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S01 · The Bootsector · L-04

Crawler Race Fix

Difficulty
easy
Type
debugging
Session
Standard
Stack
Rust rust1.75
Domain
Concurrency
Est. solve
~25 min

01 · The task

An async web crawler can't keep a straight story about which URLs it has already handled — some get visited twice, others slip through. Track down the concurrency flaw behind the inconsistency and make its bookkeeping hold up when tasks run in parallel. The bug reproduces the same way on every run, not as a flake.

02 · Constraints

  • Seeded scheduler

    A seeded scheduler reproduces the concurrency bug deterministically.

  • Deterministic grading

    Graded by replaying fixed inputs; behavior must be reproducible.

03 · Skills exercised

data-racesTwo threads touching the same memory without synchronization — a correctness bug.deterministic-concurrencyReproducing concurrency bugs reliably via a seeded scheduler.synchronizationCoordinating threads so shared state stays consistent.

How to playDaemon

The whole level runs through the local promptly daemon — it pulls the workspace, captures your token usage, tests locally, and submits the ranked run.

Full setup guide →Manage devices

  1. Pair this machine

    One-time — approve the code under your devices.

    promptly pair

  2. Pull the workspace

    Downloads the starter kit into crawler/ and starts the solve clock — cd in before the next step.

    promptly init crawler

  3. Start the captured session

    Then drive your AI tool — the daemon meters every prompt and token live.

    promptly start

  4. Submit for ranked grading

    Runs the full hidden suite and ends the attempt.

    promptly submit

Or combine steps 02–03 into one commandShortcut

promptly play pulls the workspace, launches the daemon, and starts the captured session in a single step — the same as running init then start. Then cd crawler and drive your harness.

promptly play crawler