Bug#1025349: gdm3: should define the display-manager alias for start dependencies

[Luca Boccassi]

On Mon, 5 Dec 2022 00:15:01 +0000 Simon McVittie <smcv at debian.org>
wrote:
> On Sat, 03 Dec 2022 at 01:03:38 +0100, Samuel Thibault wrote:
> > If gdm happens to manage to start quicker than console-setup,
> > console-setup's setupcon call will fail. So we need a dependency
between
> > the two, to make console-setup start "Before" display managers.
This can
> > be set with
> > 
> > Before=display-manager.service
> > 
> > since display managers usually set this alias. But gdm3 apparently
> > explicitly removes this with the
debian/patches/92_systemd_unit.patch
> > patch:
> > 
> > * Don't install the display-manager.service alias, which is managed
> >   jointly by all Debian display managers via a debconf question
> 
> Upstream gdm3.service (or possibly gdm.service, I think we do some
renaming
> for historical reasons) has
> 
>     [Install]
>     Alias=display-manager.service
> 
> so that `systemctl enable gdm3.service` will create a symlink
> /etc/systemd/system/display-manager.service ->
/lib/systemd/system/gdm3.service.
> This assumes that only one display manager can be enabled at a time,
and
> preferably only one *installed* at a time: if both gdm3 and lightdm
try
> to do that, if I understand correctly, the result will be
unpredictable.
> 
> In Debian, instead of gdm3.service having that, the debconf question
> creates that symlink programmatically if and only if gdm3 is the
selected
> display manager (in particular, the symlink isn't created if you have
> installed both gdm3 and lightdm, and chosen to use lightdm instead).

The same can be achieved by using the Alias mechanism. Simply run
'systemctl enable <selected>.service' instead of creating the symlink
manually.

The problem with manually creating things in maintainer scripts is that
this is not compatible with building image-based systems with usr-only
and ephemeral/first-boot etc. With an alias in the unit, the preset
runs on first boot and re-creates it if needed, for any unit, for any
purpose. Without an alias, a bespoke workaround needs to be added
explicitly for this unit.

Please consider switching the postinst to use this mechanism instead.