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: solvers.go
    path of file: ./repos/certmagic
    
    the code of the file until where you have to start completion: // Copyright 2015 Matthew Holt
//
// 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 CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package certmagic

import (
	"context"
	"crypto/tls"
	"encoding/json"
	"fmt"
	"log"
	"net"
	"net/http"
	"path"
	"runtime"
	"strings"
	"sync"
	"sync/atomic"
	"time"

	"github.com/libdns/libdns"
	"github.com/mholt/acmez"
	"github.com/mholt/acmez/acme"
	"github.com/miekg/dns"
)

// httpSolver solves the HTTP challenge. It must be
// associated with a config and an address to use
// for solving the challenge. If multiple httpSolvers
// are initiali
    
func (s *tlsALPNSolver) Present(ctx context.Context, chal acme.Challenge) error {
	// we pre-generate the certificate for efficiency with multi-perspective
	// validation, so it only has to be done once (at least, by this instance;
	// distributed solving does not have that luxury, oh well) - update the
	// challenge data in memory to be the generated certificate
	cert, err := acmez.TLSALPN01ChallengeCert(chal)
	if err != nil {
		return err
	}

	key := challengeKey(chal)
	activeChallengesMu.Lock()
	chalData := activeChallenges[key]
	chalData.data = cert
	activeChallenges[key] = chalData
	activeChallengesMu.Unlock()

	// the rest of this function increments the
	// challenge count for the solver at this
	// listener address, and if necessary, starts
	// a simple TLS server

	solversMu.Lock()
	defer solversMu.Unlock()

	si := getSolverInfo(s.address)
	si.count++
	if si.listener != nil {
		return nil // already be served by us
	}

	// notice the unusual error handling here; we
	// only continue to start a challenge server if
	// we got a listener; in all other cases return
	ln, err := robustTryListen(s.address)
	if ln == nil {
		return err
	}

	// we were able to bind the socket, so make it into a TLS
	// listener, store it with the solverInfo, and start the
	// challenge server

	si.listener = tls.NewListener(ln, s.config.TLSConfig())

	go func() {
		defer func() {
			if err := recover(); err != nil {
				buf := make([]byte, stackTraceBufferSize)
				buf = buf[:runtime.Stack(buf, false)]
				log.Printf("panic: tls-alpn solver server: %v\n%s", err, buf)
			}
		}()
		defer close(si.done)
		for {
			conn, err := si.listener.Accept()
			if err != nil {
				if atomic.LoadInt32(&si.closed) == 1 {
					return
				}
				log.Printf("[ERROR] TLS-ALPN challenge server: accept: %v", err)
				continue
			}
			go s.handleConn(conn)
		}
	}()

	return nil
}