middleware example in go

main.go

package main

import (
	"errors"
	"log"
	"net/http"
)

type handler func(writer http.ResponseWriter, request *http.Request, context map[string]string) error

type WebError struct {
	StatusCode int
	Message    string
}

func (w WebError) Error() string {
	return w.Message
}

func wrap(handlers ...handler) func(writer http.ResponseWriter, request *http.Request) {
	return func(writer http.ResponseWriter, request *http.Request) {
		context := make(map[string]string)
		for _, handler := range handlers {
			err := handler(writer, request, context)
			if err != nil {
				var webError WebError
				if errors.As(err, &webError) {
					// You could also marshal a JSON object here if your api deals with JSON.
					writer.WriteHeader(webError.StatusCode)
					_, _ = writer.Write([]byte(webError.Error()))
				}
				break
			}
		}
	}
}

func tester(writer http.ResponseWriter, _ *http.Request, _ map[string]string) error {
	_, _ = writer.Write([]byte("Yay!"))
	return nil
}

func main() {
	http.HandleFunc("/", wrap(authenticate, authorize("admin"), tester))
	http.HandleFunc("/health", healthCheck) // no need for middleware here
	log.Println("Starting")
	_ = http.ListenAndServe(":8080", nil)
}

func healthCheck(writer http.ResponseWriter, _ *http.Request) {
	_, _ = writer.Write([]byte("ok"))
}

func authorize(s string) handler {
	return func(writer http.ResponseWriter, request *http.Request, context map[string]string) error {
		if context["claim"] != s {
			return WebError{
				StatusCode: http.StatusUnauthorized,
				Message:    "No go bro",
			}
		}

		return nil
	}
}

func authenticate(_ http.ResponseWriter, request *http.Request, context map[string]string) error {
	query := request.URL.Query()["claim"]
	if len(query) > 0 {
		context["claim"] = query[0]
		return nil
	} else {
		return WebError{
			StatusCode: http.StatusUnauthorized,
			Message:    "Missing claim bro",
		}
	}
}