Example how to use sync.Once in Golang

main.go

package main

import (
	"fmt"
	"net/http"
	"strconv"
	"strings"
	"sync"
	"time"
)

func main() {
	http.ListenAndServe(":8080", &CORS{
		Handler:         http.HandlerFunc(hello),
		AllowAllOrigins: true,
		AllowMethods:    []string{"GET", "POST", "PUT", "PATCH", "DELETE"},
		AllowHeaders:    []string{"Content-Type", "Authorization"},
		ExposeHeaders:   []string{"X-Request-Id"},
		MaxAge:          time.Hour,
	})
}

func hello(w http.ResponseWriter, r *http.Request) {
	fmt.Fprintln(w, "Hello")
}

type CORS struct {
	Handler          http.Handler
	AllowAllOrigins  bool
	AllowOrigins     []string
	AllowMethods     []string
	AllowHeaders     []string
	AllowCredentials bool
	ExposeHeaders    []string
	MaxAge           time.Duration

	// computed
	allowOrigins  map[string]struct{}
	allowMethods  string
	allowHeaders  string
	exposeHeaders string
	maxAge        string

	once sync.Once
}

func (c *CORS) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	c.once.Do(func() {
		if c.AllowAllOrigins && c.AllowCredentials {
			panic("cors: misconfigured allow all origins and credentials")
		}

		c.allowMethods = strings.Join(c.AllowMethods, ",")
		c.allowHeaders = strings.Join(c.AllowHeaders, ",")
		c.exposeHeaders = strings.Join(c.ExposeHeaders, ",")

		c.maxAge = ""
		if c.MaxAge > time.Duration(0) {
			c.maxAge = strconv.FormatInt(int64(c.MaxAge/time.Second), 10)
		}

		c.allowOrigins = make(map[string]struct{})
		for _, v := range c.AllowOrigins {
			c.allowOrigins[v] = struct{}{}
		}
	})

	if origin := r.Header.Get("Origin"); origin != "" {
		h := w.Header()

		if c.AllowAllOrigins {
			h.Set("Access-Control-Allow-Origin", "*")
		} else if _, ok := c.allowOrigins[origin]; ok {
			h.Set("Access-Control-Allow-Origin", origin)
		} else {
			w.WriteHeader(http.StatusForbidden)
			return
		}

		if c.AllowCredentials {
			h.Set("Access-Control-Allow-Credentials", "true")
		}

		if r.Method == http.MethodOptions {
			if c.allowMethods != "" {
				h.Set("Access-Control-Allow-Methods", c.allowMethods)
			}
			if c.allowHeaders != "" {
				h.Set("Access-Control-Allow-Headers", c.allowHeaders)
			}
			if c.maxAge != "" {
				h.Set("Access-Control-Max-Age", c.maxAge)
			}

			if !c.AllowAllOrigins {
				h.Add("Vary", "Origin")
			}

			h.Add("Vary", "Access-Control-Request-Method")
			h.Add("Vary", "Access-Control-Request-Headers")

			w.WriteHeader(http.StatusNoContent)
			return
		}

		if c.exposeHeaders != "" {
			h.Set("Access-Control-Expose-Headers", c.exposeHeaders)
		}

		if !c.AllowAllOrigins {
			h.Set("Vary", "Origin")
		}
	}

	c.Handler.ServeHTTP(w, r)
}