LXAppearance hangs the desktop when started from TigerVNC

[MichaIng]

I'm not sure which component of the RPi repository makes the difference compared to Debian, so I'm reporting the following issue here:

When installing TigerVNC and LXDE on RPi with the RPi repository applied, starting LXAppearance within a VNC LXDE session hangs the whole desktop. The mouse pointer gets a loading circle, no LXAppearance appears and any desktop interaction is frozen, aside of moving the mouse cursor.

Checking processes on a second session reveals that it hangs on dbus-launch and killing this dbus-launch child makes LXAppearance appear and function normally, no further issues until it is opened again:

This does not happen on any other SBC or PC with Debian, not tested on RPi 64-bit OS on my end. It only happens within virtual TigerVNC desktop sessions, not RealVNC, nor local desktop.

Neither TigerVNC nor LXDE session logs show anything related, but I can of course provide logs and info if required, but I suggest to simply try to replicate on a fresh Raspberry Pi OS 32-bit Lite with the above two packages installed an simply use default setup/configs, no modifications. I think TigerVNC will invoke LXDE automatically, but in case, this is how I do currently (was playing around much, all without any effect on the issue):
cat /root/.vnc/xstartup

#!/bin/dash
exec lxsession -s LXDE -e LXDE

tigervncserver :1 -verbose

[MichaIng]

I learned that there is no interest in native LXDE support and that the LXDE packages from archive.raspberrypi.org are highly modified on different layers (LXDE-pi session with no lxappearance support at all but own appearance tool), but hopefully someone has still an idea what the issue can be here.

I tried to replicate on Raspberry Pi OS 32-bit with desktop and on regular RPi desktop as well as when manually switching session profile from LXDE-pi to LXDE, lxappearance starts, even that on LXDE-pi session its switches have no effect and chosen icon set/theme etc are not applied or selection saved.

What I see as error message:

** (lxappearance:1350): WARNING **: 19:07:57.543: lxappearance.c:67: Failed to connect to the session message bus: Failed to connect to socket /run/user/0/bus: Connection refused

The exact same shows up when installing lxde on Raspberry Pi OS Lite AFTER killing dbus-launch, so that it hangs is a difference an I try to figure out which package or config makes the difference and why on regular Debian (any other SBC or machine) this issue does not exist.

[MichaIng]

Another day of testing and debugging and finally I found a solution for hanging lxappearance on TigerVNC LXDE sessions. Don't ask me for details why this is required in this specific circumstance, but apt install dbus-user-session solves the issue.

• Basically without this package, when starting an X session, a dbus-daemon + dbus-launch instance is started with a specific ID. On lxappearance start, the exact same dbus-launch (same cmd options, same ID) is attempted to be started again, which hangs. Only a single instance is allowed, which is somewhat explained in the package details.
• With this package, the dbus-daemon instance is started a systemd --user child process, the dbus-launch is not visible. This seems to allow the concurrent dbus-launch instance(s) then.

Here the different process hierarchy can be seen:

If someone has some deeper insights why this is required for lxappearance within TigerVNC LXDE session only, while in no other case a similar issue was observed, I would be very interested.

When I find time, I do some investigation what RealVNC virtual desktops do differently so that it is not an issue there, but for today I'm happy to have found a solution and mark this issue hence as closed.