[Bug 2128606] Re: Correctly copy Architecture-Variant field

[Andreas Hasenack]

Please add a test plan covering release upgrades from plucky to questing
using this dpkg build.

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https://bugs.launchpad.net/bugs/2128606

Title:
  Correctly copy Architecture-Variant field

Status in dpkg package in Ubuntu:
  In Progress
Status in dpkg source package in Questing:
  Fix Committed
Status in dpkg source package in Resolute:
  In Progress

Bug description:
  [Impact]
  amd64v3 packages are differentiated from amd64 packages by

  Architecture-Variant: amd64v3

  When APT sees amd64v3 packages, it considers them an upgrade to amd64
  packages, so when amd64v3 is enabled, a dist-upgrade first upgrades
  you to amd64v3.

  Inadvertently, dpkg did not copy the field when installing the
  package, causing apt to consider the installed amd64v3 package as
  amd64 again, and the next dist-upgrade trying to "upgrade" them again.

  [Test plan]
  Check that the field appears in dpkg --status when installing an amd64v3 package
  Check that the field disappears in dpkg --status when installing an amd64 package.

  If no amd64v3 packages are involved the field always remains NULL (no
  Architecture-Variant is written to the status file); so no visible
  change is seen on any system in the real world now; as amd64v3 is opt-
  in and not yet announced.

  [Where problems could occur]
  The field could be garbage if not initialized (covered by "install an amd64 package"). The default value of the field is NULL; this is not a concern:

  The field is used here:

  src/deb/build.c
  487:                 pkg->available.arch_variant ? pkg->available.arch_variant : pkg->available.arch->name, DEBEXT);

  ^ this is unaffected by the change as available always contains the
  field if availabe

  lib/dpkg/parse.c
  66:  { FIELD("Architecture-Variant"),      f_charfield,       w_charfield,      PKGIFPOFF(arch_variant)   },

  - w_charfield will now write that field if set (it checks if the field
  value is null and skips it, which always happened so far)

  src/main/unpack.c
  908:  pkg->installed.arch_variant = pkg->available.arch_variant;

  ^ here we copy the available value to the installed database, this is
  the new line. It's a no-op everywhere that doesn't have amd64v3
  packages about to be installed (copying default NULL to NULL, nothing
  else set the field of course).

  lib/dpkg/dpkg-db.h
  124:  const char *arch_variant;

  ^ declaration of the field inside the struct

  The only thing that reads the field outside of dpkg is apt when it
  sees if the installed version is the same as an available version:
  There it checks if they have matching variants. A variant is always
  preferred over an architecture.

  If no variants where ever configured or force-installed, variants do
  not appear anywhere on the system.

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