[Bug 1748581] Re: ubuntu-release-upgrader will need to know how to switch snaps between Ubuntu release channels during an upgrade

[Stéphane Graber]

Hi,

The logic that was added for 18.10 seems to assume that:
 - The user only ever cares about the "latest" track
 - The user only ever cares about the "stable" channel

In the even where the user has chosen a different track or channel, the
upgrader resets the user to latest track, stable channel and
ubuntu-18.10 branch.

This may very much surprise users of other channels/tracks and may cause
breakages as moving from a newer release of a snap to an older one, as
the current logic is doing, is likely to result in an older version of a
snap trying to read newer data.


In the case where a snap is already installed, the upgrader should check what track and channel is in use for the snap, then attempt to switch to an "ubuntu-XX.XX" branch of that track/channel combination if it exists, if it doesn't, assume the user knows what they're doing and leave them alone.

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https://bugs.launchpad.net/bugs/1748581

Title:
  ubuntu-release-upgrader will need to know how to switch snaps between
  Ubuntu release channels during an upgrade

Status in ubuntu-release-upgrader package in Ubuntu:
  Fix Released
Status in ubuntu-release-upgrader source package in Cosmic:
  Fix Released

Bug description:
  As part of the plan for including snaps in Ubuntu images by default
  (https://wiki.ubuntu.com/UbuntuSeededSnaps), we intend for the snap
  installs to be pinned to (possibly empty) per-Ubuntu-release channels
  in the Snap Store.

  This means that as part of a dist upgrade, ubuntu-release-upgrader
  will need to switch the channels of any relevant installed snaps from
  the channel for one release, to the channel for the next release, with
  snap refresh.

  (In many cases both channels will be empty and this will involve
  downloading no new snaps, just changing the pin on the client.)

  Since 18.04 is the first release with snaps by default, this doesn't
  need to be implemented in 18.04; but will need to be supported by the
  release upgrader in 18.10 at latest.

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