A run in a shared test VM dies: "X connection to :2 broken"

Symptoms

A game running on its own Xvfb display in a VM that other sessions also use dies in the middle of a run; its log ends with "X connection to :2 broken (explicit kill or server shutdown)", and the other display's Xvfb has a new process number.

Cause

Another session's stop script ran a bare pkill Xvfb, which ends every Xvfb in the machine (observed: POD's run/vm/inside.sh stop, twice in one evening). A related trap, pkill -f matching the ssh command that holds it, is its own entry.

What to do

Scripts on a shared machine stop only their own processes: pkill -f "Xvfb :1", not pkill Xvfb. For a long run, start the X server under a name of its own (a symlink: ~/bin/Xglide), which nobody else's pkill Xvfb matches. Give each session its own display and Wine prefix.