Supply #f as cleanup function in multicast udp-dataspace, instead of void; notes on cleanup functions and visibility-restrictions

syndicate/HOWITWORKS.md

@@ -509,7 +509,31 @@ interested in the `present` constructor.
 # TODO
 
  - describe the cleanup function associated with a handler in the real implementation
+    - `relay.rkt` uses it. When an inner actor asserts interest in an
+      inbound assertion-set, the relay process pivots into the outer
+      dataspace's context, and adds a new endpoint that relays events
+      to the inner dataspace. The cleanup function attached to that
+      endpoint retracts (from the inner dataspace) any matching
+      assertions left over at the time the endpoint is removed.
+    - that appears to be it! Nowhere else in the code is a
+      `skeleton-interest` constructed with a non-`#f` cleanup
+      function.
  - figure out and describe scoped assertions / visibility-restrictions
+    - (partial/sketchy answer:) It's to deal with the fact that
+      multiple endpoints may overlap. Within a single dataspace, an
+      assertion matching both endpoints will trigger each of them.
+      When relaying, the relay maintains an endpoint in the outer
+      space for each in the inner space. When both outer endpoints are
+      triggered, if they were to naively relay the matching assertion,
+      the problem isn't so much that they'd double up (because
+      dataspaces deduplicate!), the problem is that they don't have
+      enough information to reconstruct the triggering outer assertion
+      perfectly! So a visibility-restriction causes an assertion to
+      *only* trigger inner endpoints that capture *exactly* the
+      captures of the outer endpoint. One of the outer endpoints will
+      trigger its "matching" inner endpoint, but not the inner
+      endpoint of the other endpoint, even though you might expect the
+      relayed assertion to do so.
 
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syndicate/mc/udp-dataspace.rkt

@@ -89,7 +89,7 @@
                                   (current-actor)
                                   (lambda ()
                                     (assert! (mcds-relevant term peer))))))
-                             void))
+                             #f))
                   (add-endpoint! (current-facet)
                                  "udp-dataspace (mcds-inbound (observe ...))"
                                  #t