MariaDB upgrade fails: The user specified as a definer ('mariadb.sys'@'localhost') does not exist

Symptoms

• Upgrade MySQL databases fail with the following error:

  # MYSQL_PWD=cat /etc/psa/.psa.shadow mysql_upgrade -uadmin
  Phase 3/7: Fixing views
  mysql.user
  Error : The user specified as a definer ('mariadb.sys'@'localhost') does not exist
  error : Corrupt
  Phase 4/7: Running 'mysql_fix_privilege_tables'
  ERROR 1449 (HY000) at line 8: The user specified as a definer ('mariadb.sys'@'localhost') does not exist

• mariadb.sys user does not exist:

  # plesk db "select host,user from mysql.user where user='mariadb.sys';"
  #

Cause

The user mariadb.sys no longer exists, but is specified as a definer.

Resolution

1. Connect to the server via SSH

2. Rename the user.frm file:

   # mv /var/lib/mysql/mysql/user.frm{,.bak}

3. Restart the MariaDB service:

   # systemctl restart mariadb

4. Upgrade the MySQL databases to recreate the user table:

   # MYSQL_PWD=`cat /etc/psa/.psa.shadow` mysql_upgrade -uadmin --force

Note: In case the warning "This installation of MariaDB is already upgraded" is displayed, run the command with the option --force.