[Bug 1785096] Update Released

[Steve Langasek]

The verification of the Stable Release Update for ubuntu-release-
upgrader has completed successfully and the package has now been
released to -updates.  Subsequently, the Ubuntu Stable Release Updates
Team is being unsubscribed and will not receive messages about this bug
report.  In the event that you encounter a regression using the package
from -updates please report a new bug using ubuntu-bug and tag the bug
report regression-update so we can easily find any regressions.

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https://bugs.launchpad.net/bugs/1785096

Title:
  dist-upgrader should clean up downloaded debs

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

Bug description:
  During the release upgrade process packages for the new release are
  downloaded to the apt configuration "Dir::Cache::archives" (usually
  /var/cache/apt/archives/), but those are not removed from disk until
  some time later. With the dist-upgrader growing support for
  transitioning from debian packages to snaps for some software it's
  possible that there may not be enough free space on disk (as the
  upgrader doesn't currently calculate the space need which is bug
  1783597) to support installing the snaps. Subsequently, it'd be good
  to remove the debs from disk after the upgrade has completed since
  they are likely on /var which is where the snaps will need space.

  [Test Case]
  1) Run 'du -sh /var/cache/apt/archives'
  2) Upgrade from Ubuntu 16.04 to Ubuntu 18.04
  3) Run 'du -sh /var/cache/apt/archives'
  4) Observe that /var/cache/apt/archives is taking up more space than it did.

  With the version of the dist-upgrader in -proposed
  /var/cache/apt/archives should be nearly empty - some debs may be left
  because they are left over from regular updates in Ubuntu 16.04 and
  were not part of the release upgrade process.

  [Regression Potential]
  It's possible that the debs are removed too early and if there were a failure to upgrade it'd be hard to recover from. One way to test this would be to use the apt-clone file from 1766890 which would create an upgrade failure.

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