fix(redis): stop lock waiters pinning executor threads (LAB-3596) - #290
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PerRequestRedisBackend.acquire_lock ran redis-py's blocking Lock.acquire inside asyncio.to_thread, so every waiter held an executor thread for up to blocking_timeout. The default executor has min(32, cpu_count + 4) threads - 8 on a 4-vCPU GitHub-hosted runner - so ten concurrent misses on one cold key pinned all of them; the holder's own get/set/release (also to_thread calls) queued behind the waiters, every waiter timed out at 5 s and recomputed. test_concurrent_access in tests/integration/test_redis_integration.py failed on 4 of 5 matrix versions with 9 unique results and 8 lock-timeout warnings. Each acquisition attempt is now one non-blocking SET NX round-trip via to_thread, paced by asyncio.sleep on the event loop, with the same give-up rule as redis-py's own Lock.acquire. No executor thread is held across a wait. Regression test (runs on pull requests): tests/unit/backends/test_redis_backend.py::TestRedisLockWaitersDoNotPinExecutorThreads pins the default executor at 2 threads and runs 4 contenders - red on the old implementation, green on this one.
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Included review availability: 0 reviews are currently available. Your included PR review attempts over the past 7 days set your current allowance at 2 reviews per hour. WalkthroughRedis lock acquisition now retries non-blocking attempts and sleeps on the event loop. Tests verify executor usage, waiter deadlines, single-attempt acquisition, and token compatibility. ChangesRedis lock polling
Priority: ⬇️ Low Estimated code review effort: 3 (Moderate) | ~25 minutes Change: Bug fix Sequence Diagram(s)sequenceDiagram
participant acquire_lock
participant Executor
participant Redis
acquire_lock->>Executor: schedule non-blocking SET NX
Executor->>Redis: attempt lock acquisition
Redis-->>Executor: acquired or unavailable
acquire_lock->>acquire_lock: await asyncio.sleep(0.1) when unavailable
Merge Risk: 🟡 Moderate · up to A cancelled lock waiter can temporarily leave the Redis lock held, blocking other contenders until it expires. Address cancellation cleanup before merging. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
Full details: Docstring CoverageExplanation Docstring coverage is 29.41% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 17 functions across 3 files. (1 skipped: 1 unsupported.)
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Inline comments:
In `@docs/features/distributed-locking.md`:
- Line 261: Update the distributed-locking documentation statement about
executor usage to clarify that a waiter uses an executor thread only during each
SET NX round trip, while asyncio.sleep does not occupy one between attempts.
In `@src/cachekit/backends/redis/provider.py`:
- Line 388: Update the lock acquisition flow around lock.acquire to run the
acquisition in a task and await it through asyncio.shield; when cancellation
occurs, wait for the acquisition task to finish, release the Redis lock if it
succeeded, then re-raise CancelledError. Add a test covering cancellation while
acquisition is in progress and verifying that a later successful acquisition is
released.
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docs/features/distributed-locking.mdsrc/cachekit/backends/redis/provider.pytests/unit/backends/test_redis_backend.pytests/unit/test_wrapper_lock_bare_key.py
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…3596) - assert a waiter yields False on its own deadline while the holder still holds, retries before giving up, and never attempts past blocking_timeout - assert blocking_timeout=None is a single SET NX - reset only Lock.lua_release between tests; extend/reacquire are unused here - docs: a waiter holds no executor thread while waiting between attempts; each SET NX round-trip still uses one briefly - neutral wording for the executor-size example in both docstrings
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Summary
Fixes a distributed-lock design flaw where lock waiters could exhaust the asyncio default executor thread pool, causing the exact cache stampede the locking feature is meant to prevent (LAB-3596).
Problem
The Redis
acquire_lockimplementation ran redis-py's blockingLock.acquireinsideasyncio.to_thread(). Each waiter would pin one executor thread for the entireblocking_timeoutduration while polling. The default executor only hasmin(32, cpu_count + 4)threads (8 on a 4-vCPU host), so once concurrent misses on a single key exceeded that count:get/set/releaseoperations (alsoto_threadcalls) queued behind the waitersChanges
src/cachekit/backends/redis/provider.pyLock.acquirecall with a polling loop of non-blockingSET NXattempts (blocking=False).asyncio.sleepon the event loop instead of blocking inside an executor thread, so a waiter never holds a thread while waiting.uuid-based token for the whole acquisition (across all attempts) so release still correctly matches the holder.blocking_timeout=Nonemeans a single attempt.to_threadround-trip only lasts as long as one Redis call, not the full timeout.Tests
TestRedisLockWaitersDoNotPinExecutorThreads, which pins the executor to 2 threads and runs 4 contenders. This test is red on the old blocking implementation (two waiters starve and time out) and green with the event-loop-based wait._FakeRedisimplementingSET NX PXand the release script for isolated testing.acquiresignature intest_wrapper_lock_bare_key.pyto accept the newtokenargument.Docs
distributed-locking.mdthat failed acquisitions retry every 0.1s viaasyncio.sleep, so a waiter never holds an executor thread.Summary
This PR fixes a thread starvation issue in the Redis distributed locking implementation where lock waiters could pin executor threads, potentially causing a cache stampede — the very problem distributed locking is meant to prevent.
Problem
The previous implementation ran redis-py's blocking
Lock.acquireinsideasyncio.to_thread. When the number of concurrent misses on a single key exceeded the default executor's thread pool size (min(32, cpu_count + 4), e.g. 8 on a 4-vCPU host), each waiter would occupy one executor thread for the entireblocking_timeoutduration. This caused the lock holder's ownget/set/releaseoperations (alsoto_threadcalls) to queue behind the waiters, resulting in every waiter timing out and independently recomputing the cached value — a stampede.Changes
provider.py: Documentation/comment clarification around the executor thread starvation behavior (the fix ensures waits happen viaasyncio.sleepon the event loop between non-blockingSET NXattempts, so a waiter never holds an executor thread while waiting).distributed-locking.md: Clarified the docs describing that waiters sleep on the event loop between retry attempts rather than holding an executor thread.tests/unit/backends/test_redis_backend.py: Added test coverage to verify the corrected behavior:_FakeRedisto record the monotonic timestamp of everySET NX(acquire attempt).Falseon its own deadline (not waiting for the holder's release), makes multiple retry attempts, and never issues an attempt pastblocking_timeout.blocking_timeout=None) makes exactly oneSET NXattempt.Impact
Lock waiters now wait on the event loop between non-blocking acquire attempts instead of blocking executor threads, preventing thread pool exhaustion and the resulting recompute stampede under high contention on a single key.
Summary by CodeRabbit
Bug Fixes
Documentation
Tests