[Bug 1799736] Re: Restore the ability to produce core16 images

[Robert C Jennings]

New bug #1807750 filed against ubuntu-image for the sudo dependency.
Marking this as verification-done to unblock other SRUs and because the
change introduced by this SRU does allow for building of core16 images
using the steps outlined in the test case (and in the buildds once
ubuntu-image is fixed).

** Tags removed: verification-failed verification-failed-xenial
** Tags added: verification-done verification-done-xenial

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Title:
  Restore the ability to produce core16 images

Status in livecd-rootfs package in Ubuntu:
  Fix Released
Status in livecd-rootfs source package in Xenial:
  Fix Committed
Status in livecd-rootfs source package in Bionic:
  Confirmed

Bug description:
  [Impact]

  Ubuntu Core is an all-snap Ubuntu variant that we are building through
  cdimage. In the past there was only one core series, 16, and all the
  images we were building were based on that series. Thanks to that all
  the support for building ubuntu-core images in livecd-rootfs through
  ubuntu-image was only needed in later series, as all were build using
  devel. Currently we have two different ubuntu-core image sets based of
  different bases: core16 and core18 images. As a convention, we have
  decided that cdimage (and or other components) will be building
  ubuntu-core images based of the suite that the build is being run for.
  Since core16 is based on xenial and core18 on bionic, we have decided
  the following scheme:

   * Suite: xenial -> ubuntu-core 16 image
   * Suite: bionic or later -> ubuntu-core 18 image

  For this to work, we need to have all the needed changes in xenial and
  bionic. livecd-rootfs on xenial is missing all the ubuntu-image build-
  enablement changes, so these need to be backported. Besides that, both
  xenial and bionic need to get backports of the per-distro-series core-
  series selection.

  Without these fixes in xenial, no ubuntu-core 16 images can be built
  anymore. This is needed in case we'd have need for any security-
  vulnerability fix image re-releases.

  [Test Case]

  For xenial:

   * On a xenial system install latest xenial-proposed livecd-rootfs
   * Create a temporary directory and cd into it
   * Copy the /usr/share/livecd-rootfs/live-build/auto directory into the current directory
   * sudo PROJECT=ubuntu-core SUITE=xenial ARCH=amd64 PROPOSED=0 IMAGEFORMAT="ubuntu-image" lb config
     -> Make sure that the configured model assertion is for core16
   * sudo PROJECT=ubuntu-core SUITE=xenial ARCH=amd64 PROPOSED=0 IMAGEFORMAT="ubuntu-image" lb build
     -> Make sure the build runs and finishes correctly
   * Boot test the newly created image with kvm
     -> Make sure the image is still bootable

  Also, since we are basically starting to use ubuntu-image with this
  xenial upload of livecd-rootfs (which was in universe for this
  series), we need to make sure the package is still installable with
  just main enabled.

   * On a clean chroot (without livecd-rootfs or any of its dependencies installed)
   * Modify sources.list to only have main enabled
   * sudo apt update
   * sudo apt install livecd-rootfs
     -> Make sure the package and its dependencies install correctly

  [Regression Potential]

  As part of the backport only new code is added - code that is executed
  in certain code paths only. Most regressions might appear in the
  ubuntu-core image generation, which was not available for xenial. But
  in case the code has been added in the wrong way, potentially this
  might result in livecd-rootfs not being able to generate proper
  classic images anymore. A run of lb config and lb build for a non
  ubuntu-image project is recommended.

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