Make base_ubuntu_warden_rosetta host-architecture agnostic so it can run in CI

[mkocher]

Human Summary

Make building the rosetta stemcell possible in CI by not relying on any of the arm binaries. Verified with a build and test deployment with the new stemcell.

AI Summary

The stage executed arm64 binaries while doing its work — the tar self-test, and dpkg -x in the chroot, which shells out to the just-swapped GNU tar — so it only ran on an Apple Silicon host. Download all the arm64 debs first, extract them from outside the chroot with the builder image's tar, then swap and statically verify (e_machine == 183) every replacement, including tar and the systemd daemons. The tar round-trip now runs only behind an exec probe and skips loudly otherwise, in the stage and in rosetta_spec.rb, so the stage can build on an x86-64 CI worker.

Also cover stage_collection's rosetta insertion, which had no unit test.

[coderabbitai]

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Walkthrough

The Rosetta stage now uses a host-architecture-agnostic package flow. It downloads and extracts arm64 packages outside the chroot, installs verified arm64 binaries and systemd components, and preserves amd64 binaries. The tar execution self-test runs only when the host can execute arm64 binaries. Specifications now verify Rosetta stage ordering and conditional tar validation.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 75.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 4 functions across 3 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: making the Rosetta stage host-architecture agnostic for CI.
Description check	✅ Passed	The description provides detailed human and AI summaries, explains the implementation, identifies the added tests, and states that build and deployment verification was completed. It does not explicit…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Description check

Explanation

The description provides detailed human and AI summaries, explains the implementation, identifies the added tests, and states that build and deployment verification was completed. It does not explicitly document the merge-forward branch process or AI comment resolution, but the core change and verification details are complete.

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• Commit unit tests in branch resolute-better-rosetta

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@stemcell_builder/stages/base_ubuntu_warden_rosetta/apply.sh`:
- Around line 267-291: Separate arm64 execution detection from tar validation in
the probe logic around run_in_chroot: use an independent minimal arm64
executable such as ld-linux-aarch64.so.1 to set the capability marker, then
require /usr/bin/tar --version and the existing round-trip self-test to succeed
when execution is supported, failing rather than skipping if tar is
nonfunctional. Apply the same host-capability versus tar-health distinction in
the Rosetta behavior covered by rosetta_spec.rb.

Apply the same fix in `@bosh-stemcell/spec/stemcells/rosetta_spec.rb` around lines
89 - 92: The spec has the same ambiguity and can skip on runtime failures
instead of reporting a broken arm64 tar.

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ℹ️ Review info

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Configuration used: Organization UI

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Run ID: ec7e7f2a-1656-44d7-a90a-0179081180f5

📥 Commits

Reviewing files that changed from the base of the PR and between 0862547 and 7f5d9a0.

📒 Files selected for processing (3)

• bosh-stemcell/spec/bosh/stemcell/stage_collection_spec.rb
• bosh-stemcell/spec/stemcells/rosetta_spec.rb
• stemcell_builder/stages/base_ubuntu_warden_rosetta/apply.sh

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[aramprice]

Cursor / Gemini Pro 3.1 Review

The PR looks great overall! The approach to extract debs outside the chroot and statically verify the arm64 binaries is clever and robust.

I agree with the feedback from coderabbitai: the capability probe and tar health check should be separated. If tar itself is broken (e.g., missing shared libraries or loader), /usr/bin/tar --version will fail, but the current script will interpret that failure as 'the host cannot execute arm64 binaries' and skip the test rather than failing the build.

To fix this, you can use an independent minimal arm64 executable like /lib/ld-linux-aarch64.so.1 --help to set the capability marker. Then, require /usr/bin/tar --version and the existing round-trip self-test to succeed when execution is supported.

In stemcell_builder/stages/base_ubuntu_warden_rosetta/apply.sh:

probe_marker=/tmp/arm64-exec-probe
run_in_chroot $chroot "
  rm -f $probe_marker
  /lib/ld-linux-aarch64.so.1 --help >/dev/null 2>&1 && : > $probe_marker
  true
"

if [ -f "$chroot$probe_marker" ]; then
  run_in_chroot $chroot "
    set -e
    /usr/bin/tar --version >/dev/null
    rm -rf /tmp/tar-selftest
    # ... rest of the test

And similarly in bosh-stemcell/spec/stemcells/rosetta_spec.rb:

      it "extracts an archive it just created" do
        if command("/lib/ld-linux-aarch64.so.1 --help").exit_status != 0
          skip("this build host cannot execute arm64 binaries; the tar " \
               "round-trip is the only check skipped, all static assertions ran")
        end

        expect(subject.exit_status).to eq(0)
      end

Other than that, the changes are well-structured and the comments are excellent.

[mkocher]

Agreed, that is better. I've pushed your suggested fix.

PR updated

[Copilot]

Pull request overview

Makes Rosetta stemcell builds architecture-agnostic for x86-64 CI workers.

Changes:

• Extracts ARM64 packages outside the chroot and statically verifies binaries.
• Conditionally runs ARM64 tar execution tests.
• Adds Rosetta stage-order coverage.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
stemcell_builder/stages/base_ubuntu_warden_rosetta/apply.sh	Implements host-independent ARM64 package extraction and verification.
bosh-stemcell/spec/stemcells/rosetta_spec.rb	Skips execution checks when ARM64 is unavailable.
bosh-stemcell/spec/bosh/stemcell/stage_collection_spec.rb	Tests Rosetta stage insertion and ordering.

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[aramprice]

Nit: lets clean up the docs links

[aramprice]

docs links updated