Go fly.io sticky session handler

sticky.go

package fly

import (
	"net/http"
	"os"
	"time"
)

const (
	flyMachineId = "fly-machine-id"
	flyReplay    = "Fly-Replay"
)

func Sticky(next http.HandlerFunc) http.HandlerFunc {
	mid := os.Getenv("FLY_MACHINE_ID")

	return func(w http.ResponseWriter, r *http.Request) {
		if mid != "" {
			cookie, err := r.Cookie(flyMachineId)
			if err == nil && cookie.Value != mid {
				w.Header().Set(flyReplay, mid)
				w.WriteHeader(http.StatusTemporaryRedirect)
				return
			}

			http.SetCookie(w, &http.Cookie{
				Name:    flyMachineId,
				Value:   mid,
				Path:    "/",
				Expires: time.Now().Add(24 * time.Hour),
			})
		}

		next(w, r)
	}
}