Comment 3 for bug 1880339

I had a version of this bug.

- starting from 18.04 LTS, with all package updates

- software-updater didn't offer 20.04 (surprised me this many months after 20.04 was released)

- followed https://ubuntu.com/blog/how-to-upgrade-from-ubuntu-18-04-lts-to-20-04-lts-today

- all went well until process "focal" (a python script) took 90 minutes of CPU with no progress

- it had two defunct children (which means that the code hadn't reaped them -- not good)

- I did an strace on focal and it seemed to be doing short write(1) system calls

- the last thing in the terminal sub-window was processing triggers for sgml.

I did "killall focal" and the update ground to a halt.

I tried "apt update" and "apt full-upgrade" and got (only) a long long list of things to autoremove.

I rebooted (it worked) and did an autoremove.

Now I wonder how many other package triggers never ran and how to fix that.

Fundamentally, I think that the problem flows from this bug.
There are several other reports that might be related:
https://launchpad.net/ubuntu/+source/sgml-base/+bugs?field.tag=apport-package