Add ensure-same which takes in refrs and checks they are eq?

This is to be used to take a number of refrs to actors and verify that
they are all the exat same object. It will error if they are not,
otherwise it returns one of the refrs.

goblins/actor-lib/joiners.rkt

@@ -17,9 +17,12 @@
 (provide all-of all-of* any-of)
 
 (require "../core.rkt"
+         "../ocapn/captp.rkt"
+         "../ocapn/structs-urls.rkt"
          "cell.rkt"
          "methods.rkt"
-         racket/set)
+         racket/set
+         racket/match)
 
 (define (all-of* promises)
   (define-cell waiting
@@ -89,9 +92,25 @@
                ($ waiting new-waiting)])))))
   join-promise)
 
+(define (ensure-same . refrs-vows)
+  (define (all-eq? items)
+    (match items
+      [(list item) item]
+      [(list 1st 2nd rest ...)
+       (if (eq? 1st 2nd)
+           (all-eq? (cons 2nd rest))
+           (error "items given do not resolve to the same"))]))
+
+  (on (all-of* refrs-vows)
+      (lambda (refrs)
+        (pk 'refrs refrs)
+        (all-eq? refrs))
+      #:promise? #t))
+
 (module+ test
   (require "../vat.rkt"
            rackunit
+           racket/async-channel
            racket/match)
 
   (define a-vat
@@ -139,4 +158,35 @@
   (test-true
    "any-of breaks promise if no fulfilled promises are given"
    (match (run-joiner-get-result any-of 7 9 11)
-     [(vector 'broken _err) #t])))
+     [(vector 'broken _err) #t]))
+
+  (define b-vat (make-vat))
+  (define ((^my-actor _bcom))
+    'hello)
+
+  (define my-actor (b-vat 'spawn ^my-actor))
+  (define my-actor-1 my-actor)
+  (define my-actor-2 my-actor)
+  (define another-actor (a-vat 'spawn ^my-actor))
+  (define result-ch (make-async-channel))
+
+  (a-vat 'run
+         (lambda ()
+           (on (ensure-same my-actor my-actor-1 my-actor-2)
+               (lambda (result)
+                 (async-channel-put result-ch `(result ,result))))))
+
+  (check-equal?
+   (sync/timeout 1 result-ch)
+   (list 'result my-actor))
+
+  (a-vat 'run
+         (lambda _
+           (on (ensure-same my-actor my-actor-1 another-actor)
+               #:catch
+               (lambda (err)
+                 (async-channel-put result-ch `(err ,(exn-message err)))))))
+
+  (check-equal?
+   (sync/timeout 1 result-ch)
+   (list 'err "items given do not resolve to the same")))