code: func upgradeToVer3(s Session) {
  	// Version 3 fix tx_read_only variable value.
  	sql := fmt.Sprintf("UPDATE HIGH_PRIORITY %s.%s set variable_value = '0' where variable_name = 'tx_read_only';",
  		mysql.SystemDB, mysql.GlobalVariablesTable)
  	mustExecute(s, sql)
  }
docs: // upgradeToVer3 updates to version 3.