fix(checkout-api): resolve checkout-race-condition

[Spkap]

Summary

Resolve checkout-race-condition for checkout-api with a validated ReplayX patch candidate.

Changed Files

• demo_app/src/inventory/reserve-stock.ts
• demo_app/src/checkout/submit-order.ts

Validation

• Concurrent checkout repro: exit 0
• Sequential checkout sanity: exit 0

Rollback

Revert the live inventory guard and concurrent checkout settlement handling.

[Copilot]

Pull request overview

This PR addresses the seeded checkout-race-condition in the demo checkout-api flow by preventing inventory from being driven negative under concurrent checkout attempts and by making the concurrent repro runner tolerant of expected OutOfStock outcomes.

Changes:

• Add a post-delay “live inventory” availability guard in reserveStock to prevent overselling after a stale pre-check.
• Return reservation tokens/snapshot versions based on the committed record version rather than the pre-check snapshot.
• Update the concurrent checkout scenario to use Promise.allSettled and only fail on non-OutOfStock errors.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
demo_app/src/inventory/reserve-stock.ts	Adds a live inventory check after the async delay and aligns returned reservation metadata to the committed record state.
demo_app/src/checkout/submit-order.ts	Treats OutOfStock as an expected concurrent outcome via allSettled, while still surfacing unexpected failures.

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[Copilot]

The inline comment says the delay is an “Intentional bug” that makes the pre-check stale, but the new post-delay record.available guard now prevents the stale-check oversell. Please update/remove this comment so it reflects the current behavior (e.g., that the delay simulates concurrency and the second check is the actual safety guard).

[Copilot]

This treats OutOfStock as a non-fatal error by matching on error.message.startsWith("OutOfStock:"), which is brittle (any message change or non-Error throw will change behavior). Prefer a typed signal (e.g., an exported OutOfStockError class, error.name === "OutOfStockError", or a small isOutOfStockError() helper co-located with reserveStock) so the concurrent settlement logic isn’t coupled to a string prefix.