annatchijova·annatchijova/SKILLS/concurrency-reasoning/SKILL.md

concurrency-reasoning

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Reason about concurrent execution in terms of shared mutable state, invariants, and happens-before ordering — not in terms of "adding a lock" or "it works on my machine". Use whenever code can run more than once at a time: threads, async tasks, goroutines, workers, multiple processes or replicas, retries, webhooks, cron jobs, signal handlers, or a user who double-clicks. Trigger on "race condition", "thread-safe", "add a mutex", "deadlock", "it only fails under load", "it worked locally", "the job ran twice", "duplicate charge", "lost update", "exactly-once", "idempotent", "retry storm", "this test is flaky under parallelism", "async", or a singleton/cache/counter being initialized or mutated from more than one place. Covers ordering and visibility, critical-section scope, undeclared concurrency, idempotency, deadlock and liveness, and why a passing test proves only one interleaving. Sibling of atomic-state-mutation, which covers the database-transaction case specifically.