Ensure patched assertions are added before being removed, to avoid glitching

syndicate/HOWITWORKS.md

@@ -458,6 +458,10 @@ check followed by zero or more (move, check) pairs.
     removeAssertion :: Index -> V -> 1
     removeAssertion index v = adjustAssertion index v -1
 
+Care must be taken when applying entire *patches* to ensure that added
+assertions are processed before removed assertions; otherwise, actors
+will observe glitching in certain cases.
+
 **Definition.** The procedure `sendMessage` delivers a message `v` to
   event handlers in the given index.
 

syndicate/dataspace.rkt

@@ -368,15 +368,18 @@
     ;; (for [((a c) (in-bag/count ds-assertions))] (log-info "  . ~v = ~v" a c))
     ;; (for [((a c) (in-bag/count delta))] (log-info "  → ~v = ~v" a c))
     (define rt (dataspace-routing-table ds))
+    (define pending-removals '())
     (define new-cleanup-changes
       (for/fold [(cleanup-changes (actor-cleanup-changes ac))] [((a count) (in-bag/count delta))]
         (match (bag-change! ds-assertions a count)
-          ['present->absent (remove-assertion! rt a)]
+          ['present->absent (set! pending-removals (cons a pending-removals))]
           ['absent->present (add-assertion! rt a)]
           ;; 'absent->absent absurd
           ['present->present (void)]) ;; i.e. no visible change
         (define-values (updated-bag _summary) (bag-change cleanup-changes a (- count)))
         updated-bag))
+    (for [(a (in-list pending-removals))]
+      (remove-assertion! rt a))
     (set-actor-cleanup-changes! ac new-cleanup-changes)))
 
 (define (run-scripts! ds)

syndicate/examples/box-and-client.rkt

@@ -16,4 +16,6 @@
                   (log-info "client: box has gone"))
        (on (asserted (box-state $v))
            (log-info "client: learned that box's value is now ~v" v)
-           (send! (set-box (+ v 1)))))
+           (send! (set-box (+ v 1))))
+       (on (retracted (box-state _))
+           (log-info "client: box state disappeared")))