send a HANDSHAKE_DONE frame after handshake completion (as a server)

session.go

@@ -603,16 +603,14 @@ func (s *session) handleHandshakeComplete() {
 
 	s.connIDGenerator.SetHandshakeComplete()
 	s.sentPacketHandler.SetHandshakeComplete()
-	// The client completes the handshake first (after sending the CFIN).
-	// We need to make sure it learns about the server completing the handshake,
-	// in order to stop retransmitting handshake packets.
-	// They will stop retransmitting handshake packets when receiving the first 1-RTT packet.
+
 	if s.perspective == protocol.PerspectiveServer {
 		token, err := s.tokenGenerator.NewToken(s.conn.RemoteAddr())
 		if err != nil {
 			s.closeLocal(err)
 		}
 		s.queueControlFrame(&wire.NewTokenFrame{Token: token})
+		s.queueControlFrame(&wire.HandshakeDoneFrame{})
 	}
 }
 
@@ -855,6 +853,7 @@ func (s *session) handleFrame(f wire.Frame, pn protocol.PacketNumber, encLevel p
 		err = s.handleNewConnectionIDFrame(frame)
 	case *wire.RetireConnectionIDFrame:
 		err = s.handleRetireConnectionIDFrame(frame)
+	case *wire.HandshakeDoneFrame:
 	default:
 		err = fmt.Errorf("unexpected frame type: %s", reflect.ValueOf(&frame).Elem().Type().Name())
 	}