Request ID Propagation Middleware

package main

import (
	"context"
	"crypto/rand"
	"encoding/hex"
	"fmt"
	"net/http"
)

type requestIDKey struct{}

func validRequestID(id string) bool {
	if len(id) == 0 || len(id) > 128 {
		return false
	}
	for _, char := range id {
		switch {
		case char >= 'a' && char <= 'z':
		case char >= 'A' && char <= 'Z':
		case char >= '0' && char <= '9':
		case char == '-' || char == '_':
		default:
			return false
		}
	}
	return true
}

func RequestIDMiddleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		id := r.Header.Get("X-Request-ID")
		if !validRequestID(id) {
			buffer := make([]byte, 8)
			if _, err := rand.Read(buffer); err != nil {
				http.Error(w, "cannot create request id", http.StatusInternalServerError)
				return
			}
			id = hex.EncodeToString(buffer)
		}
		w.Header().Set("X-Request-ID", id)
		ctx := context.WithValue(r.Context(), requestIDKey{}, id)
		next.ServeHTTP(w, r.WithContext(ctx))
	})
}

func RequestIDFrom(ctx context.Context) string {
	id, _ := ctx.Value(requestIDKey{}).(string)
	return id
}

func main() {
	handler := RequestIDMiddleware(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		fmt.Fprintln(w, "request", RequestIDFrom(r.Context()))
	}))
	http.Handle("/", handler)
	_ = http.ListenAndServe(":8080", nil)
}