gui-agent sometimes fails with "_qubes-video-ext is not available"

[marmarek]

Qubes OS release

Qubes OS 4.3

Brief summary

Recently starting gui-agent become unreliable.

Steps to reproduce

1. Start VM a couple of times? Maybe few at the same time? Not sure what conditions need to be met.

Expected behavior

gui-agent starts correctly every time

Actual behavior

Sometimes it fails with:

[2025-08-05 01:33:40] [    6.333779] qubes-gui[715]: Ok, somebody connected.
[2025-08-05 01:33:40] [    6.379061] qubes-gui[715]: _qubes-video-ext is not available

Additional information

Related to QubesOS/qubes-gui-agent-linux#236. Since there was a simpler, more reliable version of that PR already, I'm going to revert to it. It's not worth spending more time on debugging this race, just to avoid one harmless warning.

[HW42]

Was there something in Xorg's log? The driver should log "Failed to register _qubes-video-ext extension" if it fails to register it. Otherwise this sounds like the agent might be able to connect before the X server is fully initialized (but that code is after mkhandles (which includes wait_for_unix_socket), so some stuff is definitively running). That might have other fun interactions.

[marmarek]

I haven't found anything wrong there. I still suspect a race (note the unix socket is connected by input driver, while the extension is registered by the video driver).

Anyway, I said I'm okay with simpler workaround before: QubesOS/qubes-gui-agent-linux#236 (comment)
And now I request again you do not spend time on debugging this further.

[qubesos-bot]

Automated announcement from builder-github

The package gui-agent-linux has been pushed to the r4.3 testing repository for the Debian template.
To test this update, first enable the testing repository in /etc/apt/sources.list.d/qubes-*.list by uncommenting the line containing bookworm-testing (or appropriate equivalent for your template version), then use the standard update command:

sudo apt-get update && sudo apt-get dist-upgrade

Changes included in this update