E2 — Build reproducibility

Gates: BC3. Independent of E0/E1; can run in parallel.

Question

Can the target build be made bit-reproducible across independent machines at acceptable cost? K-of-N builder agreement is meaningless without it, and without K-of-N the release gate degrades to a single signature.

Method

1. Take the current build as-is. Build the same commit on three machines differing in OS version, CPU, filesystem ordering, and locale.
2. Compare output digests. For each divergence, identify the cause: embedded timestamp, path leakage, non-deterministic ordering, toolchain version drift, network-fetched dependency.
3. Classify each cause: cheap to fix, expensive to fix, or structural.
4. Estimate the cost of reaching bit-reproducibility under a hermetic derivation model, and separately under the existing build with fixes applied.

Pre-registered thresholds

• Divergences all cheap-to-fix: proceed with K-of-N in the release policy.
• Any structural divergence: K-of-N is unavailable. The release gate must be specified for a single trusted builder, and docs/release-policy.md needs rewriting before implementation, not after.

Note

This is the one binding constraint that is not novel. Reproducible-builds work, guix challenge, and rebuilderd have prior art and tooling. Reuse it rather than measuring from scratch — the contribution here is the claim log, not the rebuild comparison.