fix: lock ordering & cleanup (also contains atomic checkpoints) #38
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Add advisory locks to serialize concurrent await_event and emit_event operations on the same event. This prevents a race condition where: 1. Task A checks if event exists (not yet) 2. Task B emits the event and wakes waiters (none yet) 3. Task A registers as a waiter (missed the wake) The lock_event() function uses pg_advisory_xact_lock with hashed queue_name and event_name to ensure atomicity. Also changes emit_event to first-writer-wins semantics (ON CONFLICT DO NOTHING) to maintain consistency - subsequent emits for the same event are no-ops. Tests: - test_event_functions_use_advisory_locks: Verifies both functions call lock_event - test_event_race_stress: Stress test with 128 concurrent tasks x 4 rounds - test_event_first_writer_wins: Renamed from test_event_last_write_wins
Problem 1. Deadlock risk: Functions that touch both tasks and runs could deadlock if they acquired locks in inconsistent order 2. Scattered cleanup logic: Terminal task cleanup (deleting waiters, emitting parent events, cascading cancellation) was duplicated across multiple functions 3. Incomplete cascade cancellation: Auto-cancelled tasks (via max_duration) didn't cascade cancel their children Solution Lock Ordering: All functions now acquire locks in consistent order (task first, then run): - complete_run, fail_run, sleep_for, await_event: lock task FOR UPDATE before locking run - claim_task: lock task before run when handling expired claims - emit_event: lock sleeping tasks before waking runs (via locked_tasks CTE) Cleanup Consolidation: New cleanup_task_terminal() function handles: - Deleting wait registrations for the task - Emitting completion event for parent ($child:<task_id>) - Optionally cascading cancellation to children Used by: complete_run, fail_run, cancel_task, cascade_cancel_children, claim_task Other improvements: - emit_event early return when event already exists (optimization) - sleep_for now takes task_id parameter for proper lock ordering Tests - New lock_order_test.rs with 6 tests verifying lock ordering works correctly - New test_cascade_cancel_when_parent_auto_cancelled_by_max_duration in fanout_test.rs - New test helpers: single_conn_pool(), RunInfo, get_runs_for_task()
virajmehta
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GabrielBianconi
approved these changes
Dec 24, 2025
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Problem
Solution
Lock Ordering: All functions now acquire locks in consistent order (task first, then run):
Cleanup Consolidation: New cleanup_task_terminal() function handles:
Used by: complete_run, fail_run, cancel_task, cascade_cancel_children, claim_task
Other improvements:
Tests
This PR also contains #39