Transition of LXD from deb to snap

[Steve Langasek]

Hi Stéphane,

On Mon, Aug 20, 2018 at 05:13:11PM -0400, Stéphane Graber wrote:

<snip>

> The package in its preinst will attempt to contact the snap store,
> holding package upgrade until it's reachable, then ask the user about
> what track they'd like to use, followed by installing the LXD snap,
> waiting for it to be online and automatically run the migration tool.
> 
> Should any error happen, the deb content will remain on disk (as all
> this is done in preinst). We can then provide individual instructions
> for failed migrations and update lxd.migrate to account for any cases
> that it couldn't handled.

<snip>

> The current plan is to have Ubuntu LTS releases default to installing
> from the most recent LTS track (currently "3.0") while non-LTS Ubuntu
> releases should default to installing from the "latest" track.

> The debconf prompt will always be shown, so this will only affect the
> default selection.

It is impossible to enforce that debconf prompts are shown in all cases. 
Unattended-upgrades are a thing.  Manually setting
DEBIAN_FRONTEND=noninteractive is a thing.  DEBIAN_PRIORITY=critical is a
thing (and while it's probably reasonable for the "cannot talk to the store"
error prompt to be shown at critical priority, I think track selection
fits the definition of a high-priority debconf prompt, not critical).

This is a general rule anyway, but please be sure that the package behavior
is sane when the debconf prompts are not shown.

- What should the behavior be if the store cannot be reached and the prompt
  cannot be shown?  Should the preinst loop indefinitely (causing
  unattended-upgrades to hold the apt lock forever until the admin
  intervenes), or should the preinst abort (causing an apt transaction
  failure)?
- If it should abort, you may find /usr/sbin/update-secureboot-policy in
  shim-signed to be useful prior art. ("$seen_key")
- If it should loop forever, please ensure the maintainer script outputs
  sufficient information to stdout/stderr for the apparent hang to be
  debuggable from just the apt log.  (But maybe don't generate output for
  every loop iteration, since making /var/log ENOSPC is not the ideal way
  for the admin to discover the problem either.)

> A branch of the stable channel of those tracks will be created and
> closed per policy on seeded snaps (allowing to push emergency snaps to
> those users bypassing the upstream).

Excellent!

-- 
Steve Langasek                   Give me a lever long enough and a Free OS
Debian Developer                   to set it on, and I can move the world.
Ubuntu Developer                                   https://www.debian.org/
slangasek at ubuntu.com                                     vorlon at debian.org
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