tests: verify worker platform selection without QEMU

[cpuguy83]

Problem

The cross platform integration test built packages for a foreign architecture (arm64 on amd64 CI runners), which required QEMU emulation. QEMU frequently crashes in CI during this test (e.g. https://github.com/project-dalec/dalec/actions/runs/26779889971/job/78941218259?pr=1068).

Approach

Replace it with a native test that proves what the test actually cares about: the frontend selects the correct platform-specific worker image for a requested build platform — without any emulation or rebuilding the world.

• A custom multi-platform worker image is built as an OCI layout index with one manifest per real platform the distros ship (linux/amd64 and linux/arm64). Each manifest embeds a /platform-marker file naming its own platform.
• The index is registered with the build session (SolveOpt.OCIStores) and supplied as the distro worker context via an oci-layout: named context.
• For each requested build platform, the test solves the worker target and asserts the returned image's /platform-marker matches the requested platform.

The distro worker target short-circuits when a worker context image is provided and returns it directly, so nothing is executed (no rpmbuild/dpkg, no emulation) — the worker image is only inspected. Requesting arm64 on an amd64 runner therefore needs no QEMU.

Real amd64/arm64 platforms are used (rather than a made-up platform) because the worker handler resolves the real distro image config at the requested platform, so the requested platform must actually exist on that image.

[Copilot]

Pull request overview

This PR updates the Linux integration “cross platform” test selection to avoid QEMU emulation on amd64 CI runners by preferring a same-arch, different-variant target (linux/amd64/v3) instead of cross-arch (e.g., arm64).

Changes:

• Add linux/amd64/v3 to the per-distro Platforms lists used by integration tests.
• Update testTargetPlatform selection logic to prefer same-architecture platforms (different variant) to avoid QEMU on amd64 runners.
• Extend the platform plumbing assertion to verify both TARGETARCH and TARGETVARIANT are passed through.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
test/target_ubuntu_test.go	Adds linux/amd64/v3 to Ubuntu test platform matrix.
test/target_rockylinux_test.go	Adds linux/amd64/v3 to Rocky Linux 8/9 test platform matrices.
test/target_azlinux_test.go	Adds linux/amd64/v3 to Azure Linux 3 test platform matrix.
test/target_almalinux_test.go	Adds linux/amd64/v3 to AlmaLinux 8/9 test platform matrices.
test/linux_target_test.go	Changes cross-platform target selection to prefer same-arch variants and asserts TARGETVARIANT plumbing.

[cpuguy83]

Took this in a new direction.

Why: the existing cross platform test built a package/container for a foreign architecture and ran build steps inside that foreign-arch worker. On a single-arch CI runner that requires QEMU
emulation, which crashes intermittently and makes the test flaky.

What changed: instead of doing a real foreign-arch build, the test now verifies the thing that was actually at risk of regressing — that the frontend selects the correct platform-specific
worker base image for the requested build platform — without executing any foreign-arch code.

A small multi-platform "marker" image is built from the real distro worker base, injecting a /platform-marker file into each manifest that names its platform. The test then requests the
worker target for both linux/amd64 and linux/arm64 and reads the marker back to confirm the right manifest was selected. The marker is content baked into each manifest, so it proves the
actual base bytes were fetched — not just that the result was labeled with the requested platform.

Selection is checked through both resolution paths:

• worker context — the marker index is supplied as the worker build context (short-circuits worker building); nothing executes.
• source policy — a BuildKit source policy rewrites the distro base image to the marker index, exercising the normal llb.Image worker-build path. Cross-arch worker assembly runs natively
  via dnf/apt --forcearch into a mounted foreign-arch root, so no emulation is needed.

Both run natively regardless of the runner's architecture — no QEMU.

Coverage notes:

• TARGETARCH/TARGETVARIANT argument passthrough is already covered without QEMU by TestPassthroughVars (via debug/resolve).
• The previous test's end-to-end assertions on package output naming and the final container image-config platform are no longer exercised by this test. A genuinely-different-arch real
  build is only achievable without QEMU using a native sub-architecture (e.g. 386 on amd64), which is a possible future follow-up gated on per-distro 32-bit tooling availability.

[Copilot]

Pull request overview

Copilot reviewed 9 out of 10 changed files in this pull request and generated 2 comments.