package sgml-base 1.29.1 failed to install/upgrade: triggers looping, abandoned

[Filipe Dias Baumgratz]

During the upgrade from ubuntu 18.04 LTS to 20.04 the installation crashed because it could not install the package sgml-base. I rebooted the computer and got the bug I am reporting.

I have attached the print of the error.

ProblemType: Package
DistroRelease: Ubuntu 20.04
Package: sgml-base 1.29.1
ProcVersionSignature: Ubuntu 5.4.0-31.35-generic 5.4.34
Uname: Linux 5.4.0-31-generic x86_64
ApportVersion: 2.20.11-0ubuntu27
Architecture: amd64
CasperMD5CheckResult: skip
Date: Sat May 23 17:29:08 2020
ErrorMessage: triggers looping, abandoned
InstallationDate: Installed on 2020-04-20 (32 days ago)
InstallationMedia: Ubuntu 18.04.4 LTS "Bionic Beaver" - Release amd64 (20200203.1)
PackageArchitecture: all
Python3Details: /usr/bin/python3.8, Python 3.8.2, python3-minimal, 3.8.2-0ubuntu2
PythonDetails: /usr/bin/python2.7, Python 2.7.18rc1, python-is-python2, 2.7.17-4
RelatedPackageVersions:
 dpkg 1.19.7ubuntu3
 apt 2.0.2ubuntu0.1
SourcePackage: sgml-base
Title: package sgml-base 1.29.1 failed to install/upgrade: triggers looping, abandoned
UpgradeStatus: Upgraded to focal on 2020-05-23 (0 days ago)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[D. Hugh Redelmeier (hugh-mimosa)]

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