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: auth.go
    path of file: ./repos/kubeedge/vendor/github.com/docker/docker/registry
    
    the code of the file until where you have to start completion: package registry // import "github.com/docker/docker/registry"

import (
	"net/http"
	"net/url"
	"strings"
	"time"

	"github.com/docker/distribution/registry/client/auth"
	"github.com/docker/distribution/registry/client/auth/challenge"
	"github.com/docker/distribution/registry/client/transport"
	"github.com/docker/docker/api/types"
	"github.com/docker/docker/api/types/registry"
	"github.com/pkg/errors"
	"github.com/sirupsen/logrus"
)

// AuthClientID is used the ClientID used for the token server
const AuthClientID = "docker"

type loginCredentialStore struct {
	authConfig *types.AuthConfig
}

func (lcs loginCredentialStore) Basic(*url.URL) (string, string) {
	re
    
func loginV2(authConfig *types.AuthConfig, endpoint APIEndpoint, userAgent string) (string, string, error) {
	var (
		endpointStr          = strings.TrimRight(endpoint.URL.String(), "/") + "/v2/"
		modifiers            = Headers(userAgent, nil)
		authTransport        = transport.NewTransport(newTransport(endpoint.TLSConfig), modifiers...)
		credentialAuthConfig = *authConfig
		creds                = loginCredentialStore{authConfig: &credentialAuthConfig}
	)

	logrus.Debugf("attempting v2 login to registry endpoint %s", endpointStr)

	loginClient, err := v2AuthHTTPClient(endpoint.URL, authTransport, modifiers, creds, nil)
	if err != nil {
		return "", "", err
	}

	req, err := http.NewRequest(http.MethodGet, endpointStr, nil)
	if err != nil {
		return "", "", err
	}

	resp, err := loginClient.Do(req)
	if err != nil {
		err = translateV2AuthError(err)
		return "", "", err
	}
	defer resp.Body.Close()

	if resp.StatusCode == http.StatusOK {
		return "Login Succeeded", credentialAuthConfig.IdentityToken, nil
	}

	// TODO(dmcgowan): Attempt to further interpret result, status code and error code string
	return "", "", errors.Errorf("login attempt to %s failed with status: %d %s", endpointStr, resp.StatusCode, http.StatusText(resp.StatusCode))
}