Comment 27 for bug 1592669

Hi Paride,
based on my experience and on what I read on these bug reports, the steps to repro are:

- have an Ubuntu system with mysql-server-5.7 version 5.7.30 installed
- systemctl stop mysql
- systemctl disable mysql
- apt install mysql-server-5.7 (in order to upgrade it to the latest version, which is 5.7.31 now)
=> postinst scripts fail with the mentioned error:

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

Workaround:
- systemctl enable mysql (<= important!)
- systemctl start mysql
- apt install mysql-server-5.7

Anyway, there's a thorough analysis here and, if I understand this LP page, the fix was delivered to 16.09 (and hence inherited from subsequent releases) but not to 16.04, correct me if I'm wrong.