MySQL does not start after replacing MariaDB: MySQL has been frozen to prevent damage to your system

Symptoms

MySQL does not start after replacing MariaDB. Service status shows this:

# service mysql status
...
systemd[1]: Stopped MySQL Community Server.
systemd[1]: Starting MySQL Community Server...
mysql-systemd-start[132431]: MySQL has been frozen to prevent damage to your system. Please see /etc/mysql/FROZEN for help.
systemd[1]: mysql.service: Control process exited, code=exited, status=1/FAILURE

Cause

Unclear.

Replacing MariaDB with MySQL has not been tested with Plesk and is not recommended.

Resolution

Install MariaDB back:

1. Remove MySQL packages:

   # dpkg -l | grep "mysql-" | egrep -vE '(plesk|perl)' | awk {'print $2'} | xargs -i dpkg -r --force-depends {}

2. Fix broken packages to install MariaDB back:

   # apt --fix-broken install

3. Umask MariaDB service:

   # systemctl unmask mariadb

4. Recreate the missing directory for MariaDB pid files:

   # mkdir /var/run/mariadb

5. Apply the correct permissions

   # chown mysql:mysql /var/run/mariadb

6. Start MariaDB service:

   # systemctl start mariadb