[Bug 1882794] Re: do-release-upgrade should allow other tools to block or warn before upgrades

[Paul Goins]

This would be immensely helpful.  Please note that I'm commenting as a
member of Canonical's BootStack team, where we regularly use Juju, i.e.
case 1 listed above.

When I'm managing a cloud with Juju, and need to do a series upgrade,
Juju requires that certain hooks are run prior to performing the do-
release-upgrade.  If these hooks are not fired, the effects can be
fairly serious.

A key example is the use of Python virtualenvs in many charms,
especially "reactive" Juju charms.  These virtualenvs link into the
system Python, and special care must be taken at upgrade time for the
virtualenvs to be recreated against the new Ubuntu release's Python,
else the virtualenv will become unusable after the upgrade is completed.
That's part of the reason for Juju's "upgrade-series" commands.

However, it is very easy for an engineer (read: me) to accidentally miss
a step, perform the do-release-upgrade, and realize after the fact that
my action resulted in breaking management of the Juju-managed
applications deployed on the target machine.  And this is actually worse
than just that, as often you can't even cleanly remove the affected
applications because of the broken virtualenvs.  The solution is often
to completely redeploy the affected machine by force, which may also has
impact on related machines which couldn't communicate with the
forcefully redeployed unit during teardown.

The above may not be clear to those who aren't deeply familiar with
Juju, but put more simply: not being able to enforce running of Juju's
actions prior to upgrade is a risk to Juju-deployed clouds.

If we could have a way for Juju to install a hook, or even simply a "do
not run" breadcrumb, so that do-release-upgrade could be blocked until
Juju has had a chance to run its pre-release hooks, that would allow
Juju to have much more robust series upgrades of Juju managed machines.

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https://bugs.launchpad.net/bugs/1882794

Title:
  do-release-upgrade should allow other tools to block or warn before
  upgrades

Status in ubuntu-release-upgrader package in Ubuntu:
  New

Bug description:
  do-release-upgrade should allow other tools to block or warn before it
  proceeds.

  A few use cases for this:

   1) Juju managed machines - Juju requires the user to run
      'upgrade-series X prepare' before the upgrade starts.  This allows
      charms running on the machine to prepare for the upcoming Ubuntu upgrade.

   2) Paranoid/careful sysadmins (c.f. the 'molly-guard' package)

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