Carry a module on the tool runner only when the runner has something of it to serve, so a part on its own credential has one writer (issue 1329)

internal/broker/users.go

@@ -206,7 +206,7 @@ func Users(r Records) ([]Principal, error) {
 		if runtimeHere {
 			var carried []Declared
 			for _, d := range r.Assigned[node] {
-				if d.RunsAs == "" {
+				if d.RunsAs == "" && runnerServes(d) {
 					// Never a restricted stream for what the tool runner carries (novox/hq ADR 0376 §1).
 					d.Hears = ComposedHears(d)
 					carried = append(carried, d)
@@ -391,3 +391,14 @@ func boundBucketsOf(r Records) []string {
 	sort.Strings(out)
 	return out
 }
+
+// runnerServes says the machine's tool runner has something of this module to serve (novox/hq ADR 0436 §1,
+// ADR 0437 §4, issue 1329): tools or loads it answers, a seat it holds, event families a session waits on,
+// or a check its health asks. A module with none of these is a process of its own on its own credential, and
+// carrying it would hand every session on the machine the runner's account with that module's state, seat
+// traffic and streams. A module with no account of its own (NoAccount) is spoken for by the runner, so it
+// stays carried, and so does the runtime module itself. Whether the module has its own broker secret is not
+// the test: a module with tools and its own credential is still carried (ADR 0175).
+func runnerServes(d Declared) bool {
+	return d.Module == RuntimeModule || d.NoAccount || len(d.Serves) > 0 || len(d.Holds) > 0 || len(d.Waits) > 0 || len(d.Checks) > 0
+}