apt update: Can not add an empty (zero-length) key to the cache

[Luís Infante da Câmara]

When running sudo apt update, I get the following warning after the "Fetched" message:

Can not add an empty (zero-length) key to the cache

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: apt 2.0.9
ProcVersionSignature: Ubuntu 5.15.0-41.44~20.04.1-generic 5.15.39
Uname: Linux 5.15.0-41-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.24
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: ubuntu:GNOME
Date: Tue Jul 19 16:44:36 2022
InstallationDate: Installed on 2022-04-26 (84 days ago)
InstallationMedia: Ubuntu 20.04.4 LTS "Focal Fossa" - Release amd64 (20220223)
SourcePackage: apt
UpgradeStatus: No upgrade log present (probably fresh install)

[Julian Andres Klode (juliank)]

This string does not exist in apt or any of the other Ubuntu code I have checked out. If you can reproduce it, do check where it is coming from and reassign it there.

[Luís Infante da Câmara (luis220413)]

It is coming from the appstream source package.

Steps to reproduce:
1. Create a minimal Ubuntu 20.04 chroot with all packages that install files in /etc/apt/apt.conf.d on my Ubuntu 20.04 VM installed (that is, update-notifier-common, debconf, aptdaemon, apt-config-icons, apt-config-icons-hidpi, packagekit, appstream, apt, unattended-upgrades, ubuntu-advantage-tools, snapd and command-not-found).
2. Get the source code for all packages in the chroot with apt source SOURCE_PACKAGE_NAMES.
3. Run the following command:

$ grep -Fr 'Can not add an empty (zero-length) key to the cache'
appstream-0.12.10/.pc/01_less-long-token-noise.patch/src/as-cache.c: "Can not add an empty (zero-length) key to the cache");
appstream-0.12.10/src/as-cache.c: "Can not add an empty (zero-length) key to the cache");

[Matthias Klumpp (ximion)]

This issue does not longer exist in appstream since jammy - so the quickest fix is to simply upgrade to that release.

[Luís Infante da Câmara (luis220413)]

This just happened again.