check-all's guard printed 'rm -f /tmp/.X11-unix/X<n>' and nothing else. Removing
only the socket leaves /tmp/.X<n>-lock behind; Xvfb then exits 1 immediately and
the next command still reports no display, which reads as the restart having
failed for a deeper reason rather than as an incomplete recipe.
Hit three times across this project, each time following the printed recipe. The
fix was visible in Xvfb's own stderr the whole time and nobody had read it --
the guard was written to save exactly that step, so an incomplete guard is worse
than none here.
44a6cb6065