X11 fowarding - handle DISPLAY if screen number not present.

X11 forwarding doesn't care about the screen number, so handle DISPLAY
environment variables without that present correctly.

E.g., both "localhost:10.0" and "localhost:10" are now handled, whereas
previously only the first form would have been accepted.

Bug 6532.

NEWS

@@ -26,6 +26,8 @@ documents those changes that are of interest to users and administrators.
  -- Fix sacct verbose about time and states queried.
  -- burst_buffer/cray - allow 'scancel --hurry <jobid>' to tear down a burst
     buffer that is currently staging data out.
+ -- X11 forwarding - allow setup if the DISPLAY environment variable lacks
+    a screen number. (Permit both "localhost:10.0" and "localhost:10".)
 
 * Changes in Slurm 18.08.5-2
 ============================

src/common/x11_util.c

@@ -95,21 +95,25 @@ extern int x11_get_display_port(void)
 		exit(-1);
 	}
 
+	/*
+	 * Parse out port number
+	 * Example: localhost/unix:89.0 or localhost/unix:89
+	 */
 	port_split = strchr(display, ':');
 	if (!port_split) {
 		error("Error parsing DISPLAY environment variable. "
 		      "Cannot use X11 forwarding.");
 		exit(-1);
 	}
 
+	/*
+	 * Handle the "screen" portion of the display port.
+	 * Xorg does not require a screen to be specified, defaults to 0.
+	 */
 	port_split++;
 	port_period = strchr(port_split, '.');
-	if (!port_period) {
-		error("Error parsing DISPLAY environment variable. "
-		      "Cannot use X11 forwarding.");
-		exit(-1);
-	}
-	*port_period = '\0';
+	if (port_period)
+		*port_period = '\0';
 
 	port = atoi(port_split) + X11_TCP_PORT_OFFSET;