I’ve read some interesting debugging stories recently, so here’s one of mine. Hopefully it might help others who have similar issues.
Recently my laptop started taking much longer than usual to turn off: instead of ~1 second it would take ~20. Around the same time I had the exact same problem on my work machine which runs a different version of Ubuntu. So it seemed likely that the issue was something in my configuration rather than a real bug.
I started by enabling persistant systemd logs so that I could
read the logs recorded during a shutdown. Once that was set up I rebooted the
laptop, then checked the logs with journalctl --boot=-1. Skimming through the
last few items I noticed that there was a 28 second wait before the line
Stopped LSB: "privilege escalation detection system" which seemed suspicious.
Some more googling and I found this askUbuntu question by someone
having the same issue as me. It turns out that I’d unwittingly installed
something called “ninja” that watches out for suspicious looking
programs running as root. I suspect that on both of my machines I had attempted
to install ninja-the-build system (apt-get install ninja-build)
and instead installed ninja-the-monitoring-daemon (apt-get install ninja).
Moral of the story: check out unknown packages using apt-cache show before you install them!