Please tell me if this forum is the right place to ask for help in this situation!
The nub of the problem seems to be that no socket exists to connect to mysql-server.
Nextcloud server was running without a hitch on Ubuntu Server 20.04 with php7.4
During the upgrade I accepted the default to save existing configurations.
The default /var/run/ I have discovered, is populated by links to /run . Mysql fails to create a socket there, or anywhere else.
In my efforts to recover access to mysql the package manager has frozen the faulty installation:
pi@raspberrypi:~$ sudo cat /etc/mysql/FROZEN This MySQL or variant installation has entered "frozen mode". Maintainer scripts will avoid making changes or starting the daemon until manually released from this state. See /usr/share/doc/mysql-common/README for general information about this mode.
In this particular case, an incompatible downgrade attempt has been detected. This can be resolved in one of two ways:
l. Change the contents of /var/lib/mysql/ to contain database data that is compatible with the currently installed MySQL or variant daemon version. For example: you could restore from a backup. Alternatively you could do a dump using a future version binary and then a restore using the current version binary.
2. Switch to a MySQL or variant daemon version that is compatible with the data currently in /var/lib/mysql/. For example, if you have attempted a downgrade from mysql-server-5.7 to mysql-server-5.6, you could "apt install mysql-server-5.7" again.
Please resolve this situation and only then remove the/etc/mysql/FROZEN symlink. You can then run "dpkg-reconfigure <package>" where <package> should usually be in the form ‹variant>-server-‹version>. pi@raspberrypi:~$
Other error messages:
Failed to stop mysql-service. Unit mysql-service not loaded.
/etc/mysql/mysql.cnf does not exist.
I have trawled the internet looking for a solution, at least one that I could understand and carry out.
Thanks