konsole badly broken in yakkety: crashes, leaves process behind on exit using 100% cpu

[Sergio Callegari]

The "konsole" application seems to be in very bad shape in yakkety. Possibly, this is not due to the konsole itself but the qt/plasma5/plasma framework combination. Unfortunately, plasma 5 does not seem to be able to stabilize.

To reproduce:

from a terminal type in "konsole"
exit the konsole with the (X) button

either:

- the konsole crashes (typically with a message about a corrupted double linked list)

or

- the konsole seems to quit fine, but in fact it does not. CPU usage goes up to 100% and top reveal a konsole process causing it. This process does not seem to be related to any window that is still active. Killing the process puts cpu usage back to normal.

Given the importance of terminal applications, please give this bug appropriate attention.

ProblemType: Bug
DistroRelease: Ubuntu 16.10
Package: konsole 4:16.04.3-0ubuntu1
ProcVersionSignature: Ubuntu 4.8.0-22.24-generic 4.8.0
Uname: Linux 4.8.0-22-generic x86_64
NonfreeKernelModules: nvidia_uvm nvidia
ApportVersion: 2.20.3-0ubuntu8
Architecture: amd64
CurrentDesktop: KDE
Date: Tue Oct 18 17:53:34 2016
EcryptfsInUse: Yes
SourcePackage: konsole
UpgradeStatus: Upgraded to yakkety on 2016-10-18 (0 days ago)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Sergio Callegari (callegar)]

Seems to be bug https://bugs.kde.org/show_bug.cgi?id=343803.

The bug seems to be triggered by the combination of Qt, plasma 5 *and* the legacy nvidia 340 driver.

It is unfortunate that KDE people are marking this as "resolve" when clearly it is not.

It is also unfortunate that they mark it as "upstream" (indicating that it is a bug in the nvidia stack) which suggests that they intend to do nothing about it, when in fact:

- it is unclear if the bug was actually fixed with more modern nvidia proprietary drivers. The bug may be caused by some race and it may be merely the different timings in the more modern nvidia drivers or in noveau avoid triggering it;

- it should be totally possible to have a Qt terminal that does not cause the problem, as Qterminal shows.

[Sergio Callegari (callegar)]

Bug was also reported to nvidia:

https://devtalk.nvidia.com/default/topic/879586/kf5-konsole-15-04-and-15-08-consumes-100-cpu-on-close-only-with-proprietary-nvidia-driver/

Note that a comment there states: "From what I've seen it may not be the nVidia driver which is at fault. There seems to be a complicated threading issue which might or might not be caused by the driver...it may just be this driver exposes the issue differently."

The fact that some people saw the issue even in Xenial with Nvidia 340 and I did not, as well as the fact that some report that on more modern graphics cards the issue disappeared with nvidia 361.18 and reappeared in 361.28 seems to confirm that this may just be a race somewhere (Qt?) and not a bug in the nvidia drivers.