Add retainMessage to LWT to properly handle message retention (#234)

* Add retainMessage to LWT to properly handle message retention if specified in connect

* Add will retain flag on missed test

---------

Co-authored-by: Derek Duncan <derekduncan@gmail.com>

server.go

@@ -1285,6 +1285,10 @@ func (s *Server) sendLWT(cl *Client) {
 		return
 	}
 
+	if pk.FixedHeader.Retain {
+		s.retainMessage(cl, pk)
+	}
+
 	s.publishToSubscribers(pk)                      // [MQTT-3.1.2-8]
 	atomic.StoreUint32(&cl.Properties.Will.Flag, 0) // [MQTT-3.1.2-10]
 	s.hooks.OnWillSent(cl, pk)
@@ -1477,6 +1481,9 @@ func (s *Server) sendDelayedLWT(dt int64) {
 		if dt > pk.Expiry {
 			s.publishToSubscribers(pk) // [MQTT-3.1.2-8]
 			if cl, ok := s.Clients.Get(id); ok {
+				if pk.FixedHeader.Retain {
+					s.retainMessage(cl, pk)
+				}
 				cl.Properties.Will = Will{} // [MQTT-3.1.2-10]
 				s.hooks.OnWillSent(cl, pk)
 			}

server_test.go

@@ -2586,6 +2586,46 @@ func TestServerSendLWT(t *testing.T) {
 	require.Equal(t, packets.TPacketData[packets.Publish].Get(packets.TPublishBasic).RawBytes, <-receiverBuf)
 }
 
+func TestServerSendLWTRetain(t *testing.T) {
+	s := newServer()
+	s.Serve()
+	defer s.Close()
+
+	sender, _, w1 := newTestClient()
+	sender.ID = "sender"
+	sender.Properties.Will = Will{
+		Flag:      1,
+		TopicName: "a/b/c",
+		Payload:   []byte("hello mochi"),
+		Retain:    true,
+	}
+	s.Clients.Add(sender)
+
+	receiver, r2, w2 := newTestClient()
+	receiver.ID = "receiver"
+	s.Clients.Add(receiver)
+	s.Topics.Subscribe(receiver.ID, packets.Subscription{Filter: "a/b/c", Qos: 0})
+
+	require.Equal(t, int64(0), atomic.LoadInt64(&s.Info.PacketsReceived))
+	require.Equal(t, 0, len(s.Topics.Messages("a/b/c")))
+
+	receiverBuf := make(chan []byte)
+	go func() {
+		buf, err := io.ReadAll(r2)
+		require.NoError(t, err)
+		receiverBuf <- buf
+	}()
+
+	go func() {
+		s.sendLWT(sender)
+		time.Sleep(time.Millisecond * 10)
+		w1.Close()
+		w2.Close()
+	}()
+
+	require.Equal(t, packets.TPacketData[packets.Publish].Get(packets.TPublishRetain).RawBytes, <-receiverBuf)
+}
+
 func TestServerSendLWTDelayed(t *testing.T) {
 	s := newServer()
 	cl1, _, _ := newTestClient()