[Bug 1907713] Re: aufs support is deprecated and should be handled on upgrade

[Michael Hudson-Doyle]

Although there's no reason for the deb's preinst to be checking
/var/snap/docker/common/var-lib-docker/aufs surely?

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https://bugs.launchpad.net/bugs/1907713

Title:
  aufs support is deprecated and should be handled on upgrade

Status in docker.io package in Ubuntu:
  New
Status in ubuntu-release-upgrader package in Ubuntu:
  New
Status in docker.io source package in Hirsute:
  New
Status in ubuntu-release-upgrader source package in Hirsute:
  New

Bug description:
  The Ubuntu kernel team has continued to carry aufs support as a sauce
  patch but there is no known application for this filesystem anymore
  except for docker deployments using this deprecated storage option.

  We can't expect the kernel team to carry this deprecated filesystem
  indefinitely merely because docker.io upstream hasn't provided a
  migration path for existing installs.  If we can't do a smooth
  conversion on upgrade of the docker.io package from aufs to another
  storage backend, then at the very least we need to draw a line and
  block the upgrades between releases.

  So for hirsute we should make use of aufs in the docker config to
  result in blocking the upgrade in preinst.

  We should also add a quirk to ubuntu-release-upgrader, so that on
  release upgrades this case can be detected before starting the upgrade
  and causing maintainer script failures in the middle.

  We don't really want this change to be SRUed back to prior releases,
  but this is subject to being revisited if the focal HWE kernel needs
  to drop aufs support.

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