livecd-rootfs should replicate apt preferences from the host environment at image build time

[Steve Langasek]

Now that -proposed has NotAutomatic: yes set, packages are not pulled in from -proposed unless explicitly requested by version number on the apt commandline, or if the priority of (packages in) the pocket is changed via apt preferences.

When livecd-rootfs autopkgtests are run on autopkgtest.ubuntu.com, we will typically request a particular package from -proposed be installed but install all other packages from the release pocket. This configuration applies to the host environment, but packages within the target chroot will still be pulled only from the release pocket.

A recent snapd update triggered a test of livecd-rootfs. However, the test failed because the snapd used *within* the target squashfs for preseeding was the version from the release pocket - and the snapd update was a cherry-pick specifically to fix a regression when building images due to changes to the seed which depended on specific snapd features.

When we have a package pin for -proposed, we should use that pin both in the host environment (used for e.g. selecting different versions of livecd-rootfs itself), but also in the target chroot.

The package pins must be cleared from the final image.

[Michael Hudson-Doyle (mwhudson)]

I foresee this being very annoying to implement but maybe I'll be surprised!

[Julian Andres Klode (juliank)]

I'm concerned if the builders this runs on also have let's call it compatible pinning or whether it should be restricted to the testing scenarios affected, but your call.