How to fix «Could not open file <...> for error logging: Permission denied» / «Could not create unix socket lock file /var/run/mysqld/mysqld.sock.lock» for MySQL?

MySQL
1

I have fixed it by disabling AppArmor:

mkdir -p /etc/default/grub.d
echo 'GRUB_CMDLINE_LINUX_DEFAULT="$GRUB_CMDLINE_LINUX_DEFAULT apparmor=0"' \
  | sudo tee /etc/default/grub.d/apparmor.cfg
update-grub
reboot

wiki.debian.org/AppArmor/HowToUse#Disable_AppArmor
serverfault.com/a/755948