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: mysql.go
    path of file: ./repos/trojan-go/statistic/mysql
    
    the code of the file until where you have to start completion: package mysql

import (
	"context"
	"database/sql"
	"fmt"
	"strings"
	"time"

	// MySQL Driver
	_ "github.com/go-sql-driver/mysql"

	"github.com/p4gefau1t/trojan-go/common"
	"github.com/p4gefau1t/trojan-go/config"
	"github.com/p4gefau1t/trojan-go/log"
	"github.com/p4gefau1t/trojan-go/statistic"
	"github.com/p4gefau1t/trojan-go/st
    
func (a *Authenticator) updater() {
	for {
		for _, user := range a.ListUsers() {
			// swap upload and download for users
			hash := user.Hash()
			sent, recv := user.ResetTraffic()

			s, err := a.db.Exec("UPDATE `users` SET `upload`=`upload`+?, `download`=`download`+? WHERE `password`=?;", recv, sent, hash)
			if err != nil {
				log.Error(common.NewError("failed to update data to user table").Base(err))
				continue
			}
			if r, err := s.RowsAffected(); err != nil {
				if r == 0 {
					a.DelUser(hash)
				}
			}
		}
		log.Info("buffered data has been written into the database")

		// update memory
		rows, err := a.db.Query("SELECT password,quota,download,upload FROM users")
		if err != nil || rows.Err() != nil {
			log.Error(common.NewError("failed to pull data from the database").Base(err))
			time.Sleep(a.updateDuration)
			continue
		}
		for rows.Next() {
			var hash string
			var quota, download, upload int64
			err := rows.Scan(&hash, &quota, &download, &upload)
			if err != nil {
				log.Error(common.NewError("failed to obtain data from the query result").Base(err))
				break
			}
			if download+upload < quota || quota < 0 {
				a.AddUser(hash)
			} else {
				a.DelUser(hash)
			}
		}

		select {
		case <-time.After(a.updateDuration):
		case <-a.ctx.Done():
			log.Debug("MySQL daemon exiting...")
			return
		}
	}
}