Sketchy role handler update. Pragmatic, rather than principled.

os2.rkt

@@ -343,15 +343,37 @@
     [(kill pid-to-kill reason)
      (do-kill (or pid-to-kill pid) reason state)]))
 
+(define (topics-equal? ta tb)
+  ;; TODO: OK, if we had a couple of simple topics here, we'd be done
+  ;; by asking (and (specialization? ta tb) (specialization? tb ta)),
+  ;; but because we have sets of implicitly-unioned topics, things get
+  ;; jolly awkward. For now, we punt, trusting the user to not supply
+  ;; an incompatible set of topics on an endpoint update. This is
+  ;; definitely an interim position: full presence will require a
+  ;; serious treatment of topic unions via anti-unification.
+  #t)
+
 (define (do-subscribe pid pre-eid topics hs state)
   (cond
    [(hash-has-key? (vm-processes state) pid)
     (define new-eid (eid pid pre-eid))
+    (define old-endpoint (hash-ref (vm-endpoints state) new-eid #f))
     (define new-endpoint (endpoint new-eid topics hs))
-    (let* ((state (notify-route-additions state new-endpoint))
-	   (state (generic-update-process state pid (add-process-eid new-eid)))
-	   (state (install-endpoint state new-eid new-endpoint)))
-      state)]
+    (if old-endpoint
+	;; We are *updating* an existing endpoint's behaviour.
+	(if (topics-equal? (endpoint-topics old-endpoint)
+			   (endpoint-topics new-endpoint))
+	    (let* ((state (install-endpoint state new-eid new-endpoint)))
+	      state)
+	    (error 'do-subscribe
+		   "Topics must be equal when updating an endpoint: ~v vs ~v"
+		   old-endpoint
+		   new-endpoint))
+	;; We are installing a *new* endpoint.
+	(let* ((state (notify-route-additions state new-endpoint))
+	       (state (generic-update-process state pid (add-process-eid new-eid)))
+	       (state (install-endpoint state new-eid new-endpoint)))
+	  state))]
    [else state]))
 
 (define (generic-update-process state pid updater)