Fix chunk delete deadlock ordering - #2570
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oldnicke
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July 5, 2026 13:09
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#3387) (#3393) * fix(retain): match chunk-delete link endpoints through indexable joins (#3387) The ordered memory_links pre-delete in delete_chunks_by_ids matched link endpoints with `tu.id = ml.from_unit_id OR tu.id = ml.to_unit_id`. An OR spanning two columns of ml cannot be driven from either endpoint index, so PostgreSQL made memory_links the outer relation of a nested-loop semi join and sequentially scanned the whole table on every delete — O(links x units), with no bank_id predicate, so every bank scanned every other bank's rows. Past a few million links that exceeded the asyncpg command timeout and delta retain failed with a bare TimeoutError (str(TimeoutError()) is empty, so it logged as "Task execution failed: batch_retain, error:" with nothing after). Splitting the OR into a UNION of two single-column joins makes each half an index scan. Measured on PG18 with the current index set, 300k units / 1.5M links, deleting the links of 3 chunks (150 units): before 20,420 ms Seq Scan, 224.9M rows removed by join filter after 31 ms two index scans, same 1,464 rows 10 chunks took 49.8s before — already over the 60s default at a table size well below production. The row set is identical (EXCEPT in both directions returns nothing), and the deterministic ORDER BY + FOR UPDATE that #2570 added stay in ordered_links, which still locks the rows in that order. * test(memories): update store doubles for the per-bank capability API #3388 moved the store capability to writes_memory_rows_in_sql_for(bank_id) and added drop_bank_storage, but two duck-typed test doubles still carried the old surface, so test-api shard 2 has been red on main since it merged: test_integrity_violation_not_retried — SimpleNamespace(writes_memory_rows_in_sql=True) -> AttributeError: no attribute 'writes_memory_rows_in_sql_for' test_list_banks_non_sql_store._NonSqlStore — teardown's delete_bank routes the drop through the store for a non-SQL bank -> AttributeError: no attribute 'drop_bank_storage' Both doubles are hand-rolled rather than subclasses of the store base, which is why the refactor could not update them mechanically.
nicoloboschi
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) (#3406) The deadlock #2570 targeted still fired a few times a day because the insert-side lock ordering was only partial: 1. _bulk_insert_links sorted on (from, to) — two of the four columns in the unique index (from, to, link_type, COALESCE(entity_id, nil)). A temporal and a semantic edge on the same pair compared equal, so a stable sort left them in input order and concurrent inserts could take the two index entries in opposite orders. 2. That order also disagreed with chunk_storage.delete_chunks_by_ids, which normalises direction via LEAST/GREATEST. Two different total orders can still cycle. Sort both paths on one canonical key — the full, direction-normalised unique key — by extracting _lock_order_key and pointing the insert sort at it. The delete side already uses exactly this order and is unchanged.
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Summary
Why
Concurrent
DELETE FROM chunks ...statements can let PostgreSQL's FK cascade lock overlappingmemory_linksrows in executor-chosen order. When two writers delete overlapping chunk graphs, that can produce AB/BA row-lock deadlocks. Explicitly deleting the affected links in a stable order removes the unordered cascade as the first lock owner.Fixes #2560.
Supersedes #2569, which was closed when the fork branch was renamed.
Validation
uv run --project hindsight-api-slim pytest -q hindsight-api-slim/tests/test_chunk_storage_delete_ordering.pyuv run --project hindsight-api-slim ruff format --check hindsight-api-slim/hindsight_api/engine/retain/chunk_storage.py hindsight-api-slim/tests/test_chunk_storage_delete_ordering.pyuv run --project hindsight-api-slim ruff check hindsight-api-slim/hindsight_api/engine/retain/chunk_storage.py hindsight-api-slim/tests/test_chunk_storage_delete_ordering.py