Enforce server max packet (#121)

* Enforce Server Maximum Packet Size on client read
* Fix tests

clients.go

@@ -383,6 +383,10 @@ func (cl *Client) ReadFixedHeader(fh *packets.FixedHeader) error {
 		return err
 	}
 
+	if cl.ops.capabilities.MaximumPacketSize > 0 && uint32(fh.Remaining+1) > cl.ops.capabilities.MaximumPacketSize {
+		return packets.ErrPacketTooLarge // [MQTT-3.2.2-15]
+	}
+
 	atomic.AddInt64(&cl.ops.info.BytesReceived, int64(bu+1))
 	return nil
 }

clients_test.go

@@ -350,6 +350,22 @@ func TestClientReadFixedHeaderDecodeError(t *testing.T) {
 	require.Error(t, err)
 }
 
+func TestClientReadFixedHeaderPacketOversized(t *testing.T) {
+	cl, r, _ := newTestClient()
+	cl.ops.capabilities.MaximumPacketSize = 2
+	defer cl.Stop(errClientStop)
+
+	go func() {
+		r.Write(packets.TPacketData[packets.Publish].Get(packets.TPublishQos1Dup).RawBytes)
+		r.Close()
+	}()
+
+	fh := new(packets.FixedHeader)
+	err := cl.ReadFixedHeader(fh)
+	require.Error(t, err)
+	require.ErrorIs(t, err, packets.ErrPacketTooLarge)
+}
+
 func TestClientReadFixedHeaderReadEOF(t *testing.T) {
 	cl, r, _ := newTestClient()
 	defer cl.Stop(errClientStop)

server.go

@@ -1126,12 +1126,15 @@ func (s *Server) DisconnectClient(cl *Client, code packets.Code) error {
 
 	// We already have a code we are using to disconnect the client, so we are not
 	// interested if the write packet fails due to a closed connection (as we are closing it).
-	_ = cl.WritePacket(out)
+	err := cl.WritePacket(out)
 	if !s.Options.Capabilities.Compatibilities.PassiveClientDisconnect {
 		cl.Stop(code)
+		if code.Code >= packets.ErrUnspecifiedError.Code {
+			return code
+		}
 	}
 
-	return code
+	return err
 }
 
 // publishSysTopics publishes the current values to the server $SYS topics.

server_test.go

@@ -2291,6 +2291,21 @@ func TestServerRecievePacketDisconnectClientZeroNonZero(t *testing.T) {
 	require.Equal(t, packets.TPacketData[packets.Disconnect].Get(packets.TDisconnectZeroNonZeroExpiry).RawBytes, buf)
 }
 
+func TestServerRecievePacketDisconnectClient(t *testing.T) {
+	s := newServer()
+	cl, r, w := newTestClient()
+
+	go func() {
+		err := s.DisconnectClient(cl, packets.CodeDisconnect)
+		require.NoError(t, err)
+		w.Close()
+	}()
+
+	buf, err := io.ReadAll(r)
+	require.NoError(t, err)
+	require.Equal(t, packets.TPacketData[packets.Disconnect].Get(packets.TDisconnect).RawBytes, buf)
+}
+
 func TestServerProcessPacketDisconnect(t *testing.T) {
 	s := newServer()
 	cl, _, _ := newTestClient()