Dear Debian Team,

On a kiosk system (Intel i915 primary, DisplayLink gen1 / udl secondary
driven via reverse PRIME), Xorg segfaults at address 0x0 when the USB
display is unplugged, re-plugged, and unplugged again. The first unplug
(boot-probed GPU screen) is handled cleanly; removing the GPU screen
that was re-created via hotplug crashes the server:

  [Xorg.0.log excerpt: "xf86: remove device 1 ..." → "(EE) Segmentation
   fault at address 0x0" — log attached]

Fixed upstream on the 21.1 stable branch (shipped in 21.1.23, already
in
unstable) via MR
https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/2146:
  - 1443fd34 "randr: clear primary screen's primaryOutput when the
    output is deleted" (use-after-free on DisplayLink GPU unplug)
  - 75d70612 / 907c5019 (xf86CompatOutput NULL guards)

The affected files are otherwise identical between 21.1.16 and 21.1.23,
so the cherry-picks apply cleanly.

Please consider these for a trixie point release — this crash takes
down production point-of-sale terminals on a cable disconnect.

Regards

Michael