race-condition

Two or more concurrent contexts — threads, async tasks, processes, requests — read and write the same state without coordination. Tests pass on a lightly-loaded machine; production corruption appears under real traffic.

Symptoms

• Counters that "mostly" work but occasionally lose increments.
• Check-then-act sequences (e.g., "if not exists, create") where two contexts both pass the check and both create.
• Shared caches or singletons mutated from request handlers without a lock.
• Data-corruption bugs that only reproduce under load or with specific timing.

What to do

• Identify every piece of shared mutable state. Draw the boundary: who reads it, who writes it, from which contexts.
• Replace check-then-act with atomic operations: compare-and-swap, INSERT ... ON CONFLICT, unique indexes, setnx.
• Prefer immutable data or message-passing over shared mutation. Actor-style patterns remove most races by construction.
• When a lock is unavoidable, hold it for the shortest possible span and document what state it protects.
• Test concurrency explicitly. A single-threaded test proves nothing about a multi-threaded code path. Use stress tests or pytest-asyncio / concurrent harnesses.