[Bug 1768379] Re: python3-minimal should predepend on versioned version of python3-minimal

[Brian Murray]

I also performed an upgrade of an Ubuntu Desktop system from xenial w/
-proposed enabled to artful so that python3-minimal from artful-proposed
would be used and the upgrade proceeded without error.

bdmurray at clean-xenial-amd64:/tmp/ubuntu-release-upgrader-355quvv3$ grep -C10 python3-minimal /var/log/dist-upgrade/apt-term.log 
Processing triggers for mime-support (3.59ubuntu1) ...
Processing triggers for hicolor-icon-theme (0.15-0ubuntu1) ...
Processing triggers for cups (2.1.3-4) ...
Updating PPD files for postscript-hp ...
Setting up libssl1.0.0:amd64 (1.0.2g-1ubuntu13.6) ...
Setting up libpython3.6-minimal:amd64 (3.6.3-1ubuntu1) ...
Setting up libexpat1:amd64 (2.2.3-1) ...
Setting up python3.6-minimal (3.6.3-1ubuntu1) ...
Processing triggers for libc-bin (2.26-0ubuntu2.1) ...
(Reading database ... 209379 files and directories currently installed.)
Preparing to unpack .../python3-minimal_3.6.3-0ubuntu3_amd64.deb ...
Unpacking python3-minimal (3.6.3-0ubuntu3) over (3.5.1-3) ...
Setting up python3-minimal (3.6.3-0ubuntu3) ...
(Reading database ... 209379 files and directories currently installed.)
Preparing to unpack .../python3_3.6.3-0ubuntu3_amd64.deb ...
running python pre-rtupdate hooks for python3.6...
Unpacking python3 (3.6.3-0ubuntu3) over (3.5.1-3) ...
Preparing to unpack .../mime-support_3.60ubuntu1_all.deb ...
Unpacking mime-support (3.60ubuntu1) over (3.59ubuntu1) ...
Selecting previously unselected package libpython3.6-stdlib:amd64.
Preparing to unpack .../libpython3.6-stdlib_3.6.3-1ubuntu1_amd64.deb ...
Unpacking libpython3.6-stdlib:amd64 (3.6.3-1ubuntu1) ...
Selecting previously unselected package python3.6.

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https://bugs.launchpad.net/bugs/1768379

Title:
  python3-minimal should predepend on versioned version of
  python3-minimal

Status in python3-defaults package in Ubuntu:
  Triaged
Status in ubuntu-release-upgrader package in Ubuntu:
  Invalid
Status in python3-defaults source package in Artful:
  Fix Committed
Status in ubuntu-release-upgrader source package in Artful:
  Invalid
Status in python3-defaults source package in Bionic:
  Fix Committed
Status in ubuntu-release-upgrader source package in Bionic:
  Invalid
Status in python3-defaults package in Debian:
  Fix Released

Bug description:
  [Impact]
  Release upgrades from Ubuntu 16.04 LTS to either Ubuntu 17.10 or Ubuntu 18.04 LTS can fail if python3-minimial is unpacked before python3.x-minimal so a system won't have a usable python3 symlink. Then upgrades will fail because py3clean, used in prerm scripts of python3 packages, will fail with "not found".

  [Test Case]
  1) on a Ubuntu 16.04 LTS system with python3-apt installed download debs for Ubuntu 17.10 or 18.04 LTS of python3-minimal and python3-apt
  2) dpkg --unpack python3-minimal...
  3) dpkg --unpack python3-apt...

  Observe the following Traceback:

  Preparing to unpack .../python3-apt_1.4.0_beta3build2_amd64.deb ...
  /var/lib/dpkg/info/python3-apt.prerm: 6: /var/lib/dpkg/info/python3-apt.prerm: py3clean: not found
  dpkg: warning: subprocess old pre-removal script returned error exit status 127
  dpkg: trying script from the new package instead ...
  /var/lib/dpkg/tmp.ci/prerm: 6: /var/lib/dpkg/tmp.ci/prerm: py3clean: not found
  dpkg: error processing archive /home/jak/Downloads/python3-apt_1.4.0_beta3build2_amd64.deb (--unpack):
   subprocess new pre-removal script returned error exit status 127
  /var/lib/dpkg/info/python3-apt.postinst: 6: /var/lib/dpkg/info/python3-apt.postinst: py3compile: not found
  dpkg: error while cleaning up:
   subprocess installed post-installation script returned error exit status 127

  With the deb python3-minimal from -proposed you won't be able to
  unpack python3-minimal without first installing python3.6-minimal.
  While that may qualify as verification it'd still be good to test
  unpacking python3-apt. To do that you'll need to install
  python3.6-minimal from the target release by editing you
  /etc/apt/sources.list to the new release, running 'apt-get update',
  and then 'apt-get install python3.6-minimal'. After that's all done
  you can try steps 2 and 3 from the test case again and you should not
  receive a Traceback.

  [Regression Potential]
  Its possible this could cause or reveal different upgrade failures so some extensive (desktop, cloud, server) upgrade testing should be done.

  
  [A Portion of the Original Description (it was a huge blob of failure)]
  do-release-upgrade -d  on my 16.04 LTS unity laptop lead to an install failure:

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