Judge a revision that changes only the bus's user list as no move, and say so beside it (issue 1945; the review's A2)

The user list is a file in a directory the bus's container mounts, so it read as moving data and held every new
credential for a person's yes, against ADR 0539 clause 7; mesh-sdk's UserListOnly lets the assigner write such a
revision at once.

cmd/mesh-controller/composed_judge.go

@@ -67,10 +67,45 @@ func judgeMove(ctx context.Context, at composedPlace, key string, to sdkdelivery
 		out.Words = fmt.Sprintf("the composition %s runs on %s (%s) is gone", to.Module, at.node, short(strings.TrimPrefix(have.Composed, "sha256:")))
 		return out, nil
 	}
+	if onlyTheUserList(from, to) {
+		// Only the bus's user list (ADR 0539 clause 7): a file change and a reload of the bus, no move of it, written
+		// into its want at once.
+		out.Judgement, out.UserListOnly, out.Words = sdkdelivery.JudgedNeither, true, "only the bus's user list changes"
+		return out, nil
+	}
 	out.Judgement, out.Words = judged(from, to)
 	return out, nil
 }
 
+// busUsersSuffix ends the id of the bus module's user list as the composer writes it: `<module>.bus-users`.
+const busUsersSuffix = ".bus-users"
+
+// onlyTheUserList says two compositions of a module differ, and only in its file of the bus's user list.
+func onlyTheUserList(from, to sdkdelivery.Composed) bool {
+	if from.Data != to.Data {
+		return false
+	}
+	before, after := resourcesByID(from), resourcesByID(to)
+	if len(before) != len(after) {
+		return false
+	}
+	changed := false
+	for id, now := range after {
+		was, ok := before[id]
+		if !ok {
+			return false
+		}
+		if bytes.Equal(was.raw, now.raw) {
+			continue
+		}
+		if !strings.HasSuffix(id, busUsersSuffix) || now.kind != string(sdkdecl.TypeFile) {
+			return false
+		}
+		changed = true
+	}
+	return changed
+}
+
 // composedByDigest finds, in the node's bucket, the composition of module at fingerprint fp whose digest is digest: the
 // key's current value or one of the revisions it keeps.
 func composedByDigest(ctx context.Context, at composedPlace, module, fp, digest string) (sdkdelivery.Composed, bool, error) {