Kernel updates marked as "manually installed", prevents old kernels cleanup

[Mauro]

Actually, I'm not 100% sure it's Discover's fault, but I'm quite confident it could the source of the problem.

On 2021-01-27, when trying to install the latest kernel version for my Kubuntu 20.04 installation, I went out of space in my /boot partition. Looking at the installed kernels, I saw that I had 6 (SIX!) old kernels installed, including the one I was running on.
I knew I had all the automatic kernel update and cleaning in place thanks to unattended-upgrades, because I looked at it some time ago, so, I took back the Ubuntu documentation page at https://help.ubuntu.com/community/RemoveOldKernels and re-read it from scratch.
Substantially this sentence is enlightening:

Note: The following methods will only remove kernels that are marked as being automatically installed as described above.

My problem was that all the installed kernels where marked as MANUALLY INSTALLED:

$ apt-mark showauto 'linux-image-.*'
linux-image-generic
$ apt-mark showmanual 'linux-image-.*'
linux-image-5.4.0-54-generic
linux-image-5.4.0-56-generic
linux-image-5.4.0-58-generic
linux-image-5.4.0-59-generic
linux-image-5.4.0-62-generic
linux-image-5.4.0-64-generic
linux-image-5.4.0-65-generic

(note: the last kernel version was the one I was not able to completely install due to insufficient disk space)

Also, looking at /etc/apt/apt.conf.d/01autoremove-kernels I had:

rc linux-image-5.4.0-42-generic 5.4.0-42.46 amd64 Signed kernel image generic
rc linux-image-5.4.0-48-generic 5.4.0-48.52 amd64 Signed kernel image generic
rc linux-image-5.4.0-51-generic 5.4.0-51.56 amd64 Signed kernel image generic
rc linux-image-5.4.0-52-generic 5.4.0-52.57 amd64 Signed kernel image generic
rc linux-image-5.4.0-53-generic 5.4.0-53.59 amd64 Signed kernel image generic
rc linux-image-5.4.0-60-generic 5.4.0-60.67 amd64 Signed kernel image generic
ii linux-image-5.4.0-62-generic 5.4.0-62.70 amd64 Signed kernel image generic
ii linux-image-5.4.0-64-generic 5.4.0-64.72 amd64 Signed kernel image generic
iF linux-image-5.4.0-65-generic 5.4.0-65.73 amd64 Signed kernel image generic
ii linux-image-generic 5.4.0.65.68 amd64 Generic Linux kernel image

So, some kernel versions were indeed automatically pruned in the past (those marked with "rc"), but other are missing there and indeed they were marked as "manually installed".

Running "apt autoremove" manually of course does not remove old kernels, for the same reason.

I'm pretty sure the problem lies here: some kernel version were automatically installed by unattended-upgrades (and these where automatically purged), some others were installed by me by hitting "update all" button in Discover, when it periodically notifies me about new updates.

So, looking again at the Ubuntu wiki page, I see:

In Ubuntu 16.04 kernels installed by Software Updater are marked as being automatically installed. In Ubuntu 14.04 only kernels installed by unattended-upgrades are marked as being automatically installed. See bug #1439769 for details.

So, I suspect such a "bug" is still present in Kubuntu 20.04 Discover, which marks all kernels installed through it as "manually installed".

This problem has also been reported already at:
https://bugs.kde.org/show_bug.cgi?id=430296
https://github.com/hughsie/PackageKit/issues/450

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: plasma-discover 5.18.5-0ubuntu0.1
ProcVersionSignature: Ubuntu 5.4.0-65.73-generic 5.4.78
Uname: Linux 5.4.0-65-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.16
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: KDE
Date: Sun Feb 7 14:47:01 2021
InstallationDate: Installed on 2020-10-05 (124 days ago)
InstallationMedia: Kubuntu 20.04.1 LTS "Focal Fossa" - Release amd64 (20200731)
SourcePackage: plasma-discover
UpgradeStatus: No upgrade log present (probably fresh install)

[Launchpad Janitor (janitor)]

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

[Mauro (mauromol)]

Probably a duplicate of: https://bugs.launchpad.net/ubuntu/+source/apt/+bug/1914278