[LightDM] Fwd: Can't seem to change default session (user-session)

[MegaBrutal]

Hi all,

My use case is the following:
I need to build a restrictive Ubuntu system on which users have very
limited space to tamper. Actually, the only thing they are allowed to
do is to start a limited set of applications. I found that Openbox
would be an ideal choice for a restrictive desktop environment. So I
installed the openbox package, lightdm, and the unity greeter, then
tried to set openbox as the default session by adding "user-session"
to the LightDM config. For some reason, either LightDM or the greeter
still insists to log me in to the Unity ("ubuntu" session), which of
course fails, since Unity is not even installed.

I see these messages in /var/log/lightdm/lightdm.log:
[+2315.98s] DEBUG: Session pid=2825: Authentication complete with
return value 0: Success
[+2315.98s] DEBUG: Authenticate result for user op: Success
[+2315.98s] DEBUG: User op authorized
[+2315.99s] DEBUG: Greeter requests default session
[+2315.99s] DEBUG: Seat seat0: Failed to find session configuration ubuntu
[+2315.99s] DEBUG: Seat seat0: Can't find session 'ubuntu'
[+2315.99s] DEBUG: Greeter start authentication
[+2315.99s] DEBUG: Session pid=2828: Started with service 'lightdm',
username '(null)'
[+2315.99s] DEBUG: Session pid=2825: Exited with return value 0
[+2315.99s] DEBUG: Seat seat0: Session stopped

What is interesting, is that I have a working system, where users are
logged in to Openbox correctly, but I can't seem to be able to find
the difference. I really can't point out what's wrong with my build,
and what's right with the other one.

Here is the output of "lightdm --show-config" on the failing system:

# 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  greeter-session=unity-greeter
E  xserver-command=X -core
F  allow-guest=false
F  greeter-hide-users=true
F  greeter-show-manual-login=true
G  user-session=openbox

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-unity-greeter.conf
E  /usr/share/lightdm/lightdm.conf.d/50-xserver-command.conf
F  /etc/lightdm/lightdm.conf.d/50-session.conf
G  /etc/lightdm/lightdm.conf

Originally I put "user-session" into 50-session.conf, now it's in
/etc/lightdm/lightdm.conf, but neither works.

Here is the config from the system which works correctly:
# lightdm --show-config
   [Seat:*]
A  allow-guest=false
C  greeter-hide-users=true
C  greeter-show-manual-login=true
D  greeter-wrapper=/usr/lib/lightdm/lightdm-greeter-session
E  guest-wrapper=/usr/lib/lightdm/lightdm-guest-session
F  user-session=openbox
G  greeter-session=unity-greeter
H  xserver-command=X -core

   [LightDM]
B  backup-logs=false

Sources:
A  /usr/share/lightdm/lightdm.conf.d/50-disable-guest-login.conf
B  /usr/share/lightdm/lightdm.conf.d/50-disable-log-backup.conf
C  /usr/share/lightdm/lightdm.conf.d/50-disable-user-list.conf
D  /usr/share/lightdm/lightdm.conf.d/50-greeter-wrapper.conf
E  /usr/share/lightdm/lightdm.conf.d/50-guest-wrapper.conf
F  /usr/share/lightdm/lightdm.conf.d/50-openbox.conf
G  /usr/share/lightdm/lightdm.conf.d/50-unity-greeter.conf
H  /usr/share/lightdm/lightdm.conf.d/50-xserver-command.conf

I really can't see any relevant difference aside from that the config
files are named differently.

~/.dmrc files are purged, but anyway they are ignored, I also tried to
set openbox there.

What makes it even more strange is that unity-greeter seems to pick up
the default session correctly. I found this line in
/var/log/lightdm/seat0-greeter.log:
[+0.24s] DEBUG: Connected version=1.18.2 default-session=openbox
show-manual-login=true hide-users=true has-guest-account=false
show-remote-login=true

I also wonder why this log file is named "seat0-greeter.log", while on
the working system, the same logfile is named "x-0-greeter.log".

Any insights would be appreciated.


Greetings,
MegaBrutal