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: krb5.go
    path of file: ./repos/sliver/vendor/github.com/jackc/pgx/v5/pgconn
    
    the code of the file until where you have to start completion: package pgconn

import (
	"errors"
	"fmt"

	"github.com/jackc/pgx/v5/pgproto3"
)

// NewGSSFunc creates a GSS authentication provider, for use with
// RegisterGSSProvider.
type NewGSSFunc func() (GSS, error)

var newGSS NewGSSFunc

// RegisterGSSProvider registers a GSS auth
    
func (c *PgConn) gssAuth() error {
	if newGSS == nil {
		return errors.New("kerberos error: no GSSAPI provider registered, see https://github.com/otan/gopgkrb5")
	}
	cli, err := newGSS()
	if err != nil {
		return err
	}

	var nextData []byte
	if c.config.KerberosSpn != "" {
		// Use the supplied SPN if provided.
		nextData, err = cli.GetInitTokenFromSPN(c.config.KerberosSpn)
	} else {
		// Allow the kerberos service name to be overridden
		service := "postgres"
		if c.config.KerberosSrvName != "" {
			service = c.config.KerberosSrvName
		}
		nextData, err = cli.GetInitToken(c.config.Host, service)
	}
	if err != nil {
		return err
	}

	for {
		gssResponse := &pgproto3.GSSResponse{
			Data: nextData,
		}
		c.frontend.Send(gssResponse)
		err = c.frontend.Flush()
		if err != nil {
			return err
		}
		resp, err := c.rxGSSContinue()
		if err != nil {
			return err
		}
		var done bool
		done, nextData, err = cli.Continue(resp.Data)
		if err != nil {
			return err
		}
		if done {
			break
		}
	}
	return nil
}