Race Condition

You might say

Coupons occasionally oversell while one test run looks normal. Reproduce the race with concurrent timing, fix it, and verify repeatedly.

A race condition occurs when the ordering of concurrent operations changes the final result.Two requests can read and issue the last coupon at nearly the same time, so the read-write order decides whether overselling occurs. Race conditions are broader than data races; request ordering, file state, or event timing can also race, and one passing test does not prove absence.

Know first

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Why identical requests occasionally oversell under different timing

Concurrent coupon claimLast coupon

Request Asees 1 left

Request Balso sees 1 left

claim resultsometimes succeeds twice

EvidenceIdentical inputs differ across runs

The result depends on request timing.

How to turn an occasional race into reproducible evidence

Problem and key mechanism: Both requests start from one coupon, and different read-write interleavings sometimes issue two; have the concurrent test record each read and write's request ID, value, and time so timing evidence returns to the same run.

Verify stability across repeated runs: After an atomic update or correctly scoped lock, repeated runs yield one success and stock 0. This does not prove every race is gone.

A second scenario: avatar management: If a user uploads a new avatar and immediately cancels it, different upload and delete completion orders can make the avatar reappear. Record the timing and repeat the test to verify the final state always follows the last action.

You can say this to an AI Agent

Create a repeatable concurrent test for the last coupon so two requests claim from the same initial state. Record request IDs, read values, writes, and timing. Reproduce the race, then fix it with an atomic update or correctly scoped lock. Repeat the run and verify exactly one success and remaining stock 0 every time; do not treat one pass as proof.

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Coupon overselling happens only occasionally under high concurrency. What best gathers race evidence?

A. Run once serially and treat a normal result as proof that no race existsB. Hide the oversell message and let users retry until one succeedsC. Repeat the same concurrent run and record request IDs and timing for every read and write