is only
in unstable).

Norbert, if I'm correct, then you should be able to fix this on your
system with `dpkg-reconfigure lightdm` or by correcting the symlink by hand.

    S