Extend grpc-gateway generated functions with custom decode logic in proto.Message

Question

In the course of work there appeared a necessity to perform strings.TrimSpace on all fields with protobuf type string

As a result it was decided to write a plugin that generates the necessary functions for each structure: trimAll (to create a function to trim fields of type string) and UnmarshalJSON (we thought that this is the function used by grpc-gateway to translate from http.Request.Body to proto.Message).

func (m *GetLogbookCall_Request) trimAll() {
    if m == nil {
        return
    }
    m.Id = strings.TrimSpace(m.Id)
}

func (m *GetLogbookCall_Request) UnmarshalJSON(data []byte) error {
    err := proto.Unmarshal(data, m)
    if err != nil {
        return err
    }

    m.trimAll()

    return nil
}

But nothing worked as a result)

The question is: Is there any way to add custom logic when translating http.Request.Body to proto.Message?

PS:

Here are the functions generated by the grpc-gateway plugin.

func request_AccountService_UpdatePassword_0(ctx context.Context, marshaler runtime.Marshaler, client AccountServiceClient, req *http.Request, pathParams map[string]string) (proto.Message, runtime.ServerMetadata, error) {
    var protoReq account.UpdatePasswordCall_Request
    var metadata runtime.ServerMetadata

    if err := marshaler.NewDecoder(req.Body).Decode(&protoReq); err != nil && err != io.EOF {
        return nil, metadata, status.Errorf(codes.InvalidArgument, "%v", err)
    }

    msg, err := client.UpdatePassword(ctx, &protoReq, grpc.Header(&metadata.HeaderMD), grpc.Trailer(&metadata.TrailerMD))
    return msg, metadata, err

}

Answer 1

I found a solution to my problem. You can write a custom Marshaler and then use it to create a proxy server.

type Marshaler struct {
    runtime.JSONPb
}

func (m *Marshaler) NewDecoder(r io.Reader) runtime.Decoder {
    d := json.NewDecoder(r)
    return Decoder{
        decoder: &runtime.DecoderWrapper{
            Decoder:          d,
            UnmarshalOptions: m.UnmarshalOptions,
        },
    }
}

type Decoder struct {
    decoder *runtime.DecoderWrapper
}

func (d Decoder) Decode(v interface{}) error {
    err := d.decoder.Decode(v)
    if err != nil {
        return err
    }
    
    // Write your own custom logic here. In my case it's use TrimAll method

    type Trimmable interface {
        TrimAll()
    }

    if v, ok := v.(Trimmable); ok {
        v.TrimAll()
    }

    return nil
}

    mux := runtime.NewServeMux(
        runtime.WithMetadata(func(ctx context.Context, r *http.Request) metadata.MD {
        runtime.WithMarshalerOption(runtime.MIMEWildcard, &Marshaler{
            runtime.JSONPb{
                MarshalOptions: protojson.MarshalOptions{
                    EmitUnpopulated: true,
                },
            },
        }),
    )