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 checkout-race-condition incident in the demo checkout-api flow by preventing overselling during concurrent reservations and by making the concurrent repro scenario tolerate expected “out of stock” outcomes.

Changes:

• Add a commit-time inventory availability check in reserveStock to prevent concurrent oversell.
• Return reservation metadata based on the committed inventory record version (not the pre-check snapshot).
• Update the concurrent checkout scenario to use Promise.allSettled and only fail the scenario for 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 post-delay guard against stale snapshot checks and returns committed snapshot version/token.
demo_app/src/checkout/submit-order.ts	Treats OutOfStock as an expected concurrent outcome while still surfacing unexpected failures.

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[Copilot]

The concurrent-staleness comment immediately above this delay reads like the race is still an intentional bug, but the new post-delay availability guard changes that story. Please update the comment to describe the current behavior (snapshot pre-check can be stale, but the commit-time record check prevents overselling) so future readers don't assume the function is still intentionally incorrect.

[Copilot]

Using error.message.startsWith("OutOfStock:") to classify expected rejections is brittle (it couples control flow to a string that can change and doesn’t work for non-Error rejections). Prefer a structured signal (e.g., a dedicated OutOfStockError class, error.name, or an error code field) so this filtering remains stable as messages evolve.