fix: harden single-machine load handling

server/hub_tenant_test.go

@@ -102,3 +102,48 @@ func TestHubRejectsUnroutedClientMessageInsteadOfAcceptingWithoutSeq(t *testing.
 		t.Fatal("timeout waiting for unrouted message response")
 	}
 }
+
+func TestHubReturnsServiceUnavailableWhenTopicClientQueueIsFull(t *testing.T) {
+	h := &Hub{
+		topics:     &sync.Map{},
+		routeCli:   make(chan *ClientComMessage, 1),
+		shutdown:   make(chan chan<- bool),
+		routeSrv:   make(chan *ServerComMessage),
+		join:       make(chan *ClientComMessage),
+		meta:       make(chan *ClientComMessage),
+		unreg:      make(chan *topicUnreg),
+		userStatus: make(chan *userStatusReq),
+		rehash:     make(chan bool),
+	}
+	topic := &Topic{
+		tenantID: 7, name: "grpBusy", clientMsg: make(chan *ClientComMessage, 1),
+		exit: make(chan *shutDown, 1),
+	}
+	topic.clientMsg <- &ClientComMessage{Id: "occupy"}
+	h.topicPut(types.TopicKey{TenantID: 7, Topic: topic.name}, topic)
+
+	go h.run()
+	done := make(chan bool)
+	defer func() {
+		h.topicDel(types.TopicKey{TenantID: 7, Topic: topic.name})
+		h.shutdown <- done
+		<-done
+	}()
+
+	sess := &Session{tenantID: 7, send: make(chan any, 1)}
+	now := time.Now().UTC()
+	h.routeCli <- &ClientComMessage{
+		Id: "pub-overload", TenantID: 7, RcptTo: topic.name, Original: topic.name,
+		Timestamp: now, sess: sess,
+	}
+
+	select {
+	case raw := <-sess.send:
+		msg, ok := raw.(*ServerComMessage)
+		if !ok || msg.Ctrl == nil || msg.Ctrl.Code != http.StatusServiceUnavailable {
+			t.Fatalf("unexpected response: %#v", raw)
+		}
+	case <-time.After(time.Second):
+		t.Fatal("timeout waiting for overload response")
+	}
+}