grpc：服务器无法编码响应：rpc错误：代码=内部desc = grpc：编组时出错：原型：元帅以nil调用

Question

The following RPC should ideally receive a message and marshal to JSON. But, the following error is encountered: ERROR: 2018/08/12 13:43:07 grpc: server failed to encode response: rpc error: code = Internal desc = grpc: error while marshaling: proto: Marshal called with nil

func (s *beaconServer) Transmit(ctx context.Context, batch *pb.Batch) (*pb.Empty, error) {
    var empty *pb.Empty

    var messageJSON bytes.Buffer
    marshaler := &jsonpb.Marshaler{
        OrigName: true,
    }
    err := marshaler.Marshal(&messageJSON, batch)
    if err != nil {
        return empty, err
    }

    log.Println(string(messageJSON.Bytes()))
    return empty, nil
}

.. returns

2018/08/12 14:24:09 beacon.go:34: {"stream_id":"abc11","event_type":"e","events":[{"file_path":"/tmp/python.py","location":"256","count":"30"},{"file_path":"/tmp/temp.py","location":"253","count":"305"}],"start_time":"2038-01-19 03:14:07","end_time":"2038-01-19 03:14:27"}
ERROR: 2018/08/12 14:24:09 grpc: server failed to encode response:  rpc error: code = Internal desc = grpc: error while marshaling: proto: Marshal called with nil

Answer 1

The error is probably not caused by the marshallers in this code block

When you do var empty *pb.Empty you are creating an uninitialised variable *pb.Empty which is nil. It's very likely that something upstream that tries to marshal this empty is causing the error.

Runnable example here: https://play.golang.org/p/QsNCWXM615Q

If you want to initialise empty, then you should do return new(pb.Empty), nil