How an old problem with multiple (Debian / Linux) sessions seems to require an old fix.

One of the facts which I recently updated my postings with is, that I have the ability to run multiple sessions on one PC, and to switch back and forth between them, by using the key-combinations <Ctrl>+<Alt>+F7, <Ctrl>+<Alt>+F8, etc. According to that update, each session could theoretically use either ‘Plasma 5′, or ‘LXDE’ as its desktop manager, even though I’d choose an actual LXDE session for only one of my defined usernames.

When I was recently testing this ability, I found that a Plasma 5 session which had locked its screen (which I had switched away from), using the built-in Plasma 5 locker, would start to consume 1 CPU core at 100%, as was visible from within the other session. And, it appears that This is a bug which has been known for a long time, on computers that have the proprietary NVIDIA graphics drivers, as my computer named ‘Phosphene’ does. This computer is still based on Debian 9 / Stretch. Apparently, according to common lore, what one is supposed to do about this is, to create a file named such as ‘dirk.sh’ (in my case), in the directory ‘/etc/profile.d’, which is supposed to set two environment variables globally like so:

# An attempt to prevent klocker from consuming 1 CPU core 100%
# when multiple desktop sessions are active...

export KWIN_TRIPLE_BUFFER=1
export __GL_YIELD="USLEEP"



Unfortunately, this backfired on me when I tried to implement it, in that regardless of which way I tried to do so, ‘kwin’ would just crash in the session that’s supposed to be using ‘Plasma 5′. An additional mystery I ran in to was, that my attempts to set ‘__GL_YIELD’ would simply get reset somewhere, unless I had also set ‘KWIN_TRIPLE_BUFFER’. Only if I set both, would setting either reveal as successful, using an ‘echo $…’ command. (:1) Therefore, what I really needed to do was, to turn off the Screen-Locking which is provided by Plasma 5 itself (for both my usernames), and to install and use ‘xscreensaver’ instead. However, doing that has two obvious caveats: • Under Debian 10 / Buster and later, ‘xscreensaver’ is no longer fully supported, unless one also reconfigured the new, Wayland display manager to act as an X-server proxy, And • Even when I apply this fix, which means that I’ve completely disabled ‘klocker’ in my settings, at the moment I tell Plasma 5 to launch a new, parallel session, foregoing-which causes <Ctrl>+<Alt>+F8 just to lead to a blank screen, Plasma 5 will lock the current session – Using ‘klocker’ again, and causing 100% CPU usage to become visible, again, from the second session. What I find is that, once I’ve used my Plasma 5 session to create a parallel session, I need to switch to the first session once, using <Ctrl>+<Alt>+F7, and unlock that one. After that, both my Plasma 5 sessions will only lock themselves, using ‘xscreensaver’. And aside from that short, crucial interval, I haven’t seen 100% CPU-core usage again. I should add that, for certain purposes, I sometimes choose only to install the CPU-rendered ‘xscreensaver’ packages, and deliberately do not install the hardware-accelerated ones. And in this case, the hardware-accelerated screensavers were omitted, simply because they could start running the busy-wait loop again, only this time, when invoked by ‘xscreensaver’. (Update 3/24/2021, 13h55… ) Installing Chrome on Old Debian Versions (Redirect from Installing Old Chrome Version on Debian). In recent weeks I’ve been noticing some rather odd behaviour, of the Linux version, of the most up-to-date Chrome browser. In short, every time I launched the browser on my Debian 9 / Stretch computer, that has the Plasma 5.8 Desktop Manager, certain malfunctions would set in, specific to the desktop manager. I waited for several Chrome version upgrades, but the malfunctions persisted. And, as far as I can tell, the problem boils down to the following: Google will only distribute the latest Chrome version, and when they tag the line which one is supposed to have in one’s Sources.list with ‘stable’, apparently, they mean both the Stable version of Chrome, And, for the Stable version of Debian. According to Google, Debian 10 happens to exist right now, because that is the “stable” version (of Debian), but, Debian 9 and Debian 8 don’t exist anymore. Except for the fact that many people could still have either installed. Bundling AppImages with Themes. One of the projects which I have been undertaking in recent weeks has been, to teach myself GUI programming using the Qt5 GUI Library, of which I have version 5.7.1 installed on a good, tower computer, along with the IDE “Qt Creator”. What can be observed about this already is, that under Debian 9 / Stretch, which is a specific build of Linux, in addition to just a few packages, it’s really necessary to install many additional packages, before one is ready to develop Qt Applications, because of the way Debian breaks the facility into many smaller packages. Hypothetically, if a person was using the Windows, Qt SDK, then he or she would have many of the resources all in one package. Beyond just teaching myself the basics of how to design GUIs with this, I’ve also explored what the best way is, to deploy the resulting applications, so that other people – technically, my users – may run them. This can be tricky because, with Qt especially, libraries tend to be incompatible, due to even minor version differences. So, an approach which can be taken is, to bundle the main libraries required into an AppImage, such that, when the developer has compiled everything, the resulting AppImage – a binary – is much more likely actually to run, on different versions of Linux specifically. The tool which I’ve been using, to turn my compiled binaries into AppImage’s, is called ‘linuxdeployqt‘, and is not available in the Debian / Stretch repositories. However, it does run under …Stretch. But a developer may have questions that go beyond just this basic capability, such as, what he or she can do, so that the application will have a predictable appearance – a “Style” or “Theme” – on the end-user’s Linux computer. And essentially, I can think of two ways to approach that question: The ‘official but slightly quirky way’, and ‘a dirty fix, that seems to get used often’… The official, but slightly quirky way: Within the AppImage, there will be a ‘plugins’ directory, within which there will be a ‘platformthemes’ as well as a ‘styles’ subdirectory. It’s important to note, that these subdirectories serve officially different purposes: • The ‘platformthemes’ subdirectory will contain plugins, that allow the application to connect with whatever theme engine the end-user’s computer has. Its plugins need to match libraries that the eventual user has, determining his desktop theme, And • The ‘styles’ subdirectory may contain plugins, which the end-user does not have installed, but were usually compiled by upstream developers, to make use of one specific platform-engine each. Thus, what I had in these directories, for better or worse, was as shown: dirk@Phosphene:~/Programs/build-Dirk_Roots_GUI_1-Desktop-Release/plugins/platformthemes$ ls
KDEPlasmaPlatformTheme.so  libqgtk2.so  libqgtk3.so
dirk@Phosphene:~/Programs/build-Dirk_Roots_GUI_1-Desktop-Release/plugins/platformthemes$dirk@Phosphene:~/Programs/build-Dirk_Roots_GUI_1-Desktop-Release/plugins/styles$ ls
breeze.so  libqgtk2style.so
dirk@Phosphene:~/Programs/build-Dirk_Roots_GUI_1-Desktop-Release/plugins/styles\$



The reader may already get, that this was a somewhat amateurish way, to satisfy themes on the end-user’s machine. But in reality, what this set of contents, of the AppImage, does rather well is, to make sure that the 3 main theme engines on an end-user’s computer are recognized:

1. Gtk2,
2. Gtk3,
3. Plasma 5.

And, if the application tries to make no attempts to set its own theme or style, it will most probably run with the same theme, that the end-user has selected for his desktop. But, what the point of this posting really is, is to give a hint to the reader, as to how his AppImage could set its own theme eventually. And so, according to what I just cited above, my application could choose to select “Breeze” as the Style with which to display itself, or “Gtk2″. But, here is where the official way gets undermined, at least as the state of the art was, with v5.7.1 of Qt:

• ‘Breeze’ can only be set (by the application), if the end-user’s machine is running Plasma 5 (:1), And
• ‘Gtk2′ can only be set (by the application), if the end-user’s machine supports Gtk2 themes, which many Plasma 5 computers have the additional packages installed, to do.

What this means is that, even though I could try to create a predictable experience for the end-user, what the end-user will see can still vary, depending on what, exactly, his platform is. And beyond that, even though I could set the ‘Gtk2′ Style with better reliability in the outcome, I could also just think, that the classical, ‘Gtk2′ style is a boring style, not worthy of my application. Yet, in this situation, I can only select the “Breeze” theme from within my application successfully, if the end-user is based on Plasma 5. If the end-user is not, then my application’s attempt to set “Breeze” will actually cause Qt v5.7.1 to choose the “Fusion” theme, that Qt5 always supports, that might look okay, but that is not “Breeze”…

So, what other options does the application developer have?

(Updated 9/12/2020, 18h15… )

Getting the integrated equalizer to work, from Debian Jessie, KDE 4.

I happen to have an older laptop, which I name ‘Klystron’, that is running Debian 8 / Jessie, with KDE 4 as its desktop manager. Don’t ask me why, but I tend to leave older builds of Linux running on some of my computers, just because they seem to be running without any deficiencies.

That laptop has lousy speakers. I decided a few days ago, that it would benefit, if I could get the 14-band graphical equalizer to work, that is generally available on Linux computers which, like that laptop, use the ‘PulseAudio’ sound server. However, on this old version of Linux, achieving that was a bit harder than it’s supposed to be. Yet, because I succeeded, I felt that I should share with the community, what the steps were, needed to succeed.

First of all, this is what the equalizer looks like, which I can now open on that laptop:

And it works!

In order to get this sort of equalizer working with PulseAudio, eventually, the following two modules need to be loaded:

module-equalizer-sink

module-dbus-protocol

And, if I gave the command ‘load-module…’ naively from the command-line, as user, because under my builds of Linux, PulseAudio runs in user mode, both these modules seem to load fine, without my having to install any additional packages.

On more recent builds of Linux, one needs to install the package ‘pulseaudio-equalizer’ to obtain this feature, or some similarly-named package. But, because these two modules just loaded fine under Debian / Jessie, I guess the functionality once came integrated with PulseAudio.

But I soon started to run in to trouble with these modules, and discovered why, then, the equalizer function was not set up to run out-of-the-box…

(Updated 6/26/2020, 10h30… )