Review checkout code for defects and privacy

Q: Review checkout code for defects and privacy

This question evaluates a candidate's ability to audit a supermarket checkout implementation for monetary correctness, transactional integrity, security and privacy, and resilience, assessing competencies in precise financial handling, concurrency and idempotency, threat modeling, logging and redaction, and observability.

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Question

You are reviewing a supermarket checkout implementation. Identify and prioritize issues and fixes across: (a) monetary correctness (avoid floating point; tax/discount rounding rules; coupon stacking; price overrides; refunds/voids; currency/locale/time-zone edge cases), (b) transactional integrity (idempotent payments, exactly-once order creation, inventory race conditions, TOCTOU on discounts), (c) security/privacy (PCI scope reduction, tokenization, PII redaction in logs, least-privilege DB access, input validation to prevent injection), and (d) resilience/observability (retry/backoff, dead-letter queues, audit trails). Propose concrete refactors, unit/integration tests, and a minimal threat model. Provide a short code review checklist you would use in 30 minutes and 3 high-risk bugs you would hunt first.

Review checkout code for defects and privacy

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