[Bug 2139113] Re: APT::Architecture-Variants "amd64v3" disables implicit native architecture (amd64) on minimal installs

[Shashank Taliwal]

Thanks for the clarification. I verified the package counts on the
current repository, and they are indeed nearly identical (~77k in both),
so I see that Ubuntu's current implementation is a "Complete Variant"
(Metadata Mirror).

However, I have three questions regarding robustness and configuration
for mirrors:

1. The "Missing Version" Edge Case What is the expected behavior if a
specific package version is not available in amd64v3 but is available in
amd64? (e.g., if the v3 index lags behind, or a specific package fails
to sync/build for v3). Since the default APT::Architecture-Variants
configuration drops the implicit amd64 architecture, it appears the
client would simply fail with "Package not found" rather than falling
back to the base architecture. Is there any built-in fallback mechanism
for this "Complete" model?

2. Configuring a "Partial Variant" You mentioned that "Other
repositories may declare it to be a partial variant." For internal
mirrors that intend to be Partial/Sparse (hosting only the optimized
deltas to save storage and metadata overhead), how exactly do we
configure this?

3. The Mechanism Is there a server-side metadata flag (in Release or
similar) that tells APT "Treat this as an overlay and keep the base
architecture"? Or is the only way to support a Partial/Sparse repository
to strictly require clients to manually configure Architectures: amd64
amd64v3 in their sources.list, forcing the merge client-side?

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https://bugs.launchpad.net/bugs/2139113

Title:
  APT::Architecture-Variants "amd64v3" disables implicit native
  architecture (amd64) on minimal installs

Status in dpkg package in Ubuntu:
  Invalid

Bug description:
  Package: apt
  Version: 3.1.6ubuntu2 (and dpkg 1.22.21ubuntu3.1)
  Distro: Ubuntu Questing (25.04) / Docker Image

  Description:
  When following the official instructions to enable x86-64-v3 support by creating `/etc/apt/apt.conf.d/99-arch-variants` with content `APT::Architecture-Variants "amd64v3";`, APT behaves incorrectly on minimal installations (Docker).

  Instead of treating 'amd64v3' as an overlay/variant of the native
  'amd64' architecture, APT drops the implicit 'amd64' architecture
  entirely.

  Steps to Reproduce:
  1. docker run --rm --platform linux/amd64 -it ubuntu:rolling bash
  2. echo 'APT::Architecture-Variants "amd64v3";' > /etc/apt/apt.conf.d/99arch-variants
  3. rm -rf /var/lib/apt/lists/*
  4. apt update
  5. ls -1 /var/lib/apt/lists/ | grep binary-amd64_Packages

  Expected Result:
  The 'binary-amd64' list files should exist (Base architecture).

  Actual Result:
  Only 'binary-amd64v3' list files exist. The system loses access to all packages not explicitly indexed in the v3 overlay.

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