Async dedupe race Debugging Lab

Async dedupe raceEvents & ConcurrencyHardPreparing workspaceAutosaved

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Incident Dossier

Async dedupe race

When the payments endpoint slows down, the provider fires retries within milliseconds of the original delivery. Two retries of the same event id arrive in the same event-loop tick and both pass the dedupe check before either of them marks the event as seen, so the side effect runs twice.

Initial failing signalConcurrent retries run the side effect twiceExpected side effect runs exactly once across concurrent retries · received side effect runs twice

Progress

1Reproduce the bugRun checks

2Edit the fixOpen

3Pass checksOpen

4Submit fixOpen

Reproduction Steps

Run npm test.
Notice the dedupe set is only mutated after the await in ingest.
Imagine two retries hitting the same handler on the same event loop tick.
Move the set mutation to before the await so the second concurrent caller is blocked at the check.

Focus Mode

Start

src/race.js

Likely area

src/race.js

Progressive hints

Hints reveal broad direction first, then narrow toward the suspect path without showing hidden tests or exact solution code.

TopicEvents & Concurrency

DifficultyHard

Est. time26 min

Workspacecreating

Explorer

Preparing workspace files…

Evidence Map

Failing signal loaded from challenge API
Workspace files loaded from workspace API
Results rendered only after API run/submit

Autosaved

workspace

Outputread-onlyready$ workspace boot

Workspace booting...