private String echo(String msg) throws Exception {
    return clientUGI.doAs((PrivilegedExceptionAction<String>) () -> {
      BlockingRpcChannel channel = rpcClient.createBlockingRpcChannel(
        ServerName.valueOf(HOST, rpcServer.getListenerAddress().getPort(), -1), User.getCurrent(),
        10000);
      TestProtobufRpcProto.BlockingInterface stub = TestProtobufRpcProto.newBlockingStub(channel);
      return stub.echo(null, TestProtos.EchoRequestProto.newBuilder().setMessage(msg).build())
        .getMessage();
    });
  }
Test secure client connecting to a non secure server, where we have multiple server principal
candidates for a rpc service. See HBASE-28321.
hbase-server/src/test/java/org/apache/hadoop/hbase/security/TestMultipleServerPrincipalsFallbackToSimple.java