usocket の使い方をすっかり忘れてしまっていた

サーバ側

(defparameter *port* 1958)

(defgeneric server-start (server))
(defgeneric server-stop (server))


(defclass* server ()
  ((host usocket:*wildcard-host*)
   (port *port*)
   (listen-thread nil)))

(defmethod server-start ((server server))
  (setf (listen-thread-of server)
        (sb-thread:make-thread
         (lambda ()
           (usocket:with-socket-listener (socket (host-of server) (port-of server) :reuse-address t)
             (loop (usocket:with-connected-socket (connected-socket
                                                   (usocket:socket-accept socket))
                     (let ((stream (usocket:socket-stream connected-socket)))
                       (print (eval (read stream)) stream)
                       (force-output stream))))))
         :name "nunumo:server")))

(defmethod server-stop ((server server))
  (sb-thread:terminate-thread (listen-thread-of server))
  (setf (listen-thread-of server) nil))

クライアント側

(defgeneric client-open (client))
(defgeneric client-close (client))
(defgeneric client-read (client))
(defgeneric client-write (client object))
(defgeneric client-flush (client))

(defclass* client ()
  ((host "localhost")
   (port *port*)
   (socket nil)
   (stream nil)))

(defmethod client-open ((client client))
  (setf (socket-of client)
        (usocket:socket-connect (host-of client) (port-of client))
        (stream-of client)
        (usocket:socket-stream (socket-of client)))
  client)

(defmethod client-close ((client client))
  (force-output (stream-of client))
  (usocket:socket-close (socket-of client))
  client)

(defmethod client-read ((client client))
  (read (stream-of client)))

(defmethod client-write ((client client) object)
  (print object (stream-of client)))

(defmethod client-flush ((client client))
  (force-output (stream-of client))
  client)

はい。RPC システム完成!

(let ((server (make-instance 'server))
      (client (make-instance 'client)))
  (server-start server)
  (unwind-protect
       (progn
         (client-open client)
         (client-write client '(format nil "(+ 1 2) => ~a" (+ 1 2)))
         (client-flush client)
         (client-read client))
    (server-stop server)))
;;=> "(+ 1 2) => 3"