[LightDM] lightdm doesn't work via systemd, but does via shell

[Adam Nielsen]

Hi all,

Just following up on this in case anyone else encounters a similar
issue in future.

> Basically if I run lightdm from the shell as root (via SSH) then it
> launches X11 properly and presents the greeter with a login prompt, I
> can log in, and everything works as expected.
> 
> However if I run 'systemctl start lightdm', or reboot the machine
> where systemd tries to launch lightdm during startup, then X11 loads,
> lightdm loads, but the lightdm-gtk-greeter locks up and never draws
> anything on the screen.  I'm just left sitting there at a black
> screen.

The hint here turned out to be the fact that it worked as root.  This
points at a permissions issue, and in my case checking things like
/dev/null showed the permissions to be wrong, which caused this problem.

Ultimately it was a malfunctioning udev rule I had added (my fault)
that incorrectly changed the permissions of a bunch of unrelated nodes
in /dev.  I hadn't rebooted the source machine since adding the rule,
so the first time it had been used during the boot process was on the
destination machine.  I didn't realise this at the time and it confused
me further, thinking the two machines should be identical!

Removing the errant rule and rebooting fixed the problem and got
LightDM working perfectly again.

Cheers,
Adam.