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();
    });
  }
Tests for HBASE-28321, where we have multiple server principals candidates for a rpc service.
<p>
Put here just because we need to visit some package private classes under this package.
hbase-server/src/test/java/org/apache/hadoop/hbase/ipc/TestMultipleServerPrincipalsIPC.java