add 0x04 Reason Code functionality (#395)

* add 0x04 Reason Code functionality

* fix tpackets test

* simplify logic

---------

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

packets/tpackets.go

@@ -201,6 +201,7 @@ const (
 	TDisconnect
 	TDisconnectTakeover
 	TDisconnectMqtt5
+	TDisconnectMqtt5DisconnectWithWillMessage
 	TDisconnectSecondConnect
 	TDisconnectReceiveMaximum
 	TDisconnectDropProperties
@@ -3781,6 +3782,31 @@ var TPacketData = map[byte]TPacketCases{
 				},
 			},
 		},
+		{
+			Case:    TDisconnectMqtt5DisconnectWithWillMessage,
+			Desc:    "mqtt5 disconnect with will message",
+			Primary: true,
+			RawBytes: append([]byte{
+				Disconnect << 4, 38, // fixed header
+				CodeDisconnectWillMessage.Code, // Reason Code
+				36,                             // Properties Length
+				17, 0, 0, 0, 120,               // Session Expiry Interval (17)
+				31, 0, 28, // Reason String (31)
+			}, []byte(CodeDisconnectWillMessage.Reason)...),
+			Packet: &Packet{
+				ProtocolVersion: 5,
+				FixedHeader: FixedHeader{
+					Type:      Disconnect,
+					Remaining: 22,
+				},
+				ReasonCode: CodeDisconnectWillMessage.Code,
+				Properties: Properties{
+					ReasonString:              CodeDisconnectWillMessage.Reason,
+					SessionExpiryInterval:     120,
+					SessionExpiryIntervalFlag: true,
+				},
+			},
+		},
 		{
 			Case: TDisconnectSecondConnect,
 			Desc: "second connect packet mqtt5",

server.go

@@ -1393,6 +1393,10 @@ func (s *Server) processDisconnect(cl *Client, pk packets.Packet) error {
 		cl.Properties.Props.SessionExpiryIntervalFlag = true
 	}
 
+	if pk.ReasonCode == packets.CodeDisconnectWillMessage.Code { // [MQTT-3.1.2.5] Non-normative comment
+		return packets.CodeDisconnectWillMessage
+	}
+
 	s.loop.willDelayed.Delete(cl.ID) // [MQTT-3.1.3-9] [MQTT-3.1.2-8]
 	cl.Stop(packets.CodeDisconnect)  // [MQTT-3.14.4-2]
 

server_test.go

@@ -3138,6 +3138,22 @@ func TestServerProcessPacketDisconnectNonZeroExpiryViolation(t *testing.T) {
 	require.ErrorIs(t, err, packets.ErrProtocolViolationZeroNonZeroExpiry)
 }
 
+func TestServerProcessPacketDisconnectDisconnectWithWillMessage(t *testing.T) {
+	s := newServer()
+	cl, _, _ := newTestClient()
+	cl.Properties.Props.SessionExpiryInterval = 30
+	cl.Properties.ProtocolVersion = 5
+
+	s.loop.willDelayed.Add(cl.ID, packets.Packet{TopicName: "a/b/c", Payload: []byte("hello")})
+	require.Equal(t, 1, s.loop.willDelayed.Len())
+
+	err := s.processPacket(cl, *packets.TPacketData[packets.Disconnect].Get(packets.TDisconnectMqtt5DisconnectWithWillMessage).Packet)
+	require.Error(t, err)
+
+	require.Equal(t, 1, s.loop.willDelayed.Len())
+	require.False(t, cl.Closed())
+}
+
 func TestServerProcessPacketAuth(t *testing.T) {
 	s := newServer()
 	cl, r, w := newTestClient()