/*
** Zabbix
** Copyright (C) 2001-2021 Zabbix SIA
**
** This program is free software; you can redistribute it and/or modify
** it under the terms of the GNU General Public License as published by
** the Free Software Foundation; either version 2 of the License, or
** (at your option) any later version.
**
** This program is distributed in the hope that it will be useful,
** but WITHOUT ANY WARRANTY; without even the implied warranty of
** MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
** GNU General Public License for more details.
**
** You should have received a copy of the GNU General Public License
** along with this program; if not, write to the Free Software
** Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
**/

package postgres

import (
	"context"
	"errors"

	"github.com/jackc/pgx/v4"
	"zabbix.com/pkg/zbxerr"
)

// oldestXIDHandler gets age of the oldest xid if all is OK or nil otherwise.
func oldestXIDHandler(ctx context.Context, conn PostgresClient,
	_ string, _ map[string]string, _ ...string) (interface{}, error) {
	var resultXID int64

	query := `SELECT greatest(max(age(backend_xmin)), max(age(backend_xid)))
				FROM pg_catalog.pg_stat_activity`

	row, err := conn.QueryRow(ctx, query)
	if err != nil {
		return nil, zbxerr.ErrorCannotFetchData.Wrap(err)
	}

	err = row.Scan(&resultXID)
	if err != nil {
		if errors.Is(err, pgx.ErrNoRows) {
			return nil, zbxerr.ErrorEmptyResult.Wrap(err)
		}

		return nil, zbxerr.ErrorCannotFetchData.Wrap(err)
	}

	return resultXID, nil
}
