[Bug 1841225] Re: deb to snap transition uses an ubuntu release channel for the core18 snap

[Łukasz Zemczak]

Verified ubuntu-release-upgrader 19.04.16.7 as per the test-case.

I set up a multipass bionic system, switched /etc/update-manager
/release-upgrades to Prompt=normal, installed ubuntu-desktop and made
sure there is no core18 snap installed. I then proceeded with running
`do-release-upgrade --frontend DistUpgradeView --proposed`. The upgrade
succeeded,confirmed that 19.04.16.7 was used [1] and checked that in the
end all snaps got installed with the right channels selected (stable for
core18 and stable/ubuntu-19.04 for others).

[1] 2019-10-09 14:14:11,811 INFO release-upgrader version '19.04.16.7'
started

** Tags removed: verification-needed verification-needed-disco
** Tags added: verification-done verification-done-disco

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https://bugs.launchpad.net/bugs/1841225

Title:
  deb to snap transition uses an ubuntu release channel for the core18
  snap

Status in ubuntu-release-upgrader package in Ubuntu:
  Fix Released
Status in ubuntu-release-upgrader source package in Disco:
  Fix Committed
Status in ubuntu-release-upgrader source package in Eoan:
  Fix Released

Bug description:
  [Impact]

  During distro upgrades, additional logic exists that replaces existing
  debs with snaps and refreshes existing, installed snaps to the branch
  of the series we're upgrading to (e.g. from stable/ubuntu-18.04 to
  stable/ubuntu-19.04 etc.). However, the upgrader tries to apply the
  logic for all the snaps it 'supports' in migration, even though snaps
  like core18 do not have per-series branches (and will never have).
  This causes non-fatal failures during installation/refresh of the
  core18 snap on upgrades. Even though this does not cause a crash or
  error, and in theory should should not cause any trouble as core18
  *should* be pulled in as a dependency of the other snaps, it is
  invalid behavior and might result in core18 not being present on the
  end system. Upgrades should not perform commands that fail, if
  possible.

  [Test Case]

  * Prepare a bionic machine
  * Make sure ubuntu-desktop and snapd are installed on it
  * Make sure core18 is *not* installed on the system - if there are any snaps installed, remove them
  * Perform an upgrade to disco (with the --proposed option, to use disco-proposed)
  * Reboot
  * Make sure that the upgrade succeeded
  * Check if core18 is installed, open /var/log/dist-upgrade/main.log and look for 'install of snap core18', make sure the command succeeded

  [Regression Potential]

  The changes have been constrained to the DistUpgradeQuirks place, only
  to the snap handling pieces. So any possible regressions should be
  visible in the snap replacement/refresh areas. Most probable ones
  would be for the snaps not being installed/refreshed properly, or
  using the wrong channels/branches.

  [Original Description]

  The release upgrader (DistUpgradeQuirks.py) tries to run the following
  command when upgrading a system without snaps installed to Ubuntu
  19.10 (eoan):

  snap install --channel stable/ubuntu-19.10 core18

  This channel does not exist and the release upgrade will fail e.g.:

  2019-08-23 03:24:45,533 DEBUG quirks: running eoanPostUpgrade
  2019-08-23 03:24:45,533 DEBUG running Quirks.eoanPostUpgrade
  2019-08-23 03:24:51,547 DEBUG install of snap core18 failed
  2019-08-23 03:25:51,348 DEBUG install of snap gnome-3-28-1804 succeeded

  The quirk should be updated so that an ubuntu release channel is not
  used when installing the core18 snap.

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