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: server.go
    path of file: ./repos/bfe/bfe_http2
    
    the code of the file until where you have to start completion: // Copyright (c) 2019 The BFE Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR C
    
func ConfigureServer(s *http.Server, conf *Server) error {
	if conf == nil {
		conf = new(Server)
	}

	if s.TLSConfig == nil {
		s.TLSConfig = new(tls.Config)
	} else if s.TLSConfig.CipherSuites != nil {
		// If they already provided a CipherSuite list, return
		// an error if it has a bad order or is missing
		// ECDHE_RSA_WITH_AES_128_GCM_SHA256.
		const requiredCipher = tls.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
		haveRequired := false
		sawBad := false
		for i, cs := range s.TLSConfig.CipherSuites {
			if cs == requiredCipher {
				haveRequired = true
			}
			if isBadCipher(cs) {
				sawBad = true
			} else if sawBad {
				return fmt.Errorf("http2: TLSConfig.CipherSuites index %d contains an HTTP/2-approved cipher suite (%#04x), but it comes after unapproved cipher suites. With this configuration, clients that don't support previous, approved cipher suites may be given an unapproved one and reject the connection.", i, cs)
			}
		}
		if !haveRequired {
			return fmt.Errorf("http2: TLSConfig.CipherSuites is missing HTTP/2-required TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256")
		}
	}

	// Note: not setting MinVersion to tls.VersionTLS12,
	// as we don't want to interfere with HTTP/1.1 traffic
	// on the user's server. We enforce TLS 1.2 later once
	// we accept a connection. Ideally this should be done
	// during next-proto selection, but using TLS <1.2 with
	// HTTP/2 is still the client's bug.

	s.TLSConfig.PreferServerCipherSuites = true

	haveNPN := false
	for _, p := range s.TLSConfig.NextProtos {
		if p == NextProtoTLS {
			haveNPN = true
			break
		}
	}
	if !haveNPN {
		s.TLSConfig.NextProtos = append(s.TLSConfig.NextProtos, NextProtoTLS)
	}
	// h2-14 is temporary (as of 2015-03-05) while we wait for all browsers
	// to switch to "h2".
	s.TLSConfig.NextProtos = append(s.TLSConfig.NextProtos, "h2-14")

	if s.TLSNextProto == nil {
		s.TLSNextProto = map[string]func(*http.Server, *tls.Conn, http.Handler){}
	}
	protoHandler := func(hs *http.Server, c *tls.Conn, h http.Handler) {
		if testHookOnConn != nil {
			testHookOnConn()
		}
		conf.ServeConn(c, &ServeConnOpts{
			Handler:    h,
			BaseConfig: hs,
		})
	}
	s.TLSNextProto[NextProtoTLS] = protoHandler
	s.TLSNextProto["h2-14"] = protoHandler // temporary; see above.
	return nil
}