code: func updateBootstrapVer(s Session) {
  	// Update bootstrap version.
  	sql := fmt.Sprintf(`INSERT HIGH_PRIORITY INTO %s.%s VALUES ("%s", "%d", "TiDB bootstrap version.") ON DUPLICATE KEY UPDATE VARIABLE_VALUE="%d"`,
  		mysql.SystemDB, mysql.TiDBTable, tidbServerVersionVar, currentBootstrapVersion, currentBootstrapVersion)
  	mustExecute(s, sql)
  }
docs: // updateBootstrapVer updates bootstrap version variable in mysql.TiDB table.