captp: Allow for non-promise-pipelined messages requesting a resolution

goblins/ocapn/captp.rkt

@@ -756,11 +756,11 @@
       [(op:deliver to-desc method
                    args-marshalled
                    kw-args-marshalled
-                   answer-pos
+                   ;; answer-pos is either an integer (promise pipelining)
+                   ;; or #f (no pipelining)
+                   (and (or (? integer?) #f)
+                        answer-pos)
                    resolve-me-desc)
-       (define-values (_answer-promise answer-resolver)
-         (install-answer! answer-pos resolve-me-desc))
-
        (define args
          (incoming-post-unmarshall! args-marshalled))
        (define kw-args
@@ -770,7 +770,26 @@
          (kws-hasheq->kws-lists kw-args))
        (define sent-promise
          (keyword-apply <- kws kw-vals target args))
-       ($ answer-resolver 'fulfill sent-promise)
+       ;; We're either resolving the to the answer promise we create
+       ;; or we're resolving to the actual object described by resolve-me-desc
+       ;;
+       ;; The former case is an indirection because messages pipelined
+       ;; to a to-be-answered object are simply sent to a local promise
+       ;; which will eventually resolve to the answer.
+       ;;
+       ;; It's possible that something more efficient could be done
+       ;; than throwing in an intermediate promise pair that we inform the
+       ;; other side of; maybe re-evaluate when we handle
+       ;; automatic-severance-on-session-disconnect.
+       (cond
+         [answer-pos
+          (define-values (_answer-promise answer-resolver)
+            (install-answer! answer-pos resolve-me-desc))
+          ($ answer-resolver 'fulfill sent-promise)]
+         [else
+          (define to-resolve
+            (maybe-install-import! resolve-me-desc))
+          (<-np to-resolve 'fulfill sent-promise)])
        (void)]
 
       ;; TODO: Here's where we have to record that a listening interest