Bug reported by Chip Coldwell to the Debian BTS 3 years ago, still applies to current git.

It's a race condition in xdm/policy.c:Willing() around line numbers 140--145, which reads

	    if ((fd = popen(willing, "r")))
	    {
		char *s = NULL;
		while(!(s = fgets(statusBuf, 256, fd)) && errno == EINTR)
			;

Here's the problem.  The "popen" call creates a child process and a
pipe to communicate with it.  If the child process exits during the
"fgets" call without generating any output, the parent process
receives SIGCHLD and the read system call gets interrupted.  Therefore
errno == EINTR, and since the child has exited the pipe never returns
any data.  xdm goes into an infinite loop.  I think the problem is
that fgets doesn't reset errno to zero; we have to do that manually.
The fix is the trivial patch below.

(The child process is the "Xwilling" script; in the case of the default
Debian configuration it is "su nobody -c /usr/X11R6/lib/X11/xdm/Xwilling")


--- xc/programs/xdm/policy.c~	2002-12-07 15:31:04.000000000 -0500
+++ xc/programs/xdm/policy.c	2004-06-24 09:56:19.000000000 -0400
@@ -140,8 +140,9 @@
 	    if ((fd = popen(willing, "r")))
 	    {
 		char *s = NULL;
+		errno = 0;
 		while(!(s = fgets(statusBuf, 256, fd)) && errno == EINTR)
-			;
+			errno = 0;
 		if (s && strlen(statusBuf) > 0)
 			statusBuf[strlen(statusBuf)-1] = 0; /* chop newline */
 		else