io: fix handling of sharing in place messages

pkgs/racket-test-core/tests/racket/place.rktl

@@ -52,8 +52,11 @@
     ;interned into the same table as us and us2
     ;because the same place sends and receives
     (test #t equal? us (car r2))
-    (test #t equal? us2 (cdr r2))))
-  
+    (test #t equal? us2 (cdr r2)))
+
+  (place-channel-put out (make-prefab-struct 'vec (vector) (vector)))
+  (test (make-prefab-struct 'vec (vector) (vector)) place-channel-get in))
+
 (let ([p (place/splat (p1 ch)
           (printf "Hello form place 2\n")
           (exit 99))])

racket/src/thread/place-message.rkt

@@ -81,11 +81,13 @@
   (define graph #f)
   (define used #f)
   (define (maybe-ph ph v)
-    (if (and used (hash-ref used ph #f))
-        (begin
-          (placeholder-set! ph v)
-          ph)
-        v))
+    (cond
+      [(and used (hash-ref used ph #f))
+       (placeholder-set! ph v)
+       ph]
+      [else
+       (hash-set! graph v #f)
+       v]))
   (define new-v
     (let loop ([v v])
       (cond