Comment 20 for bug 1605948

A very similar problem happened today to me when upgrading mysql-sever-5.7 from 5.7.30 to 5.7.31.

This is what happens in my Linux Mint 18.3 (based on Ubuntu 16.04):

Log started: 2020-07-29 11:41:51
Configurazione di mysql-server-5.7 (5.7.31-0ubuntu0.16.04.1)...
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: errore nell'elaborare il pacchetto mysql-server-5.7 (--configure):
 il sottoprocesso installato script di post-installation ha restituito lo stato di errore 1
Si sono verificati degli errori nell'elaborazione:
 mysql-server-5.7
Log ended: 2020-07-29 11:41:56

Please note my MySQL Server 5.7 service is DISABLED.
So, I followed the instructions from #7 (enable the service, start it, retry the package installation/upgrade, then disable the service again) and it fixed the upgrade process.

I suspect there is some flaw in these mysql-server-5.7 upgrade scripts that don't take into account the case in which the user might have disabled the service in order to start it just on demand, instead that on each boot. And this flaw has not been fixed yet.