fix(gengateway): correct body field decoding in opaque API mode

[kop]

References to other Issues or PRs

Fixes #6189

Have you read the Contributing Guidelines?

Yes

Brief description of what is fixed or changed

When use_opaque_api is enabled with a body field mapping (e.g., body: "field_name"), the generated code was incorrectly decoding the HTTP request body as the full request message type instead of the specific field's message type.

This fix:

• Adds GetBodyFieldType() helper to determine the body field's type
• Updates template to decode into the field type and pass it directly to the setter method

Other comments

I added OpaqueCreateProduct and OpaqueCreateProductField to opaque.proto to act as a test case.

Generated code shows the difference. Note var bodyData OpaqueCreateProductRequest vs bodyData := &OpaqueProduct{}.

func request_OpaqueEcommerceService_OpaqueCreateProduct_0(ctx context.Context, marshaler runtime.Marshaler, client OpaqueEcommerceServiceClient, req *http.Request, pathParams map[string]string) (proto.Message, runtime.ServerMetadata, error) {
	var (
		protoReq OpaqueCreateProductRequest
		metadata runtime.ServerMetadata
	)
	var bodyData OpaqueCreateProductRequest
	if err := marshaler.NewDecoder(req.Body).Decode(&bodyData); err != nil && !errors.Is(err, io.EOF) {
		return nil, metadata, status.Errorf(codes.InvalidArgument, "%v", err)
	}
	protoReq = bodyData
	if req.Body != nil {
		_, _ = io.Copy(io.Discard, req.Body)
	}
	msg, err := client.OpaqueCreateProduct(ctx, &protoReq, grpc.Header(&metadata.HeaderMD), grpc.Trailer(&metadata.TrailerMD))
	return msg, metadata, err
}

vs

func request_OpaqueEcommerceService_OpaqueCreateProductField_0(ctx context.Context, marshaler runtime.Marshaler, client OpaqueEcommerceServiceClient, req *http.Request, pathParams map[string]string) (proto.Message, runtime.ServerMetadata, error) {
	var (
		protoReq OpaqueCreateProductFieldRequest
		metadata runtime.ServerMetadata
	)
	bodyData := &OpaqueProduct{}
	if err := marshaler.NewDecoder(req.Body).Decode(bodyData); err != nil && !errors.Is(err, io.EOF) {
		return nil, metadata, status.Errorf(codes.InvalidArgument, "%v", err)
	}
	protoReq.SetProduct(bodyData)
	if req.Body != nil {
		_, _ = io.Copy(io.Discard, req.Body)
	}
	if err := req.ParseForm(); err != nil {
		return nil, metadata, status.Errorf(codes.InvalidArgument, "%v", err)
	}
	if err := runtime.PopulateQueryParameters(&protoReq, req.Form, filter_OpaqueEcommerceService_OpaqueCreateProductField_0); err != nil {
		return nil, metadata, status.Errorf(codes.InvalidArgument, "%v", err)
	}
	msg, err := client.OpaqueCreateProductField(ctx, &protoReq, grpc.Header(&metadata.HeaderMD), grpc.Trailer(&metadata.TrailerMD))
	return msg, metadata, err
}

[johanbrandhorst]

Thanks for the quick fix! The bazel error seems unrelated, I expect it will go away.