Go Middleware Chains

main.go

package main

// Loosely based on https://www.alexedwards.net/blog/making-and-using-middleware
// and https://blog.questionable.services/article/guide-logging-middleware-go/
// Similar to https://github.com/justinas/alice but more minimalist :)

import (
	"log"
	"net/http"
)

type Middleware func(http.Handler) http.Handler

type loggingInterceptor struct {
	w             http.ResponseWriter
	statusCode    int
	statusWritten bool
}

func (l *loggingInterceptor) Header() http.Header {
	return l.w.Header()
}

func (l *loggingInterceptor) Write(bs []byte) (int, error) {
	return l.w.Write(bs)
}

func (l *loggingInterceptor) WriteHeader(statusCode int) {
	if l.statusWritten {
		return
	}

	l.statusCode = statusCode
	l.w.WriteHeader(statusCode)
	l.statusWritten = true
}

func wrapResponseWriter(w http.ResponseWriter) *loggingInterceptor {
	return &loggingInterceptor{w: w, statusCode: http.StatusOK}
}

func requestLogger(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		wrapped := wrapResponseWriter(w)
		next.ServeHTTP(wrapped, r)
		log.Printf("%s %s - %d", r.Method, r.URL.Path, wrapped.statusCode)
	})
}

func authHandler(password string) Middleware {
	return func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			authHeader := r.Header.Get("Authorization")

			if authHeader != password {
				log.Println("Suspected hacking attempt detected!")
				http.Error(w, "Please present valid multipass!", http.StatusUnauthorized)
				return
			}

			next.ServeHTTP(w, r)
		})
	}
}

func messageHandler(message string) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		log.Printf("Sending '%s'", message)
		// w.WriteHeader(http.StatusOK)
		w.Write([]byte(message))
	}
}

func chain(c ...Middleware) Middleware {
	return func(next http.Handler) http.Handler {
		for i := len(c) - 1; i >= 0; i-- {
			m := c[i]
			next = m(next)
		}
		return next
	}
}

func main() {
	mux := http.NewServeMux()

	auth := authHandler("multipass")
	finalHandler := messageHandler("final")
	otherHandler := messageHandler("other")

	c := chain(requestLogger, auth)

	mux.Handle("/", c(finalHandler))
	mux.Handle("/foo", c(otherHandler))

	log.Print("Listening on :3000...")
	err := http.ListenAndServe(":3000", mux)
	log.Fatal(err)
}