Use update-alternatives(8) to set default greeter

[Daniel Richard G.]

This is a wishlist item for LightDM in Oneiric.

Currently, LightDM uses the /usr/lib/lightdm/lightdm-set-defaults command for packages to change the default user session and greeter. If unity-greeter is not installed, and /etc/lightdm/lightdm.conf specifies the default (lightdm-gtk-greeter), then installing unity-greeter will update the .conf file to use same by invoking the aforementioned command in its postinst script. (Likewise, when unity-greeter is removed, the prerm script sets things back the way they were.)

This sort of behavior is already standardized in Debian/Ubuntu via the update-alternatives(8) mechanism. I think it would be good to consider using that infrastructure rather than rolling one's own. Presumably, there would be a /usr/share/xgreeters/default-lightdm-greeter alternative that points to whatever the "best" LightDM greeter is installed; lightdm.conf would then specify "default-lightdm-greeter" unless it has been manually modified by the user to use something else explicitly.

Ideally, the same could be done for user-session, but this would require all available sessions to be represented in the update-alternatives system (not currently the case as I understand) and so would be a larger project to coordinate.

[Didier Roche-Tolomelli (didrocks)]

This is not something I can commit to for this cycle, due to the benefit/risk.
We need, if we want to do that, having this for greeter and sessions so that this work makes sense. Then, ensuring that the update-aternatives can be run as part of busybox in the casper session of the livecd (this wasn't working on lucid, hence why this solution wasn't picked at the time, maybe it changed?), enabling changing again at the end of ubiquity intallation (to set another default session, like in case of a11y choice). The other requirements (working on postinst/postrm should be straightforward).

So, in a summary: patches welcomed, but ensure we can have the same solution for sessions and greeters, as well that all works as expected for the live session and after installation.