how to implement timeout with fragmented output for drakma

Question

Drakma does not currently support timeouts for SBCL but I need a timeout which returns the content transported up to the point in time the timeout occurred.

This is my current working attempt, but is probably a rather good fit for thedailywtf instead of production code:

(defun perform-request (url parameters method)
  (let ((string nil))
    (restart-case
        (sb-ext:with-timeout 20
           (let ((stream (drakma:http-request url
                                              :method method
                                              :parameters parameters
                                              :want-stream T)))
             (unwind-protect
                 (do ((char (read-char stream nil nil)
                            (read-char stream nil nil)))
                     (nil nil)
                   (when char 
                     (push char string)))
               (close stream))))
      (ignore ()
        "")
      (default ()
        (coerce (reverse string) 'string)))))

The attempt does have some serious flaws:

• the method cannot run shorter than the given timeout
• using the hack with the timeout to return from a while-true loop and bastardizing the restart-case to get the appropriate return value just feels wrong
• the char-by-char reading of the stream causes significant load on the CPU (instead on just waiting on the completion of the loading and then doing a bulk read)

How can I properly read from the drakma stream until a certain timeout (or EOF) and then return the read in string?