The Wallet That Let It Slide Debugging Lab

The Wallet That Let It SlideDatabases & ConsistencyEasyPreparing workspaceAutosaved

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Incident Dossier

The Wallet That Let It Slide

A user has 100 units in their wallet. Two concurrent transfers of 60 units each both report success and the balance drops to -20. The check and the debit are not atomic, so each transfer observes the same pre-debit balance.

Initial failing signaltwo concurrent transfers overdraw the sourceExpected exactly 1 success, balance == 40 · received 2 successes, balance == -20

Progress

1Reproduce the bugRun checks

2Edit the fixOpen

3Pass checksOpen

4Submit fixOpen

Reproduction Steps

Run npm test.
Inspect the transfer function: the balance read, the check, and the debit are split across an await.
Two callers can read the same balance, both pass the check, and both debit.
Serialize per-account access so the check and the debit cannot interleave.

Focus Mode

Start

src/wallet.js

Likely area

src/wallet.js

Progressive hints

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

TopicDatabases & Consistency

DifficultyEasy

Est. time14 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...