[LightDM] lightdm goes into tight loop trying to create greeter sessions and fills up logs

[Adam Nielsen]

> Sorry, I thought the lightdm log was most interesting and only kept
> the X log for the one that failed.  I no longer have the corresponding
> X logs, but I can send current ones if you think it will be helpful.
> Let me know what you want; I have the X log from my xinit session on
> seat0 VT1 (since March 4) and the lightdm X logs for seat0 VT7
> (greeter only, since April 28) and seat-1 (logged in, since April 28).

Probably one X log for seat0 and one for seat-1 would be most
interesting, just to see which display devices it ends up using.

> Hmm, I don't think seat-1 logs out and back in; the log shows starting
> up until +8.87s, then from +1699.91s to +1706.86s (28m) my son logs in
> on seat-1, then the next line is at +308685.52s (85.7h), when it
> begins looping, failing to start a greeter on seat-1.

Sorry you're right, I saw the 'username' lines but didn't follow
through to the authentication lines which do only seem to appear once.

> > This itself isn't a problem, as you can run `dm-tool switch-to-greeter`
> 
> I don't think you can run another session on seat-1; AFAICT only seat0
> ever supports multiple sessions on linux, and the log seems to reflect that:

Interesting.  I think it would be worth trying the dm-tool command
anyway.  Not so much to start another session, but to see whether it
triggers your loop.  If it does trigger another loop, it means the
ultimate cause is something trying to start another login
session/greeter on the seat, and if it doesn't trigger the problem,
it's another possibility that can be crossed off the list.

My reasoning is that something could be trying to lock the display for
some reason after some time, which I think can be implemented in the
same way as starting another session on the seat (switching back to the
original session once the authentication is successful).  So confirming
or eliminating the multi-session-on-one-seat theory will be useful
either way.

> so the comment refers to this as "LP bug #1371250"; still not sure
> what "LP" refers to but maybe you know?

I'm not a LightDM dev and haven't been around long enough to know what
they used before GitHub, but a DuckDuckGo search for "lightdm bug
1371250" came up with Launchpad:

  https://bugs.launchpad.net/lightdm/+bug/1371250

> > I presume you have configured the seats manually in the X config files.  
> 
> I have not configured the seats, nor do I have an Xorg.conf per-se.  I
> thought those were obsolete (generated automatically) these days?
> Anyway, I followed multi-seat instructions I found online which
> essentially just boiled down to
> 
> # for each device corresponding to seat-1 (graphics card & USB hub)
> loginctl attach seat-1 /sys/devices/...

Interesting, I could never get that to work and had to configure things
manually.  Yes that is the 'new' way if you are lucky enough to get it
to work.  If you can post a link to the instructions you used let me
know because I would rather do that than use finicky X config files.

Essentially as far as I understand it, it tags those resources with a
seat number and then the default X config files are set to pick up
those tags.

I guess thinking about it, if you did configure the X file manually, it
would probably just pick one device instead of trying them all, but
still complain that it's already in use?  Hard to say without trying it
with dm-tool to find out.

> I'm not sure why lightdm would try to start a greeter on seat-1 when
> there is a session logged in, and seat-1 has property
> CanMultiSession=no.

LightDM wouldn't do this on its own, something else would be triggering
it.  If dm-tool switch-to-greeter actually works, then that will tell
us that it's not even trying to start another greeter but something
else is happening.

> I restarted lightdm Tuesday morning, we'll see if this problem recurrs
> in the next day or so.  Maybe one of the config changes I made will
> solve the issue?  I did add allow-user-switching=false, which I
> believe should also inhibit starting another greeter when there is an
> active login session.

Again dm-tool can confirm that theory :)

Cheers,
Adam.