disconnect politely if there are no active assignments

svn: r1076

collects/handin-client/client-gui.ss

@@ -238,6 +238,7 @@
                  (let* ([h (connect)]
                         [l (retrieve-active-assignments h)])
                    (when (null? l)
+                     (handin-disconnect h)
                      (error 'handin "there are no active assignments"))
                    (set! connection h)
                    (for-each (lambda (assign) (send assignment append assign))

collects/handin-client/client.ss

@@ -2,6 +2,7 @@
   (require (lib "mzssl.ss" "openssl"))
 
   (provide handin-connect
+	   handin-disconnect
 	   retrieve-extra-fields
 	   retrieve-active-assignments
 	   submit-assignment
@@ -42,6 +43,10 @@
 	;; Return connection:
         (make-handin r w))))
 
+  (define (handin-disconnect h)
+    (write+flush (handin-w h) 'bye)
+    (close-handin-ports h))
+
   (define (retrieve-extra-fields h)
     (let ([r (handin-r h)] [w (handin-w h)])
       (write+flush w 'get-extra-fields 'bye)