reviewing-concurrency-and-async

name	reviewing-concurrency-and-async
description	Reviews concurrent and async code for races and ordering bugs: shared mutable state without synchronization, check-then-act spanning an await, lost updates from interleaved requests, lock ordering, unawaited promises, accidental sequential awaits, non-idempotent message consumers, exactly-once assumptions, and missing cancellation/timeout propagation. Use when reviewing threads, async/await, promises, locks, queues, message handlers, or anything two callers can run at once. Skip when the code is single-threaded and synchronous with no shared mutable state, async/await, or message handling — nothing two callers race on.
provenance	{"taxonomy_version":"v0.9","built_from":[{"category":3,"source":"docs/research/cluster-1-correctness.md#3","hash":"c9755d9b409479e2fac960b39449dc1eaaf0f98e8bbe01fd2758ad97a6bd7ba5"}]}

reviewing-concurrency-and-async

What breaks when two run at once? Races, lost updates, unawaited promises, idempotency.

When to use

Reviews concurrent and async code for races and ordering bugs: shared mutable state without synchronization, check-then-act spanning an await, lost updates from interleaved requests, lock ordering, unawaited promises, accidental sequential awaits, non-idempotent message consumers, exactly-once assumptions, and missing cancellation/timeout propagation. Use when reviewing threads, async/await, promises, locks, queues, message handlers, or anything two callers can run at once. Skip when the code is single-threaded and synchronous with no shared mutable state, async/await, or message handling — nothing two callers race on.

Shape: diff — design-capable. Also works on design docs and plans: apply the same checks to the proposed states, data flows, and failure paths before any code exists. When the design doc is specifically a decision record (an ADR, RFC, or adoption/deprecation plan), also run the shared decision-record checklist on top of this lens's own topical checks: is the rationale actually recorded (not just the outcome); are the stated assumptions still current; is there a revisit-trigger; is an exit, rollback, or sunset path defined; were real alternatives weighed, not just the chosen option justified after the fact? A gap here is this lens's finding, reported the same way as a topical one — not a separate report.

Reviewer discipline

Report only real problems. If the code correctly handles the case, reply "No findings" and stop — do not invent issues. This guards against false positives on correct code; still report every genuine issue you do find, with its full detail.

Defects are the default; improvements are opt-in. By default this lens is defect-only: do not suggest changes to code that is already correct. When the team has opted up into improvement suggestions, a finding on already-correct code is admissible only as nit-severity, route: implementer (the author applies, defers, or ignores), and must clear the non-configurable anti-churn floor: it must genuinely improve — never offer a merely equivalent alternative — and must converge (once a dimension is as good as you can confidently make it, stop; never oscillate A→B then B→A, never re-order to an equivalent state). Defects keep the strict bar above regardless of this setting.

Team preferences. If the reviewed repo has .code-quality-atlas/preferences.md, apply it before reporting: a repo's .code-quality-atlas/preferences.md may set/tune this lens's thresholds or selection, but this is a floor-tier lens: it can never suppress a finding outright. The strongest override available is acknowledge — a recorded rationale that keeps the finding visible, tagged acknowledged deviation: <reason>, and non-blocking rather than removing it. Absent the file, apply this lens's defaults exactly as written above.

Pre-existing defects in touched code are surfaceable, not yours to fix. When you notice a genuine defect this change did not introduce but that sits in the code this PR actually touches — the edited function or immediately adjacent lines — you may surface it, tagged "pre-existing — not introduced by this change." Like improvements it is opt-in and default-quiet (off unless the team opts up), route: implementer, and non-blocking: it informs the author's fix-now / file-a-ticket / ignore call and never sets this PR's verdict, because the diff did not cause it. Stay scoped to code the change touches — a repo-wide hunt is the audits' job, not this review — and never let it expand the PR's scope.

Top checks

The head of the full checklist — enough for a first pass without opening any reference file:

• Is shared mutable state touched by multiple threads/tasks without synchronization?
• Any check-then-act / read-modify-write that spans an await/yield and isn't atomic?
• Could two concurrent requests interleave to break an invariant (lost update, double-spend)?
• Are locks acquired in a consistent global order, with minimal scope (deadlock avoidance)?
• Are all promises awaited/handled, and is concurrency intentional (Promise.all) vs. accidental sequential?
• For message consumers: is processing idempotent and keyed on a stable id (at-least-once-safe)?
• Does the code assume wall-clock ordering across nodes/processes (clock skew)?
• Does it assume exactly-once delivery (it doesn't exist — needs dedupe)?

Mechanizing these checks

Where a finding here is one a tool can catch deterministically, surface that as an advisory route: implementer note next to the finding: the hand review caught it this time, and wiring the matching tool from reference/tool-rules.md into CI gates it going forward. This is a suggestion to mechanize, not a defect — it never blocks a verdict, and it falls away on a repo that already runs the tool.

Process notes. If this lens misfired on this change — flagged correct code, missed an obvious issue squarely in its own scope, or its checklist didn't fit the change shape — say so in one line under synthesizing-review-findings's Process notes appendix; that is not a defect finding. Say nothing if the lens worked as intended — never invent a process note to fill the section.

Going deeper

• reference/heuristics.md — the full checklist; open it when the change sits squarely in this lens's domain.
• examples.md — concrete good/bad findings, and the output format to match.
• reference/tool-rules.md — static-analysis rules covering the mechanical subset; for wiring up linters, not needed for the judgment review itself.
• reference/sources.md — the research behind each check; for provenance, not needed during a review.