DELIMITER $$
CREATE PROCEDURE gc_vpn_for_disconn()
BEGIN
    UPDATE
    vpn_pool vp
    LEFT OUTER JOIN (
        SELECT COUNT(vpn_id) AS n_disconnected, vpn_id, cred_id
                FROM vpn_pool GROUP BY cred_id) vp2
        ON vp.vpn_id = vp2.vpn_id
    INNER JOIN vpn_cred_pool cpool
        ON cpool.cred_id = vp.cred_id
    LEFT OUTER JOIN INFORMATION_SCHEMA.PROCESSLIST conns
        ON conns.ID = vp.vpn_conn_id
    SET
            vp.vpn_node = NULL,
            vp.cred_id = NULL,
            vp.vpn_conn_id = -1,
            cpool.cred_busy = cpool.cred_busy - vp2.n_disconnected
    WHERE vp.vpn_node IS NOT NULL AND conns.ID IS NULL;
END $$

DELIMITER ;

-- Errors i get in the logs from time to time:

110728  3:45:31 [Warning] Event Scheduler: [dbuser@%][routing.gc_vpn_disconn_pass] Column 'cred_busy' cannot be null