base-apt: Inconsistent priorities between apt-cache and .deb data break offline builds

["'MOESSBAUER, Felix' via isar-users" <isar-users@googlegroups.com>]

Dear isar-devs,

we noticed that cross-architecture offline builds are currently broken
and single-architecture offline builds differ from the online
variant. One failure pattern is the following, whereby the offline
isar-mmdebstrap-host fails with an error message like this:

The following packages have unmet dependencies:
 libtext-wrapi18n-perl : Depends: libtext-charwidth-perl but it is not
installable
E: Unable to correct problems, you have held broken packages.
E: The following information from --solver 3.0 may provide additional
context:
   Unable to satisfy dependencies. Reached two conflicting decisions:
   1. libtext-wrapi18n-perl:amd64=0.06-10 is selected for install
   2. libtext-wrapi18n-perl:amd64 Depends libtext-charwidth-perl
      but none of the choices are installable:
      [no choices]

The root cause for this is that we ignore the upstream's repo priority
overrides when creating the base-apt: reprepro includedeb records the
priority from the .deb's own control field, and that becomes the
priority published in base-apt's Packages index.

When these priorities differ, the set of packages installed in offline
mode are different from the ones in online mode. This per-se is plain
wrong, but only surfaces if a now-required arch=all package depends on
its architecture specific variant which never has been fetched (because
the online bootstrap had access to the prio overrides). For non-cross
builds the result is also different, which can be checked by comparing
the SBOMs.

There are HACKY ways to re-create the upstream priorities, but IMHO our
approach for offline builds is problematic and should be changed:
Instead of explicitly recreating the base-apt repo, we better just
preserve the upstream apt lists and inject our downloads/deb dir into
the apt cache that is used by the build. This would allow us to
completely drop the base-apt.

Reproducer:

1. kas-container menu, select qemuarm64 machine, build
2. mv build build-online && mkdir build && cp -rf build-
online/downloads build/
3. kas-container build .config.yaml:offline.yml

offline.yml:
header:
  version: 14
local_conf_header:
  offline: |
    ISAR_USE_CACHED_BASE_REPO = "1"
    BB_NO_NETWORK = "1"

The initial report of this error is by Herbert.

Best regards,
Felix

-- 
Siemens AG
Linux Expert Center
Friedrich-Ludwig-Bauer-Str. 3
85748 Garching, Germany

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