Pr Review

fastcdc-rs PR review

This skill reviews a pull request against the standards this project's maintainers and active downstream users expect. The criteria below were derived from real reviewer feedback (ciehanski on PR #44 and #45) plus the determinism guarantees in CLAUDE.md.

fastcdc is a published crate (currently 4.0.1) with real downstream consumers, so the bar is "would a careful human depending on this crate be comfortable merging it into master?" — not "does it compile and pass the existing tests?"

How to run the review

1. Determine the scope of changes:
   • A specific PR: gh pr view <N> --json title,body,files,additions,deletions and gh pr diff <N>.
   • The current branch: git diff master...HEAD (or git diff for uncommitted work).
2. Read the changed files in full, not just the diff hunks — context outside the hunk often determines whether a change is correct (e.g. constructor invariants that make an edge case reachable or not).
3. Walk every criterion below against the change. For each finding, cite file:line and say why it matters to a downstream user, not just what it is.
4. Verify, don't assume: run cargo test, cargo test --features tokio, cargo test --features futures, cargo clippy, and cargo doc --features futures. Report what you actually ran and its result. If a claim in the PR body is testable (e.g. "every byte is emitted", "no perf regression"), test it or say you couldn't.
5. Summarize findings grouped as Blocking, Should-fix, and Consider, then give an explicit merge recommendation.

Review criteria

1. No silent data loss or silent failures

The chunking code's whole job is to account for every input byte. A break, return, early-exit, or swallowed error that can drop or skip data is a blocking issue.

• Trace every loop exit and error path: can any input byte fail to be emitted?
• An edge case that is "currently unreachable through the public API" is still a latent bug. It must debug_assert! (or otherwise fail loudly) and degrade safely (emit the leftover) rather than silently dropping data. (This is exactly the silent-break tail-drop ciehanski flagged on #44.)
• Confirm the chunker produces contiguous, gapless, exact coverage of the input.

2. Per-iteration / per-chunk waste

Hot-path work that could be hoisted out of the loop is a real defect in a perf-critical crate, not a nitpick.

• Look for conversions, allocations, try_into, copies, or table rebuilds happening per chunk or per byte that could be done once (the per-chunk try_into on the GEAR tables ciehanski flagged on #44).
• The core chunking loop is latency-bound on the gear-hash recurrence; be skeptical of changes that add work to it, and ask for A/B benchmark evidence (see criterion 6).

3. Public API surface and semver

This is a published crate; API changes ripple to every downstream user.

• For any new pub item (struct, fn, field, trait, enum variant): is it intended to be public, or should it be pub(crate) / private? Default to the smallest surface that works. (ciehanski: "A new public Chunker struct is introduced — should this struct be public to the consuming user?")
• Does the change alter, remove, or rename any existing public API, or change behavior of existing public API? If so it's a breaking change requiring a major version bump per semver. Flag the required version change explicitly against the current Cargo.toml version.
• Adding a public item is at least a minor bump; document the expected version change.
• New public items need rustdoc and ideally a doctest/example.

4. Determinism is a contract

Identical cut points across versions are a core guarantee (see CLAUDE.md).

• Any change that alters cut points / hashes / chunk boundaries must be intentional, called out loudly in the PR, and is itself a breaking change.
• The hardcoded fixture hashes/lengths in tests are the guardrail — they should only change deliberately. A PR that edits expected fixture values needs strong justification.
• Cross-check v2016 and v2020 produce the same cut points where the docs claim they do.

5. Test rigor — tests must actually catch regressions

• New behavior needs tests that assert the invariant that matters (e.g. every byte emitted, in order, no gaps), not just that the happy path returns something.
• A test is only credible if it fails when the code is broken. Prefer changes where the author demonstrably verified this (deliberately break it, watch it fail). When reviewing, if a test's failure mode is unclear, mentally (or actually) inject the bug and check the test would catch it.
• Be honest about coverage gaps: if a branch can't be reached through the public API and therefore isn't covered by a test, say so plainly rather than implying full coverage.
• Cover edge cases: empty input, sub-minimum-size input, all-identical bytes (worst case for finding a cut), and the real fixture.
• Run the feature-gated test matrix: default, tokio, futures (the two async features are mutually exclusive — check the cfg guards still hold).

6. Performance claims need reproducible evidence

• Any "faster" / "no regression" claim should be backed by an interleaved old-vs-new A/B measurement that asserts identical output before timing, ideally on more than one architecture (e.g. ARM + x86_64). Don't accept single-run wall-clock numbers.
• New perf-sensitive changes are a good prompt to ask whether CI should gain a cross-platform test / perf-regression workflow (ciehanski's suggestion), so future PRs have data before merge.

7. Human accountability and provenance

This was ciehanski's central concern: PRs and comments written end-to-end by an AI agent with no evident human review, landing in a depended-on crate.

• The review's job is to be the human-quality gate. Surface issues directly and concretely so a human can make the merge decision — never imply "looks good, merge it" as a substitute for maintainer judgment.
• Flag anything that looks auto-generated and unreviewed: plausible-but-wrong assertions, comments/PR text that overstate what was verified, tests that look like coverage but don't exercise the claimed invariant.
• Encourage a clear paper trail: PR description states what changed, why, what was tested, and the semver impact. Do not vouch for code paths you did not actually exercise.

Output format

## Review of <PR # / branch>

### Blocking
- file:line — issue — why it matters downstream — suggested fix

### Should-fix
- ...

### Consider
- ...

### Verification run
- cargo test: <result>
- cargo test --features tokio / futures: <result>
- cargo clippy / cargo doc: <result>
- (any perf or invariant checks performed)

### Semver impact
- <none / patch / minor / major> — reason

### Recommendation
- <merge / merge after fixes / needs work> — one-line rationale for the human maintainer

Source: nlfiedler/fastcdc-rs — distributed by TomeVault.