Bug#869535: lightdm: service fails to start due to incorrect data dir perms

["Phil Endecott" <debian_bugs_2021@chezphil.org>]

I've also seem this warning message and the root.root permissions on 
/var/lib/lightdm/data/

This system previously had a very minimal (debootstrap'd) 
Debian Bullseye installation. I then did:

# apt-get install openbox lightdm

and rebooted. X starts and shows a background image but there are 
no login controls or anything else. On this first boot, 
/var/log/lightdm/lightdm.log says:

[+0.06s] WARNING: Could not enumerate user data directory /var/lib/lightdm/data: Error opening directory '/var/lib/lightdm/data': No such file or directory

Looking at the permissions:

# ls -ld /var
drwxr-xr-x 12 root root 4096 Oct 11 16:36 /var

# ls -ld /var/lib
drwxr-xr-x 23 root root 4096 Oct 17 16:34 /var/lib

# ls -ld /var/lib/lightdm/
drwxr-x--- 4 lightdm lightdm 4096 Oct 17 16:36 /var/lib/lightdm/

# ls -ld /var/lib/lightdm/data/
drwxr-xr-x 3 root root 4096 Oct 17 16:36 /var/lib/lightdm/data/

# ls -ld /var/lib/lightdm/data/lightdm/
drwxrwx--- 2 lightdm lightdm 4096 Oct 17 16:36 /var/lib/lightdm/data/lightdm/

# ls -l /var/lib/lightdm/data/lightdm/
total 0

Observe the ownership going down the hierarchy is root then lightdm 
then root then lightdm, which seems peculiar.

On the second boot, this warning was not shown - presumably the 
missing directory was created on the first boot albeit with the 
surprising ownership. lightdm still didn't work, but that was due 
to other issues.....


Regards, Phil.