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

[Greg Klanderman]

Hi Adam, sorry I've been tied up last few days on outdoor projects now
that it finally got a bit nice in the northeast US.. I will try
dm-tool in the next few days and follow up to the other parts of your
email below..  Probably need to try with and without the
allow-user-switching=false setting.

So far with that setting, I've not had the problem recur.

Greg


>>>>> On May 1, 2020 Adam Nielsen <a.nielsen at shikadi.net> wrote:

>> 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.