How to solve InnoDB: Unable to lock ./ibdata1 mysql error?

It should say "There is another solution: the problem might be AppArmor". If you do not see an entry in /var/log/apport.log, then probably AppArmor is not the problem.

An intriguing solution, however.

First, list MySQL processes using the ps command:

ps aux | grep mysql

And then kill the process

sudo kill 56311

Believe it or not, the solution is elsewhere. The problem stems from AppArmor misconfiguration apparently.

So just do:

$ apt install apparmor-profiles

and then restart MySQL (notice how fast it'll restart).

I noticed a file missing related to AppArmor when doing:

$ systemctl status mysql.service

Voila.