Comment # 1 on bug 1166781 from Stefan Seyfried

I can reproduce it with my test data set uploaded here:

https://drive.google.com/file/d/1optsli8AWCqeNnDheRitrXC537vOOf1d/view?usp=sharing

(contains /var/lib/mysql, user root@localhost, passwd mysql)

* install 15.1 minimal server system with online repos (updates already
installed)
* zypper in mariadb
* rm -rf /var/lib/mysql
* untar above tarball in /var/lib
* systemctl start mariadb
* mysql -u root -pmysql
  => works, "show databases;"
* zypper --releasever 15.2 ref
* zypper --releasever 15.2 -v dup --no-recommends
* mysql -u root -pmysql =>
  ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using
password: YES)

Now comes the interesting part:
* systemctl stop mariadb.service
* rm -rf /var/lib/mysql
* untar tarball again
* systemctl start mariadb.service
* mysql -u root -pmysql => works.

To break it again:
* touch /var/lib/misc/.mariadb_run_update
* echo -n 10.2 /var/lib/misc/mariadb_upgrade_info
* systemctl restart mariadb
  => broken

So it is the update routine in
[Service]
ExecStartPre=/usr/lib/mysql/mysql-systemd-helper  install
ExecStartPre=/usr/lib/mysql/mysql-systemd-helper  upgrade
ExecStart=/usr/lib/mysql/mysql-systemd-helper     start

that does kill the DB users table.