On Debian systems mysql_install_db may change file permissions when upgrade
When upgrading percona-
I may create bin-log files if they were enabled in my.cnf before.
If there is no "user=mysql" option in [mysqld] section, the bin-logs are owned by root.
However when MySQL is started by mysql.server (e.g. /etc/init.d/mysql) group [mysql.server] is processed where "user=mysql" may be specified.
Root owner of the log files prevents mysqld to start. It can't read the last log due to "permission denied" error.