Golang HTTP custom error handling response

I was going through https://blog.golang.org/error-handling-and-go and at the end it gave a good example on how to handle returning errors in a cleaner way and just made something simple:

// Wrapper for handler functions.
type rootHandler func(http.ResponseWriter, *http.Request) error

// Implement the http.Handler interface.
func (fn rootHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    err := fn(w, r) // Call handler function.
    if err == nil {
        return
    }

    ...
}

// POST /api/test/
testRouter.
    Handle("/", rootHandler(create)).
    Methods("POST")

// GET /api/test/{id}/
testRouter.
    HandleFunc("/{id:[0-9]+}/", rootHandler(getByID)).
    Methods("GET")

func create(w http.ResponseWriter, r *http.Request) error {
    // CustomError implementes error
    return &CustomError{
        Kind:       EPARSE,
        Status:     http.StatusBadRequest,
        Message:    "some message",
        Op:         "create",
        Err:        err,
    }
}

This worked very well, but I would prefer not to wrap every controller method (create in this case) in rootHandler, and figured the best way is to figure out some sort of post-middleware. I've failed at trying to create a post-middlewhere which the router uses instead of each controller method, and wondering how you may go about implementing this. The closest answer on SO I could find was emmbee's answer on https://stackoverflow.com/questions/42871194/how-can-i-combine-go-middleware-pattern-with-error-returning-request-handlers except for fn in AuthMiddleware would be a controller method.

So ideally, I would have the below handler which handles the CustomError if it exists

// POST /api/test
testRouter.
    Handle("/", create).
    Methods("POST")

For context, I'm using gorilla mux and negroni.

Any ideas are appreciated! Thank you very much.

Your ideal solution will not work. The mux API supports http.Handler and func(http.ResponseWriter, *http.Request) arguments. You have a func(http.ResponseWriter, *http.Request) error. A func(http.ResponseWriter, *http.Request) error cannot be passed as one of the argument types supported by the mux API.

The only solution is to adapt each controller function to a type supported by the mux API using the rootHandler wrapper. The wrapper does not add overhead compared to the standard http.HandlerFunc wrapper.

Use a helper function to reduce the amount of code required for the wrappers:

func handle(r *mux.Router, p string, fn func(http.ResponseWriter, *http.Request) error) *mux.Route {
    return r.Handle(path, rootHandler(fn))
}

...
handle(testRouter, "/", create).Methods("POST")
handle(testRouter, "/{id:[0-9]+}/", getByID).Methods("GET")

Use a type switch in the helper function to handle different controller types:

func handle(r *mux.Router, p string, h interface{}) *mux.Route {
	switch h := h.(type) {
	case func(http.ResponseWriter, *http.Request) error:
		return r.Handle(p, rootHandler(h))
	case func(http.ResponseWriter, *http.Request):
		return r.HandleFunc(p, h)
	case http.Handler:
		return r.Handle(p, h)
	default:
		panic(fmt.Sprintf("handler type %T not supported", h))
	}
}