vglgenkey: Abort if DISPLAY env variable is unset

In an XDMCP environment, a separate instance of the display manager is
launched in a remote display session.  Depending on the system, this may
cause the display manager startup script to be invoked with the DISPLAY
environment variable unset.  If this occurred, vglgenkey preemptively
removed /etc/opt/VirtualGL/vgl_xauth_key, but there was no way for xauth
to generate a new key.  Thus, effectively the xauth key generated for
the "root" display manager was clobbered by the instance of the display
manager launched in the remote display session.

Author: dcommander

server/vglgenkey

@@ -1,4 +1,13 @@
 #!/bin/sh
+
+# Abort if the DISPLAY environment variable is unset, which may be the case if
+# the display manager is launched in a remote display session via XDMCP.
+# Otherwise this script will clobber the existing xauth key with no possibility
+# of regenerating it.
+if [ -z "$DISPLAY" ]; then
+	exit 0
+fi
+
 XAUTH=xauth
 if [ -x /usr/X11R6/bin/xauth ]; then
 	XAUTH=/usr/X11R6/bin/xauth