GDM fails to start at boot after removing LightDM

[Chris Halse Rogers]

I removed LightDM on the 14th and then didn't reboot until after upgrading to Focal. The next reboot didn't launch a display manager at all, and we tracked this down to /etc/systemd/system/display-manager.service still pointing to lightdm.service. This is probably a bug in lightdm {pre,post}rm?

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: lightdm (not installed)
ProcVersionSignature: Ubuntu 5.3.0+bcachefs.git20200106.d763e8ab-1-generic 5.3.13
Uname: Linux 5.3.0+bcachefs.git20200106.d763e8ab-1-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu15
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Fri Jan 24 11:18:25 2020
InstallationDate: Installed on 2019-08-29 (148 days ago)
InstallationMedia: Ubuntu 19.10 bcachefs (20190828)
SourcePackage: lightdm
UpgradeStatus: Upgraded to focal on 2020-01-20 (3 days ago)

[Gunnar Hjalmarsson (gunnarhj)]

Did you forget to make gdm the default DM?

sudo dpkg-reconfigure gdm3

If you did, this does not sound as a bug to me.

[Chris Halse Rogers (raof)]

I do not recall, but uninstalling lightdm should have done this automatically.

[Gunnar Hjalmarsson (gunnarhj)]

On 2020-01-24 12:56, Chris Halse Rogers wrote:
> uninstalling lightdm should have done this automatically.

Can you point at some spot in the documentation where the behavior you claim is correct has been documented? (If not, this may be considered a feature request - importance "wishlist".)

[Chris Halse Rogers (raof)]

debian/lightdm.prerm quite obviously attempts to remove itself from the display manager set.

Also, “remove this non-default package and your system stops booting¹” is a terrible experience.

¹: from an end-user perspective, booting to a black screen is the same as not booting.

[Gunnar Hjalmarsson (gunnarhj)]

On 2020-01-24 13:42, Chris Halse Rogers wrote:
> debian/lightdm.prerm quite obviously attempts to remove itself from
> the display manager set.

Can it be that that script was not updated when we switched to systemd?