lightdm-gtk-greeter/lightdm does not honor default session setting for a never logged in user

[dlefevre]

This problem applies to Ubuntu-MATE setups using out-of-the-box lightdm and lightdm-gtk-greeter settings.

To duplicate this bug:

Create a new installation of Ubuntu-MATE. Create two users (user1 and user2).

Install a second windows manager (such as openbox)

Have user1 log in using the openbox windows manager and log out.

Have user2 log in. (Note: user2 is a user who as never logged into machine and has no record of last session used.)

The expected outcome is that user2 will use the default session: MATE, but the outcome is that
user2 will be logged into an openbox session.

lightdm --show-config shows that the default is set to mate:

--
user1@tester:~$ lightdm --show-config
   [LightDM]
A backup-logs=false

   [Seat:*]
B greeter-wrapper=/usr/lib/lightdm/lightdm-greeter-session
C guest-wrapper=/usr/lib/lightdm/lightdm-guest-session
D user-session=mate
E xserver-command=X -core
F greeter-session=lightdm-gtk-greeter

Sources:
A /usr/share/lightdm/lightdm.conf.d/50-disable-log-backup.conf
B /usr/share/lightdm/lightdm.conf.d/50-greeter-wrapper.conf
C /usr/share/lightdm/lightdm.conf.d/50-guest-wrapper.conf
D /usr/share/lightdm/lightdm.conf.d/50-ubuntu-mate.conf
E /usr/share/lightdm/lightdm.conf.d/50-xserver-command.conf
F /usr/share/lightdm/lightdm.conf.d/60-lightdm-gtk-greeter.conf
--

Extra information:

user1@tester:~$ lsb_release -rd
Description: Ubuntu 16.04.1 LTS
Release: 16.04

user1@tester:~$ dpkg -l | grep lightdm
ii liblightdm-gobject-1-0:amd64 1.18.2-0ubuntu1 amd64 LightDM GObject client library
ii lightdm 1.18.2-0ubuntu1 amd64 Display Manager
ii lightdm-gtk-greeter 2.0.1-2ubuntu4 amd64 simple display manager (GTK+ greeter)
ii lightdm-gtk-greeter-settings 1.2.1-0ubuntu1 all settings editor for the LightDM GTK+ Greeter
ii ubuntu-mate-lightdm-theme 16.04.7 all LightDM theme and default configuration for Ubuntu MATE

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: lightdm-gtk-greeter 2.0.1-2ubuntu4
ProcVersionSignature: Ubuntu 4.4.0-31.50-generic 4.4.13
Uname: Linux 4.4.0-31-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
CurrentDesktop: MATE
Date: Fri Feb 24 10:28:16 2017
InstallationDate: Installed on 2017-02-24 (0 days ago)
InstallationMedia: Ubuntu-MATE 16.04.1 LTS "Xenial Xerus" - Release amd64 (20160719)
SourcePackage: lightdm-gtk-greeter
UpgradeStatus: No upgrade log present (probably fresh install)

[Jonathan (jonny-4)]

probably the same as https://bugs.launchpad.net/ubuntu/+source/lightdm-gtk-greeter/+bug/1701745, i attached a patch there

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.