[Bug 1678042] Re: removal of priority changed packages breaks builds of the core snap

[Andy Whitcroft]

The image fails to build because we attempt to remove the systemd-sysv
from the system.  We do this because there is a newer version of
systemd-sysv in an applied PPA for this build and that version is marked
as Priority: important.

  $ apt-cache show systemd-sysv | egrep Priority\|Version                               
  Priority: important
  Version: 229-4ubuntu16+ppa2
  Priority: required
  Version: 229-4ubuntu16
  Priority: required
  Version: 229-4ubuntu10
  Priority: required
  Version: 229-4ubuntu4
  $

This leads us to attempt to remove the package.  The removal only
deletes that one package so it is deemed removable and all hell breaks
loose.  The removal of this package triggers a pile of installations of
new packages presumably for dependencies with alternatives which are
satisfiable.  In this case with a complete switch to upstart.

I believe there are two main bugs in this code.  Firstly we should only
be taking into account the latest version available for Priority:
detection; otherwise we could switch a package back to required and it
would still be removed (though in this case the removal is valid based
on priority).  Secondly we should not be only considering removals when
testing the apt-get remove we should also consider whether its removal
triggers installation of new packages; this latter implies there is a
dependency but an alternative could be found.

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https://bugs.launchpad.net/bugs/1678042

Title:
  removal of priority changed packages breaks builds of the core snap

Status in live-build package in Ubuntu:
  New

Bug description:
  the fix for bug 1668847 seems to cause a removal of systemd-sysv (and
  installation of upstart) in the ubuntu core snap builds, which makes
  teh build fail now.

  a log of the last successful build of the core snap:
  https://launchpadlibrarian.net/313520770/buildlog_snap_ubuntu_xenial_amd64_core_BUILDING.txt.gz

  a log of a current failing build:
  https://launchpadlibrarian.net/313704330/buildlog_snap_ubuntu_xenial_amd64_core_BUILDING.txt.gz

  it removes locales properly but then tries to also remove systemd-sysv
  in the lb_chroot_archives step

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