On , I learnt ...

systemd timers can be stopped

Timers are systemd unit files that control services. I didn’t realise until today that, when disabling a timer, it also needs to be stopped too. This can be done with the --now option:

systemctl disable --now apt-daily.timer

Before this, running:

systemctl disable apt-daily.timer
systemctl stop apt-daily.service

didn’t guarantee that the apt-daily service had stopped, which caused intermittent lock acquisition problems when provisioning.