[Bug 2115536] Proposed package upload rejected

[Julian Andres Klode]

An upload of ubuntu-meta to focal-proposed has been rejected from the
upload queue for the following reason: "Removal of wslu potentially
breaks existing user workflows".

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https://bugs.launchpad.net/bugs/2115536

Title:
  [SRU] Allow building WSL images of Focal in the new format

Status in livecd-rootfs package in Ubuntu:
  New
Status in ubuntu-meta package in Ubuntu:
  New

Bug description:
  [ Impact ]

   * Previously, WSL instances relied on a very lightweight build pipeline which was using the CPC image.
  In 22.04 we did a first step to have WSL has its own project in livecd-rootfs, with its own seed and image to produce a rootfs, while leaving Focal for later.

   * Later, changes to livecd-rootfs and related packages for Noble and
  later were implemented to adapt the images to the new format promoted
  by WSL that allows distributions to no longer rely on MS Store, which
  is becoming deprecated for WSL. Those changes are in the process of
  being backported to Jammy.

   * Focal is the only LTS still supported via Ubuntu Pro that lacks
  those image building capabilities.

   * This Stable Release Update request aims to bring Focal to the
  current state of the art build pipeline of WSL images, allowing us to
  host the final artifacts in Canonical's infrastructure and no longer
  depend on MS Store.

   * We thus need to promote 'ubuntu-wsl' as its own project in 'livecd-
  rootfs' for focal, implement the same image building hooks already
  existing in Jammy and update 'ubuntu-meta' to reflect the changes
  already implemented in the seed related to this project.

  [ Test Plan ]

  1. Build a new Focal image, publish them on cdimage.ubuntu.com
    -> we should see one image ending up with ".wsl" for 20.04 image

  A. New installations:

  1. Ship a cloud-init file to touch a file on disk on `%USERPROFILE%\.cloud-init\Ubuntu-20.04.user-data`
  2. Download the image made available from above, double-click install it on a Windows machine with WSL 2.4.4 and later, then:
  -> Check that basic shell commands works
  -> Check that no MoTD messages complains about cloud-init failure
  -> Check that the file from the cloud-init profile was applied
  -> Run systemctl --failed and ensure nothing is listed related to WSL specific configuration itself
  -> Pro attach the machine and check that it’s attached

  [ Where problems could occur ]

  * Most of the issues could happen on initial boot. It will be really
  easy to spot any issues there with the previous test plan.

  * The set of updated defaults in 20.04 only affect WSL targets, so
  other platforms remain unaffected by the proposed changes.

  [ Other Info ]

   * As already explained in LP #2115426, despite Focal being out of
  standard support, we need to align its build pipeline to the current
  state of the art (implemented for Jammy and later), as MS Store is
  being deprecated.

   * All the changes in the packages tracked by this SRU bug are related
  to each other and need to land together otherwise building a WSl image
  of Focal would fail.

   * This report relates to LP #2080223 and LP #2091293, but the changes
  needed for 20.04 are more complex (as already mentioned in the
  former), thus I'd rather do it in a dedicated SRU.

  * The WSL seed for Focal was already aligned with Jammy and later as
  we can see in [this commit in the ubuntu-seeds
  repository](https://git.launchpad.net/~ubuntu-core-dev/ubuntu-
  seeds/+git/ubuntu/commit/?h=focal&id=70d0bae95ea450b5cb18325d52a56b9e41be9465)

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