Bug#1117610: gdm3: don't create a gdm user

[Simon McVittie]

On Wed, 08 Oct 2025 at 11:35:55 -0400, Jeremy Bícha wrote:
>GDM 49 now uses dynamic users for greeter sessions.

This causes its own problems for us if we cannot rely on libnss_systemd 
working correctly in all installations: see #1116563. But for the 
purposes of this bug let's assume that we can.

>In Debian, I don't believe we
>care about the gdm UID, so a Debian version would look like one of
>these 2 lines, depending on whether we decide to keep the
>Debian-specific group name:
>g Debian-gdm - -
>g gdm - -

Using the 'gdm' group name has the same problem as in #1107944: if your 
name is something like George David Miller, it's completely reasonable 
that you would want to use your initials, gdm, as your own login name 
(which also becomes your user's group name). So we should use Debian-gdm 
(hysterical raisins) or _gdm (as per Policy §9.2.1).

>Then there is this proposal to store dconf overrides in /etc/dconf/gdm.d/
>https://gitlab.gnome.org/GNOME/gdm/-/merge_requests/312

This is great - it would let us stop doing our downstream-specific 
things with /var/lib/gdm3/greeter-dconf-defaults.

I think we'd still need to regenerate /etc/dconf/db/gdm from 
/etc/dconf/db/gdm.d during gdm startup? But perhaps that would be 
upstreamable.

>Upgrades are an extra complication.

Yes. If the user has edited /etc/gdm3/greeter.dconf-defaults, those 
changes should be propagated into the new thing on upgrade. If it's the 
same format (which I think it is) then it could be as simple as doing a 
mv_conffile operation from /etc/gdm3/greeter.dconf-defaults to 
/etc/dconf/db/gdm.d/greeter.dconf-defaults or similar.

     smcv