tasks/DIST1_queue_race/spec.md

DIST1: Message Queue — Race Condition Repair

Goal

Fix 3 race conditions in the message queue that cause message loss, duplication, and crashes under concurrent load.

Requirements

1. put() must atomically check capacity AND add the message (no TOCTOU race)
2. get() must use an acknowledgment pattern — messages must not be removed until the consumer explicitly acknowledges successful processing
3. The priority queue must use a deterministic, type-safe tie-breaker for equal-priority messages (comparing message payloads is not safe — payloads can be any type)
4. All tests must pass: pytest tests/
5. The queue must handle 10,000 messages across 20 concurrent threads with zero loss

Supporting Documents

• mqueue/queue.py — Main queue implementation (bugs 1 and 2)
• mqueue/priority.py — Priority queue comparator (bug 3)
• mqueue/consumer.py — Consumer interface (needs ack/nack support)
• tests/test_concurrent.py — Concurrent correctness tests (fail with bugs)
• tests/test_message_loss.py — Zero-loss guarantee test

Important

The single-threaded tests (test_single_thread.py) pass even with the bugs. The concurrent tests expose the race conditions. The acknowledgment pattern means get() should return a message AND a receipt/token, and the message is only confirmed-delivered when ack(receipt) is called.