Plasma session freezes when using two sessions on different ttys

[Martin Steigerwald]

Hello!

On Plasma 5.6/5.7 – 5.7 currently partially uploaded – (before pure 5.6), KF 
5.23, and Qt 5.6.1 on Debian Sid on ThinkPad T520 with Sandybridge graphics 
and xserver-xorg-video-intel 2:2.99.917+git20160706-1, mesa 11.2.1 on libdrm 
2.4.68 when using two sessions after switching from one session to another 
sometimes the session I switched to is stuck.

Stuck means: No reaction whatsoever to mouse clicks or keyboard inputs. All 
windows are still shown just fine and there is no process at 100% cpu. The 
session seems just idle, just that it doesn´t respond to user input anymore.

I didn´t take much care to debug this yet, as I have no exact idea how and I 
wanted the session back now. So what I did is killall -u affecteduser, but I 
noticed this didn´t clear all processes. ksmserver was still there and it only 
responded to a KILL signal.

I didn´t find much in .xsession-errors that I can relate to that. Two things I 
noticed:

org.kde.plasma: unversioned plugin detected, may result in instability

But I think I have seen this for ages.


Well, before creating yet another of these tons of freeze bug reports against 
plasmashell and kwin, without being able to provide much of helpful 
information… any idea how to make a useful bug report out of it, next time it 
happens?

I found similar reports such as

Bug 354126 - Plasma 5 Screen Freezes on a regular basis...
https://bugs.kde.org/show_bug.cgi?id=354126


But I only see this when using two sessions … and then also Alt-F2 does not 
work anymore, also Alt-Shift-F12 to disable compositing produces no effect.

I could try to produce a backtrace of ksmserver, kwin, plasmashell… enable all 
debugging in kdebugdialog and looking again in ~/.xsession-errors, also 
checking kernel log (did not find anything), but is there any more systematic 
and also time-efficient approach to this?


I found some hints in the forum, but I think these are two general for my 
situation and… well trying with two clean users… it isn´t as if the problem 
reproduces everytime, so this would take a considerable amount of time.


Such a randomly appearing "I just freeze the desktop, without telling you why, 
hahaha" kind of bug is something that I´d like to squash for good, but at the 
same time seems to be quite elusive. There are a gazillion layers in the stack 
where things can go wrong and I currently know now systematic approach on how 
to find which part exactly fails.

Thanks,
-- 
Martin
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