Fix -inetd not working with xserver >= 1.19

xserver 1.19's OsInit will create a pollfd, followed by checking if fd 2 /
stderr is writable and if it is not, replacing fd 2 with /dev/null.

Since we close stderr in inetd mode to avoid xserver messages being send
to the client as vnc data, the pollfd becomes fd 2, only to be replaced
by /dev/null since a pollfd is not writable.

This commit fixes this by opening /dev/null directly after the close(2),
avoiding that the pollfd becomes fd 2.

Alan Coopersmith: Change to use dup2() for atomic switch of fd

Signed-off-by: Hans de Goede <hdegoede@redhat.com>

unix/xserver/hw/vnc/xvnc.c

@@ -572,9 +572,17 @@ ddxProcessArgument(int argc, char *argv[], int i)
 
     if (strcmp(argv[i], "-inetd") == 0)
     {
+	int nullfd;
+
 	dup2(0,3);
 	vncInetdSock = 3;
-	close(2);
+
+	/* Avoid xserver >= 1.19's epoll-fd becoming fd 2 / stderr only to be
+	   replaced by /dev/null by OsInit() because the pollfd is not
+	   writable, breaking ospoll_wait(). */
+	nullfd = open("/dev/null", O_WRONLY);
+	dup2(nullfd, 2);
+	close(nullfd);
 
 	if (!displaySpecified) {
 	    int port = vncGetSocketPort(vncInetdSock);