ADR-0024: Benchmark data publishing model

• Status: Accepted
• Date: 2026-07-04
• Authors: IBEX Harness team

Context

The benchmark workflow collects proxy load and Go microbenchmark results, validates them, and produces benchmark-data.json for the docs dashboard. An earlier design pushed benchmark JSON commits onto contributor PR branches using GITHUB_TOKEN. That caused:

1. CI cascade noise — every bot push emitted pull_request synchronize and re-ran CI, CodeQL, Semgrep, and semantic PR checks.
2. Integrity bypass risk — trusting git author email to skip comparison allowed spoofing via local commit metadata.
3. Token exposure — persist-credentials: true on PR head checkouts left credentials available to later script steps.
4. Rerun loops — cumulative PR path filters and merge-ref diffs re-triggered full benchmark collection after data-only commits.

The docs site embeds benchmark JSON at build time (docs/app/src/lib/benchmarks/published-data.ts). Preview on PRs must show fresh benchmark results without mutating the PR branch.

Decision

1) No in-PR branch writes from ibex-harness workflows

• Remove publish-benchmark-data (PR branch commits) from .github/workflows/benchmark.yml.
• PR benchmark results live in the benchmark-data workflow artifact and PR comments only.
• docs-build in .github/workflows/ci.yml overlays the latest successful artifact for the PR head SHA before building docs.

2) Integrity checks without spoofable trust

• verify_pr_benchmark_integrity.sh fetches the base ref with an explicit one-shot token header; checkout uses persist-credentials: false.
• If benchmark-data.json is unchanged from base, verification passes (artifact is the PR preview source).
• If the branch claims the current PR in committed JSON (pr_number match), committed data must equal the workflow artifact (compare_pr_benchmark_json.py).
• Do not trust git author email for bypass decisions.

3) Latest-commit benchmark gate

• detect-benchmark-trigger in .github/workflows/benchmark.yml checks out pull_request.head.sha with fetch-depth: 2 and diffs HEAD~1..HEAD so data-only commits do not re-run collection.

4) Post-merge publishing via external GitHub App (follow-up)

• Long-term publishing to main moves to a separate repository (ibex-benchmark-bot) running a dedicated GitHub App.
• Interim: open-benchmark-data-pr in .github/workflows/benchmark.yml on schedule / workflow_dispatch against main remains until the app replaces it.

5) run_number semantics

• run_number stores GITHUB_RUN_NUMBER, not the workflow run ID.
• validate_published_data.py rejects rows where run_number equals the numeric ID embedded in run_url.

Consequences

• PR branches stay free of automated benchmark commits; CI runs once per human code push.
• Docs preview on PRs depends on a successful Benchmarks workflow artifact (falls back to committed JSON if none yet).
• Fork PRs continue to use artifact download only; no branch writes.
• A follow-up milestone implements the external app for verified post-merge PRs to main.

Alternatives considered

References

• PR #177 benchmark pre-PR pipeline work
• ibex-benchmark-bot — external app specification and implementation (follow-up)