Xdummy crashes with dixGetPrivateAddr: Assertion `key->initialized' failed

Hi,

I am having issues with the Xdummy driver that I am trying to employ together with the Xpra remote display server and client for forwarding applications and desktop screens (https://xpra.org/).

I have already thoroughly investigated the issue with the xpra developer at https://xpra.org/trac/ticket/2764, so excluding an Xpra bug.

My system is an intel laptop (i7-10510U cpu (comet lake) and integrated Intel HD graphics 620) running Kubuntu 19.10. It was freshly installed in Jan/Feb, so there should be no weird leftover from previous distro release.

Operation should be as follows: xdummy gets fired up by Xpra (working in server mode), then you can launch applications on it, e.g. xpra start :101 and then DISPLAY=:101 xterm. At this point, Xpra running in client mode can attach to the xpra server to display the windows from the running applications e.g. xpra attach ssh:localhost:101.

The problem is that when the xpra client attaches, the application windows (e.g., the xterm, following the example above) are only briefly shown then the xserver crashes. In the logs, I find:

Xorg-for-Xpra-:101: ../../../../../../include/privates.h:121: dixGetPrivateAddr: Assertion `key->initialized' failed.

and a backtrace including

[ 92979.720] (EE) Backtrace:
[ 92979.720] (EE) 0: /usr/lib/xorg/Xorg-for-Xpra-S32265 (OsLookupColor+0x13c) [0x55e4bda04acc]
[ 92979.721] (EE) 1: /lib/x86_64-linux-gnu/libpthread.so.0 (funlockfile+0x60) [0x7fadd176059f]
[ 92979.721] (EE) 2: /lib/x86_64-linux-gnu/libc.so.6 (gsignal+0xcb) [0x7fadd159e3eb]
[ 92979.721] (EE) 3: /lib/x86_64-linux-gnu/libc.so.6 (abort+0x12b) [0x7fadd157d899]
[ 92979.721] (EE) unw_get_proc_name failed: no unwind info found [-10]
[ 92979.721] (EE) 4: /lib/x86_64-linux-gnu/libc.so.6 (?+0x0) [0x7fadd157d75a]
[ 92979.721] (EE) 5: /lib/x86_64-linux-gnu/libc.so.6 (__assert_fail+0x46) [0x7fadd158f006]
[ 92979.721] (EE) 6: /usr/lib/xorg/Xorg-for-Xpra-S32265 (DRIMoveBuffersHelper+0xc15) [0x55e4bd9cfaa5]
[ 92979.721] (EE) 7: /usr/lib/xorg/Xorg-for-Xpra-S32265 (DRI2Authenticate+0xa2) [0x55e4bd9d10f2]
[ 92979.721] (EE) 8: /usr/lib/xorg/Xorg-for-Xpra-S32265 (DRI2GetParam+0x944) [0x55e4bd9d24b4]
[ 92979.722] (EE) 9: /usr/lib/xorg/Xorg-for-Xpra-S32265 (SendErrorToClient+0x354) [0x55e4bd8a3f44]
[ 92979.722] (EE) 10: /usr/lib/xorg/Xorg-for-Xpra-S32265 (InitFonts+0x3b4) [0x55e4bd8a7fd4]
[ 92979.722] (EE) 11: /lib/x86_64-linux-gnu/libc.so.6 (__libc_start_main+0xf3) [0x7fadd157f1e3]
[ 92979.722] (EE) 12: /usr/lib/xorg/Xorg-for-Xpra-S32265 (_start+0x2e) [0x55e4bd891a1e]
[ 92979.722] (EE) 
[ 92979.722] (EE) 
Fatal server error:
[ 92979.722] (EE) Caught signal 6 (Aborted). Server aborting

Any idea at why this may occur, any hint at how to debug?