[Bug 1785096] [NEW] dist-upgrader should clean up downloaded debs

[Brian Murray]

Public bug reported:

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 id.

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.

** Affects: ubuntu-release-upgrader (Ubuntu)
     Importance: Undecided
         Status: Fix Released

** Affects: ubuntu-release-upgrader (Ubuntu Bionic)
     Importance: Medium
     Assignee: Brian Murray (brian-murray)
         Status: In Progress

** Also affects: ubuntu-release-upgrader (Ubuntu Bionic)
   Importance: Undecided
       Status: New

** Summary changed:

- dist-upgrader does not clean up downloaded debs
+ dist-upgrader should clean up downloaded debs

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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:
  In Progress

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 id.

  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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