debconf prompt about DM to use during natty->oneiric

Thanks for your report.

I confirm this behavior during upgrades of Natty to Oneiric.

I'm trying to think how we can handle this special case without triggering a debconf prompt. Would it be prudent to lower the debconf question priority to "medium", so that we don't see it at all? I can't think of a way for lightdm's .config to detect whether it's installed as part of a dist-upgrade to oneiric, or from scratch on an already installed system.

Converting all *dm packages to have /etc/X11/default-display-manager be handled as an alternative seems too expensive to me.

So I think we are down to two options:

 * Add an upgrade quirk to update-manager to remove gdm. Then you'd only get the question if you use apt-get, which is fair I think. However, it means that you can't restart your session if the upgrade gets interrupted, or after the upgrade finished.

 * Lower debconf priority and make lightdm the default when you install it from scratch. That would mean that lightdm would behave differently than other *dms, and would make itself the default automatically. Given that you just installed it this isn't unreasonable IMHO.

I'm currently leaning towards the second solution. Robert, Michael, WDYT?

Michael,

does update-manager set something that /var/lib/dpkg/info/lightdm.config can read to see whether this is a dist-upgrade instead of a normal package installation? Or can it be changed to export something which indicates that? Or can lightdm.config just check if update-manager is in its parental chain?

Then the db_input could be guarded:

 if [ dist_upgrade running ];
    # set default to lightdm
else
    db_input
fi

I agree that removing gdm is too dangerous. It would be nice to remove it, as upgraders really don't want two display managers lying around after upgrading. I'm guessing nothing will depend on it so an autoremove would get rid of it anyway?

So "don't ask on upgrade" is the best option in my opinion. It would be best if it did ask you if you installed it at a later point (e.g. KDE users don't want KDM automatically replaced without asking).

For my point of view getting a working system after upgrade is more important than choose a display manager!

As you can see in https://bugs.launchpad.net/ubuntu/+source/nvidia-settings/+bug/840462 , the debconf question - where i chose lightdm - left me with an unusable system (infinite loop of xserver when logging in.).

Switching back to gdm (which to found out took me hours) brought back the system.

Discussed with Michael. This should be a fair approximation of what we want:

 if [ "$2" == "" && -n "$RELEASE_UPGRADE_IN_PROGRESS" ]; then
      # do not show question, i. e. priority medium
else
     # show question with high
fi

for lightdm. For gdm we need to not ask at all if RELEASE_UPGRADE_IN_PROGRESS

Fixed in packaging bzr. Robert is going to make a new upstream release tomorrow and upload this.

This bug was fixed in the package lightdm - 0.9.5-0ubuntu2

---------------
lightdm (0.9.5-0ubuntu2) oneiric; urgency=low

  * debian/lightdm.config: When installing from scratch as part of a release
    upgrade, default to lightdm, otherwise ask. (LP: #806559)
  * Add 04_dont_write_files_as_root.patch: Do not write ~/.dmrc and
    ~/.Xauthority as root. [CVE-2011-3349]
  * Add 00upstream_unlock_fix.patch: Only unlock displays if switched to from
    greeter. Cherrypicked from upstream r1137. (LP: #844274)
 -- Martin Pitt <email address hidden> Thu, 15 Sep 2011 08:52:24 +0200

This is not fixed, or has resurfaced. I am getting this prompt when upgrading to xenial (16.4 LTS) I have no idea what was previously selected and do not want to break my system as described in the 2011-09-04 post from apos. What should I choose? How can I find out what was previously selected? Why can't the upgrade process reuse my previous selection?