some screensavers produce freezes when stopped

I'm using Ubuntu MATE 16.04, as far as I can remember, the rss-glx package was installed by default.

Reproduction:

1. Start the screensaver settings dialog: System / Preferences / Look and Fell / Screensaver.

2. From the list on the left pick an rss-glx screensaver. I did not do this test with all of them, but the following ones produce the freeze (in order of seriousness): plasma, biof, pixelcity, colorfire.

3. Let the preview run for a short while (~10 seconds was always sufficient in my case, YMMV).

4. Try to exit the dialog or switch to another screensacer (blank screen).

5. One of two things happen:

a) The screensaver window freezes up, preview animation included. Looking az the offending screensaver process in top/htop will show a high CPU usage. Also, the offending process can not be stopped with SIGTERM, you likely will need to use SIGKILL on it.

b) Complete system freeze. (I couldn't even switch to tty via CTRL-ALT-F1) About 30-40% of biof and pixelcity tests resulted in a complete freeze, and colorfire seems to consistently produce a system freeze. In colorfire's case, I had to temporarily uninstall rss-glx to be able to set the screensaver dialog to a state that will not produce a system freeze on just about any action that would normally result in colorfire stopping.

I was able to reproduce the bug with the above steps with all 4 window compositor settings available in MATE Tweak, so Compiz / Marco does not seem to be the source of the problem.

The bug also affects normal screensaver / screen lock use. I originally encountered the bug by setting the screensaver to random, then I noticed that whenever I interrupt pixelcity, it freezes and I don't get an unlock dialog.

The phenomena described above also may be several separate bugs (one per mentioned screensaver).

Also, some existing bugs (namely: #114783, #375819, #424782) seem to describe similar problems.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: rss-glx 0.9.1-6ubuntu4
ProcVersionSignature: Ubuntu 4.4.0-34.53-generic 4.4.15
Uname: Linux 4.4.0-34-generic x86_64
NonfreeKernelModules: nvidia_uvm nvidia
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
CurrentDesktop: MATE
Date: Fri Aug 26 21:05:45 2016
InstallationDate: Installed on 2016-07-08 (48 days ago)
InstallationMedia: Ubuntu-MATE 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
SourcePackage: rss-glx
UpgradeStatus: No upgrade log present (probably fresh install)