recent update messed up my system, can't update or install any package

Question

I am using Loki. Yesterday I got mysql server update and something went wrong with it, now I can't update my system nor can I install any package.

$ sudo apt upgrade
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Calculating upgrade... Done
0 to upgrade, 0 to newly install, 0 to remove and 0 not to upgrade.
1 not fully installed or removed.
After this operation, 0 B of additional disk space will be used.
Do you want to continue? [Y/n] 
Setting up mysql-server-5.7 (5.7.18-0ubuntu0.16.04.1) ...
insserv: warning: current start runlevel(s) (empty) of script `mysql' overrides LSB defaults (2 3 4 5).
insserv: warning: current stop runlevel(s) (0 1 2 3 4 5 6) of script `mysql' overrides LSB defaults (0 1 6).
mysql_upgrade: Got error: 2002: Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2) while connecting to the MySQL server
Upgrade process encountered error and will not continue.
mysql_upgrade failed with exit status 11
dpkg: error processing package mysql-server-5.7 (--configure):
 subprocess installed post-installation script returned error exit status 1
Errors were encountered while processing:
 mysql-server-5.7
E: Sub-process /usr/bin/dpkg returned an error code (1)

Please note I did try starting mysql.service before the upgrade and also checked its status which shows 'action running' by systemctl. After running upgrade command I noticed mysql.service stopped on its own. Also I checked and did found that '/var/run/mysqld/mysqld.sock' existed before running upgrade.
I can't even uninstall mysql, so I am stuck in between.

Answer 1

Try forcibly reinstalling MySQL:

sudo apt remove --purge mysql-server mysql-client mysql-common
sudo apt-get autoremove
sudo apt-get autoclean

sudo apt update
sudo apt install mysql-server
sudo mysql_install_db

Note that this will delete your databases!

Answer 2

After trial and error, I found the solution:

• enable mysql service: systemctl enable mysql.service
• and purge it again: sudo apt purge 'mysql*'

Answer 3

On my system some of the mysql service files were missing but the package maintainer versions were there, so I was able to fix it by doing the following:

sudo mv /etc/mysql/debian-start.dpkg-dist /etc/mysql/debian-start
sudo mv /etc/init.d/mysql.dpkg-dist /etc/init.d/mysql
sudo mv /etc/init/mysql.conf.dpkg-dist /etc/init/mysql.conf

This might explain why purging/reinstalling mysql fixes it since that would recreate those files.