Grpc-gateway流式HTTP响应解码失败：如何排查并解决runtime.JSONPb.Decode返回nil值的问题？

{"result":{"code":1,"msg":"1111"}}
{"result":{"code":2,"msg":"2222"}}
{"result":{"code":3,"msg":"3333"}}
{"result":{"code":4,"msg":"4444"}}
{"result":{"code":5,"msg":"5555"}}
{"result":{"code":6,"msg":"6666"}}

syntax = "proto3";

package pb;

option go_package = "/pb;pb";

import "google/api/annotations.proto";

message Req {
    int32 id = 1;
    string name = 2;
}

message Resp {
    int32 code = 1;
    string msg = 2;
}

service TestService {
    rpc QueryStreamResp(Req) returns (stream Resp) {
        option (google.api.http) = {
            post: "/query-stream-resp"
            body: "*"
        };
    };
}

func (ts *TestService) QueryStreamResp(req *pb.Req, stream pb.TestService_QueryStreamRespServer) error {
    // ... 服务端逻辑 ...
    for _, r := range result {
        log.Printf("resp: %+v", r)
        if err := stream.Send(r); err != nil {
            log.Fatal(err)
        }
        time.Sleep(100 * time.Millisecond)
    }
    // ... 服务端逻辑 ...
    return nil
}

func TestHttpRespStream(t *testing.T) {
    // ... 测试代码 ...
    jsonb := &runtime.JSONPb{}
    decoder := jsonb.NewDecoder(resp.Body)

    for {
        var result *pb.Resp
        err := decoder.Decode(result)
        // ... 测试代码 ...
    }
}

message HttpResp {
    Resp result = 1;
}

var result *pb.HttpResp
err := decoder.Decode(&result)