I’m trying to create an app using go grpc gateway but I faced with the problem. I integrate external service using it’s webhooks that use XML format. But as I understand grpc gateway doesn’t support request via XML so I decided to create a middleware that will convert XML to JSON in this way:

type Best2PayCallback struct {
    ...
}

func ConvertB2PCBToJSON(next http.Handler) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        decoder := xml.NewDecoder(r.Body)

        var callback Best2PayCallback
        if err := decoder.Decode(&callback); err != nil {
            http.Error(w, err.Error(), http.StatusInternalServerError)
            return
        }

        jsonData, err := json.Marshal(callback)
        if err != nil {
            http.Error(w, err.Error(), http.StatusInternalServerError)
        }

        r.Header.Set("Content-Type", "application/json")
        r.Body = io.NopCloser(bytes.NewReader(jsonData))

        next.ServeHTTP(w, r)
    }
}


func (s *Server) MapHandlers() {

    logger.Info("Trying to map handlers...")

    authClient := authProto.NewAuthServiceClient(s.authConn)
    mw := middleware.NewMDWManager(authClient)

    s.mux = runtime.NewServeMux(
        runtime.WithIncomingHeaderMatcher(customHeaderMatcher),
    )
    s.paymentServer = grpc.NewServer(
        grpc.ChainUnaryInterceptor(
            grpcConnector.LoggerInterceptor(),
        ),
    )
    
    paymentHandlers := paymentDelivery.NewHandlers()
    paymentProto.RegisterPaymentServiceServer(s.paymentServer, paymentHandlers)
    opts := []grpc.DialOption{grpc.WithTransportCredentials(insecure.NewCredentials())}

    err := paymentProto.RegisterPaymentServiceHandlerFromEndpoint(ctx, s.mux, s.cfg.PaymentServer.Grpc.Host, opts)
    if err != nil {
        return errors.Wrap(err, "failed to register payment gRPC service gateway")
    }

    httpMux := http.NewServeMux()
    httpMux.Handle("/b2p/listen",ConvertB2PCBToJSON(s.mux))
    httpMux.Handle("/", s.mux)
    logger.Infof("Serving http on %s", s.cfg.PaymentServer.Http.Host)
    if err := http.ListenAndServe(s.cfg.PaymentServer.Http.Host, httpMux); err != nil {
        return errors.Wrap(err, "failed to serve")
    }
}

But here’s the problem: when I try to send XML request to /b2p/listen address, it says:

invalid character '<' looking for beginning of value

And I don’t understand why. I thought that it was http package problem that it try to decode xml to json but I use application/xml header.

So what can be the problem?