Tweak kernel-to-user transitions, in prep for exception handling

os2.rkt

@@ -142,10 +142,10 @@
 	 [preaction
 	  (loop rest (perform-action pid preaction state) outbound-actions)])])))
 
-(define (run-user-code v)
+(define (send-to-user f . args)
   ;; TODO: use this hook to find all the bits of code that will need
   ;; with-handlers and crash compensation.
-  v)
+  (apply f args))
 
 (define (perform-action pid preaction state)
   (match preaction
@@ -241,19 +241,26 @@
 	       body)))
 
 (define (run-trapk state pid trap-k . args)
-  (run-ready state pid (run-user-code (apply trap-k args))))
+  (if trap-k
+      (run-ready state pid (apply send-to-user trap-k args))
+      state))
+
+(define (maybe-transition->transition t)
+  (if (transition? t)
+      t
+      (transition t '())))
 
 (define (run-ready state pid interrupt-k)
   (define old-process (hash-ref (vm-processes state) pid))
   (match-define (transition new-process-state actions)
-    (run-user-code (interrupt-k (process-state old-process))))
+    (maybe-transition->transition (send-to-user interrupt-k (process-state old-process))))
   (struct-copy vm (enqueue-actions state pid actions)
 	       [processes (hash-set (vm-processes state) pid
 				    (struct-copy process old-process
 						 [state new-process-state]))]))
 
 (define (do-spawn spawning-pid thunk k state)
-  (match-define (transition initial-state initial-actions) (run-user-code (thunk)))
+  (match-define (transition initial-state initial-actions) (send-to-user thunk))
   (define new-pid (vm-next-process-id state))
   (run-trapk (struct-copy vm (enqueue-actions state new-pid initial-actions)
 			  [processes (hash-set (vm-processes state) new-pid (process new-pid