Two For One Email Debugging Lab

Two For One EmailDatabases & ConsistencyMediumPreparing workspaceAutosaved

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Incident Dossier

Two For One Email

Two users race to register the same email address at the same moment. Both receive success responses and the database ends up with two accounts for the same email.

Initial failing signalduplicate inserts for the same emailExpected exactly 1 success · received 2 successes, store size 2

Progress

1Reproduce the bugRun checks

2Edit the fixOpen

3Pass checksOpen

4Submit fixOpen

Reproduction Steps

Run npm test.
Trace createUser: notice the exists-check runs, then an await, then the insert.
Two callers pass the check before either inserts, so both fall through.
Make the check and the insert atomic with respect to other registrations.

Focus Mode

Start

src/users.js

Likely area

src/users.js

Progressive hints

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

TopicDatabases & Consistency

DifficultyMedium

Est. time18 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...