You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: derphttp_server.go
    path of file: ./repos/sliver/vendor/tailscale.com/derp/derphttp
    
    the code of the file until where you have to start completion: // Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause

package derphttp

import (
	"fmt"
	"log"
	"net/http"
	"strings"

	"tailscale.com/derp"
)

// fastStartHeader is the header (with value "1") that signals to the HTTP
// server that 
    
func Handler(s *derp.Server) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		up := strings.ToLower(r.Header.Get("Upgrade"))
		if up != "websocket" && up != "derp" {
			if up != "" {
				log.Printf("Weird upgrade: %q", up)
			}
			http.Error(w, "DERP requires connection upgrade", http.StatusUpgradeRequired)
			return
		}

		fastStart := r.Header.Get(fastStartHeader) == "1"

		h, ok := w.(http.Hijacker)
		if !ok {
			http.Error(w, "HTTP does not support general TCP support", 500)
			return
		}

		netConn, conn, err := h.Hijack()
		if err != nil {
			log.Printf("Hijack failed: %v", err)
			http.Error(w, "HTTP does not support general TCP support", 500)
			return
		}

		if !fastStart {
			pubKey := s.PublicKey()
			fmt.Fprintf(conn, "HTTP/1.1 101 Switching Protocols\r\n"+
				"Upgrade: DERP\r\n"+
				"Connection: Upgrade\r\n"+
				"Derp-Version: %v\r\n"+
				"Derp-Public-Key: %s\r\n\r\n",
				derp.ProtocolVersion,
				pubKey.UntypedHexString())
		}

		s.Accept(r.Context(), netConn, conn, netConn.RemoteAddr().String())
	})
}