https://bugs.freedesktop.org/show_bug.cgi?id=72732
--- Comment #2 from Dave Gilbert freedesktop@treblig.org --- and now understand the perm problem; I was running
X :2
from vt 2, it was starting X visible on vt 3, when I changed to vt3 it removed the acl on /dev/dri/card0 so that it lost the perms when clients ran (although the server still had it open).
So still say it's missing that NULL check.