CORS grpc Gateway GoLang

I have a vue.js 3 frontend, and I am calling a Golang backend via grpc-gateway. I have been at this for a while but I see light at the end of the tunnel.

I am currently facing a CORS issue. However, I am reading conflicting information on how to handle it. Therefore, I want to post and hopefully it helps someone.

Here is the code on how I init my mux server for GRPC (gateway)

func RunHttpServer(server *http.Server, httpEndpoint, grpcEndpoint, swaggerPath string) (err error) {
    server.Addr = httpEndpoint

    ctx, cancel := context.WithCancel(context.Background())

    defer cancel()

    // Register gROC server endpoint
    mux := runtime.NewServeMux(
        runtime.WithErrorHandler(func(ctx context.Context,
            mux *runtime.ServeMux,
            marshaler runtime.Marshaler,
            w http.ResponseWriter, r *http.Request,
            err error,
        ) {
            s, ok := status.FromError(err)
            if ok {
                if s.Code() == codes.Unavailable {
                    err = status.Error(codes.Unavailable, ErrUnavailable)
                }
            }

            runtime.DefaultHTTPErrorHandler(ctx, mux, marshaler, w, r, err)

        }),
    )

    opts := []grpc.DialOption{
        grpc.WithTransportCredentials(insecure.NewCredentials()),
        grpc.WithChainUnaryInterceptor(),
    }

    if err = api.RegisterApiServiceHandlerFromEndpoint(ctx, mux, grpcEndpoint, opts); err != nil {
        return
    }

    swMux := http.NewServeMux()
    swMux.Handle("/", mux)
    serveSwagger(swMux, swaggerPath)

    server.Handler = swMux

    return server.ListenAndServe()

}

Here is where I believe I should add the cors config, but I am not sure this is how I set it up in the server.go file..

var httpServer http.Server

// Run Http Server with gRPC gateway
g.Go(func() error {
    fmt.Println("Starting Http sever (port {}) and gRPC gateway (port {})",
        strconv.Itoa(cfg.Server.HTTPPort),
        strconv.Itoa(cfg.Server.GRPCPort),
    )

    return rest.RunHttpServer(
        &httpServer,
        ":"+strconv.Itoa(cfg.Server.HTTPPort),
        ":"+strconv.Itoa(cfg.Server.GRPCPort),
        "/webapi",
    )
})

error in console:

Access to XMLHttpRequest at 'http://localhost:8080/v1/test' from origin 'http://localhost:9000' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin'

I am not sure where to add something like

func enableCors(w *http.ResponseWriter) {
    (*w).Header().Set("Access-Control-Allow-Origin", "*")
}

and I feel the golang GRPC gateway should have something built in but I cannot find anything?

Any advice would be greatly appreciated.

----- update 1 -----

I have tried

func enableCors(h http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        w.Header().Set("Access-Control-Allow-Origin", "http://localhost:9000")
        w.Header().Set("Access-Control-Allow-Methods", "GET, PUT, POST, DELETE, HEAD, OPTIONS")
        h.ServeHTTP(w, r)
    })
}

and

func enableCors(h http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        w.Header().Set("Access-Control-Allow-Origin", "*")
        w.Header().Set("Access-Control-Allow-Methods", "GET, PUT, POST, DELETE, HEAD, OPTIONS")
        h.ServeHTTP(w, r)
    })
}

and

func enableCors(h http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        w.Header().Set("Access-Control-Allow-Origin", "http://localhost")
        w.Header().Set("Access-Control-Allow-Methods", "GET, PUT, POST, DELETE, HEAD, OPTIONS")
        h.ServeHTTP(w, r)
    })
}

in conjuction with

func serveSwagger(mux *http.ServeMux, swaggerPath string) {
    fileServer := http.FileServer(http.Dir(swaggerPath))
    prefix := "/swagger-ui"
    mux.Handle(prefix, http.StripPrefix(prefix, fileServer))
}

and still have the same issue.. Very frustrating

Solution

Based on the latest error you've provided in a comment:

Access to XMLHttpRequest at 'localhost:8080/v1/test' from origin 'localhost:9000' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: It does not have HTTP ok status.

Your browser is sending a preflight request (OPTIONS HTTP method) to ascertain whether the desired cross origin request can be made.

And the server is responding with a non-2xx response.

I suspect this is because your enableCors functions are propagating the request to the grpc-gateway handler, which is unhappy with the OPTIONS HTTP method and returning an error status, probably:

< HTTP/1.1 501 Not Implemented
< Content-Type: application/json
< Vary: Origin
< Date: Fri, 25 Nov 2022 11:17:52 GMT
< Content-Length: 55
< 
{"code":12,"message":"Method Not Allowed","details":[]}

So to avoid that, you'd want to not propagate the request further in the case of a preflight request being made, e.g.

func enableCors(h http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        w.Header().Set("Access-Control-Allow-Origin", "http://localhost:9000")
        w.Header().Set("Access-Control-Allow-Methods", "GET, PUT, POST, DELETE, HEAD, OPTIONS")
        if r.Method == http.MethodOptions {
            w.WriteHeader(http.StatusNoContent)
            return
        }
        h.ServeHTTP(w, r)
    })
}

However, the above is likely still not a reasonable implementation of CORS handling. You should look to use an existing package for this, e.g. github.com/rs/cors, which will handle this in a reasonable fashion, and also deal with any potential gotchas, etc.

So importing github.com/rs/cors and then doing something like:

server.Handler = cors.AllowAll().Handler(swMux)

should get you started by allowing everything through. The library will allow you to tailor to specific origins, HTTP methods, etc, as required.