The u-i snap is built on xenial an uses a PPA backport of e2fsprogs

[Łukasz Zemczak]

Currently, for the ubuntu-image snap to work correctly on all supported series, we build it on xenial against the ppa:canonical-foundations/ubuntu-image PPA. The reason why we use a PPA is because to work properly (and not require sudo during core image builds), we need a newer e2fsprogs (mkfs.ext4 to be exact) that supports the -d option. Since backporting as an SRU was not an option, using the ubuntu-image PPA has been chosen as a temporary solution.

Ideally though we would want to build the u-i snap from the archive.

We also cannot use bionic or later as our build platform because of the xenial != bionic glibc mismatch. Since core16 is based on xenial, to actually build the u-i classic snap we need to explicitly stage the required glibc version inside the snap - there's even a snapcraft build failure popping up forcing us to do so. The problem with that is that after the snap is built, it basically will only run on bionic+, segfaulting terribly on earlier series. This is because of the mismatch between core16 snap libc, the bundled u-i libc and the host system libc because we're in classic confinement.

This might be fixed by just staging more packages inside the ubuntu-image snap and making sure we only use what's in u-i. Not sure how much we'd have to pull in and if that's even possible with the current classic architecture.