fix: lock knowledge_queue rows with SELECT ... FOR UPDATE SKIP LOCKED to prevent duplicate consumption (#1025) - #1031
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…consumption DbKnowledgeSource read unconsumed knowledge_queue rows with a plain SELECT and no row lock. queue_runner.run_librarian_queue holds one open transaction across the read, the full retrieval/rerank pipeline, and the write-back, committing only at the end. Two concurrent runs could therefore both read the same unconsumed rows, both pay for the expensive pipeline work, and both persist a decision envelope for the same chunk before either reached mark_consumed. Add SELECT ... FOR UPDATE SKIP LOCKED to DbKnowledgeSource._query(), matching the with_for_update() pattern already used in db.py:set_user_resource_selection. A second concurrent reader now excludes rows the first is holding instead of blocking on them or re-reading them. Postgres-only in effect (a no-op on SQLite), so existing SQLite-backed tests are unaffected. Fixes OWASP#1025
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Walkthrough
ChangesKnowledge queue locking
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: 🟡 Moderate · up to The PR prevents duplicate queue consumption, but the current head has an indentation error that prevents the affected tests from importing, and failed queue runs may retain row locks without an explicit rollback path, temporarily hiding work from other workers. Fix the test syntax and explicitly address transaction cleanup before merging. Suggested reviewers: 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
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Full details: Linked Issues checkExplanation The implementation satisfies issue Full details: Out of Scope Changes checkExplanation The changes are within scope for issue
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Inline comments:
In `@application/tests/librarian/knowledge_source_test.py`:
- Around line 209-218: Harden the test around worker2 and its transaction:
capture any exception raised by worker2, assert the thread has completed after
join rather than inferring completion from worker2_ids, and always call
sqla.session.rollback() in a finally block so worker1’s locks are released when
assertions fail.
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Addresses CodeRabbit review comment: t.join(timeout=5) alone doesn't confirm the thread finished -- if SKIP LOCKED failed and worker2 blocked instead, the test would still pass silently. Now asserts the thread is not alive, captures/asserts no exceptions from the worker, and moves the rollback into a finally block so a failed assertion can't leak a held lock into the next test.
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Inline comments:
In `@application/tests/librarian/knowledge_source_test.py`:
- Around line 214-231: Update the SKIP LOCKED test’s finally block to roll back
worker 1’s transaction first, then join worker2 without the timeout so any
blocked worker completes before teardown. Preserve the existing timeout
assertion and ensure cleanup waits for the worker2 thread after lock release.
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git add application/tests/librarian/knowledge_source_test.py git commit -m "test: rejoin worker2 after rollback on the assertion-failure path Addresses CodeRabbit review comment: if the t.is_alive() assertion fails (worker2 genuinely blocked), the finally block's rollback releases worker 1's locks and lets worker2 resume -- but tearDown() runs right after and drops the tables, racing worker2's still-running query. Join the thread again after rollback so it is guaranteed to finish before teardown." git push origin fix/1025-knowledge-queue-row-locking Signed-off-by: prajakta G kamble <prajuu2812@gmail.com>
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Inline comments:
In `@application/tests/librarian/knowledge_source_test.py`:
- Line 190: Align the sqla.session.add_all call with the surrounding method body
and the adjacent sqla.session.commit() statement so it has no unexpected extra
indentation.
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application/tests/librarian/knowledge_source_test.py
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| if "postgresql" not in str(sqla.engine.url): | ||
| self.skipTest("row-lock serialization requires Postgres (SKIP LOCKED)") | ||
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| sqla.session.add_all([_row("a"), _row("b")]) |
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🎯 Functional Correctness | 🔴 Critical | ⚡ Quick win
Fix the unexpected indentation.
Line 190 is indented more deeply than the surrounding method body. No nested block requires this indentation, so Python raises IndentationError: unexpected indent when it imports the test module. Align this call with sqla.session.commit().
Proposed fix
- sqla.session.add_all([_row("a"), _row("b")])
+ sqla.session.add_all([_row("a"), _row("b")])📝 Committable suggestion
‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.
| sqla.session.add_all([_row("a"), _row("b")]) | |
| sqla.session.add_all([_row("a"), _row("b")]) |
🧰 Tools
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[warning] 190-190: Unexpected indentation
(invalid-syntax)
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In `@application/tests/librarian/knowledge_source_test.py` at line 190, Align the
sqla.session.add_all call with the surrounding method body and the adjacent
sqla.session.commit() statement so it has no unexpected extra indentation.
Source: Linters/SAST tools
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Problem
DbKnowledgeSource(Module C's live queue reader) selects unconsumedknowledge_queuerows with a plain, unlockedSELECT.queue_runner.run_librarian_queuekeeps one transaction open across that read, the full retrieval/rerank pipeline, and the write-back — it commits only once, at the very end, aftermark_consumed.If two runs execute concurrently (an orchestrator retry overlapping a scheduled pass, or two workers), both read the same unconsumed rows, both pay for the expensive retrieval/rerank work on them, and both call
sink.write()— persisting two decision envelopes for the same chunk — before either reachesmark_consumed. Theconsumed_at IS NULLfilter inmark_consumedonly protects thetimestamp column from being written twice; it does nothing to prevent the duplicate work or the duplicate writes that already happened upstream.
Solution
Add
SELECT ... FOR UPDATE SKIP LOCKEDtoDbKnowledgeSource._query().This is the standard Postgres job-queue locking pattern, and it mirrors a locking approach already used elsewhere in this codebase (db.py:set_user_resource_selection'swith_for_update()) — the difference here is that a second reader should skip rows the first is holding rather than block waiting for them.FOR UPDATE/SKIP LOCKEDcompiles to a no-op on SQLite, so local/CI runs (SQLite-backed) are unaffected.queue_runneralready uses so a row claimed by one run cannot be claimed by another until that run commits or rolls back.Changes
application/utils/librarian/knowledge_source.pyDbKnowledgeSource._query()now claims its batch with.with_for_update(skip_locked=True). Class docstring updated with a "Concurrency" section documenting the guarantee and why the lock is held for the run's full duration.application/tests/librarian/knowledge_source_test.pyAddedtest_concurrent_readers_skip_locked_rows, which opens one worker's read (leaving its transaction open, mirroringqueue_runner's shape) and asserts that a second, concurrent worker sees neither row. Postgres-gated (skipTeston SQLite), matching the existing convention used byuser_model_test's row-lock test.Testing
python -m unittest application.tests.librarian.knowledge_source_test -v→ 10 passed, 1 skipped (the new concurrency test; SKIP LOCKED needs a real Postgres backend to exercise, so it correctly skips on the SQLite dev/CI database, same as the project's other row-lock test).with_for_update(skip_locked=True)compiles to a silent no-op against SQLite (noCompileError), confirming no other test in the suite is affected by this change.git diffreviewed line-by-line before commit.Notes for reviewers
This intentionally does not restructure
queue_runner's claim → process → commit shape into a shorter claim-then-release pattern — that would change the transaction boundary and felt like a separate discussion. Holding the lock for the run's duration is the smallest change that actually closes the race described in #1025; happy to discuss trade-offs if a shorter claim window is preferred.Fixes #1025