[Question] Behavior of empty repeated field

[Bo0km4n]

What version of gRPC are you using?

1.16.0

What version of Go are you using (go version)?

go version go1.11.1 darwin/amd64

What operating system (Linux, Windows, …) and version?

maxOS High Sierra 10.13.6

What did you do?

Hi, everyone.
I have a question about handling empty array of protobuf's repeated fields.

The following shows two codes of client and server and proto file.

server code

func launchServer() {
	fmt.Println("<<<<< server >>>>>")
	lis, err := net.Listen("tcp", ":5050")
	if err != nil {
		log.Fatal(err)
	}
	s := grpc.NewServer()
	RegisterShopAPIServer(s, newShopAPIHandler())

	reflection.Register(s)
	if err := s.Serve(lis); err != nil {
		log.Fatal(err)
	}
}

// implementation grpc shop api
type shopAPI struct{}

func newShopAPIHandler() *shopAPI {
	return &shopAPI{}
}

func (s *shopAPI) List(c context.Context, in *Empty) (*Books, error) {
	res := &Books{
		DebugStringFields: []string{},
		Books:             []*Book{
			//  {Price: 500, Name: "Hoge"},
		},
	}
	resJsonb, _ := json.Marshal(res)
	fmt.Println(string(resJsonb))
	return res, nil
}

client code

func launchClient() {
	conn, err := grpc.Dial("localhost:5050", grpc.WithInsecure())
	if err != nil {
		log.Fatal(err)
	}

	defer conn.Close()
	cli := NewShopAPIClient(conn)
	res, err := cli.List(context.Background(), &Empty{})
	if err != nil {
		log.Fatal(err)
	}
	jsonb, _ := json.Marshal(res)
	fmt.Println(string(jsonb))
}

shop proto file

syntax = "proto3";

package main;

service ShopAPI {
    rpc List(Empty) returns(Books);
}

message Empty {}

message Books {
    repeated string debug_string_fields = 1;
    repeated Book books = 2;
}

message Book {
    string name = 1;
    int64 price = 2;
}

The result of client requesting server and the output of server side.

client

{"debug_string_fields":null,"books":null}

server

<<<<< server >>>>>
{"debug_string_fields":[],"books":[]}

What did you expect to see?

I expect client side response like this:

{"debug_string_fields":[],"books":[]}

I guessed that Books fields are not null, cause assigned empty array: Books: []*Book{},.

Currently check the field of response, if the length of the array is 0, you have to write code to allocate an empty array like this:

res, err := cli.List(context.Background(), &Empty{})
	if err != nil {
		log.Fatal(err)
	}
 if len(res.Books) == 0 {
  res.Books = []*Book{}
}

This seems not to be a very smart solution. Please let me know if there is any good way.

[menghanl]

Which json library are you using?

The repeated fields are tagged with json:"...,omitempty", so it should be omitted in this case.

If this is still not helping, try the proto json package instead: https://godoc.org/github.com/golang/protobuf/jsonpb#Marshaler.Marshal

[Bo0km4n]

Thanks reply! I'm using standard json library.

Then, I removed tag which omitempty, but never changed result.
I'll try use jsonpb just now.

[Bo0km4n]

@menghanl I'm tried jsonpb. That's behavior is I'm expected!!! Thanks 👍 !