Make Old CMUCL CLX Work With Magic Cookies
Put the following forms in a file, and load it after CLX is loaded:

(in-package "XLIB")

(defun get-best-authorization (host display protocol)
  (labels ((read-short (stream &optional (eof-errorp t))
	     (let ((high-byte (read-byte stream eof-errorp)))
	       (and high-byte
		    (dpb high-byte (byte 8 8) (read-byte stream)))))
	   (read-short-length-string (stream)
	     (let ((length (read-short stream)))
	       (let ((string (make-string length)))
		 (dotimes (k length)
		   (setf (schar string k) (card8->char (read-byte stream))))
		 string)))
	   (read-short-length-vector (stream)
	     (let ((length (read-short stream)))
	       (let ((vector (make-array length :element-type '(unsigned-byte 8))))
		 (dotimes (k length)
		   (setf (aref vector k) (read-byte stream)))
		 vector))))
    ;; Original version didn't handle "localhost" correctly -- SEF.
    (if (string= host "localhost")
	(setq host (machine-instance)))
    (let ((pathname (authority-pathname)))
      (when pathname
	(with-open-file (stream pathname :element-type '(unsigned-byte 8)
				:if-does-not-exist nil)
	  (when stream
	    (let* ((host-family (ecase protocol
				  ((:tcp :internet nil) 0)
				  ;; The remaining protocols are not really supported -- SEF.
				  ((:dna :DECnet) 1)
				  ((:chaos) 2)))
		   (host-address (rest (host-address host host-family))))
	      (loop
	       (let ((family (read-short stream nil)))
		 (cond ((null family) (return (values "" ""))) ; No useful entry found. -- SEF
		       ((eql family 0)
			(let* ((address (read-short-length-vector stream))
			       (number (parse-integer (read-short-length-string stream)))
			       (auth-name (read-short-length-string stream))
			       (auth-data (read-short-length-vector stream)))
			  (when (and (= family host-family)
				     (equal host-address (coerce address 'list))
				     (= number display)
				     (string= auth-name "MIT-MAGIC-COOKIE-1"))
			    (return (values auth-name auth-data)))))
		       ;; This is the new case.  The cookie contains a string naming the
		       ;; host, then the display number, auth-name and auth-data. -- SEF
		       ((eql family 256)
			(let* ((hname (read-short-length-string stream))
			       (number (parse-integer (read-short-length-string stream)))
			       (auth-name (read-short-length-string stream))
			       (auth-data (read-short-length-vector stream)))
			  (when (and (string= hname host)
				(= number display)
				(string= auth-name "MIT-MAGIC-COOKIE-1"))
			    (return (values auth-name auth-data)))))))))))))))