fix(swr): release refresh slots stranded on stopped event loops (LAB-7295) by 27Bslash6 · Pull Request #488 · cachekit-io/cachekit-py

27Bslash6 · 2026-10-03T21:15:17Z

An async SWR refresh left pending on an event loop that stopped (sync code calling loop.run_until_complete, or closing the loop without cancelling its tasks) held its refresh slot and its key for good. Once all 32 slots were held that way, stale hits from other loops stopped refreshing until the hard TTL. Both SWR pools (L1-only and backed-mode L2) now apply the rule the TTL refresh scheduler already uses: such a refresh gives up its slot and key 30 s after admission and is cancelled.

Closes LAB-7295

Change

New _RefreshPool in decorators/wrapper.py, used by _l1_swr_pool and _l2_swr_pool. It replaces each pool's BoundedSemaphore, its task set, and (for L2) the in-flight key set and its fork reset.
- The cap is hard on threads and running loops, counted under one lock. It is best-effort on stopped loops. A task whose loop is not running keeps its slot and key for _SWR_STOPPED_LOOP_HOLD_SECONDS (30 s, the same as _TTL_REFRESH_STOPPED_LOOP_HOLD_SECONDS) from admission. The next acquire then drops it, releases its key, and cancels it on its own loop with call_soon_threadsafe (a RuntimeError from a closed loop is suppressed). A task that finished without releasing (cancelled before its first step) is dropped at once.
- Release checks ownership. release(slot) returns whether the slot was still held. A pruned task that runs again gets False, so it gives back nothing and settles no key state. That matters on L1: a refresh's version is the entry generation, and a newer refresh of the same entry shares it, so a late fail_refresh would clear the newer refresh's marker.
- The pool holds the bound task refs that _l1_swr_tasks and _l2_swr_tasks used to hold.
L1: _l1_swr_acquire passes cancel_refresh(key, version) as the prune hook. _l1_swr_refresh_async calls fail_refresh / complete_refresh only while it still holds its slot.
L2: the pool's holder carries the cache key, so the in-flight dedup runs under the pool lock, after the prune. A stale hit on the stranded key itself recovers it, and the old cross-thread check-then-add race is gone.
L2 store: cancelling a pruned revalidation stops it only if the decorated function lets CancelledError through. One that swallows the cancellation and returns would otherwise store its late result over a newer revalidation's value. _l2_swr_recompute_store_async therefore checks _l2_swr_pool.holds(slot) after the recompute and before the store. Past that check only cachekit code runs, which lets a later cancellation through. A write already sent when its loop stops is not recalled.
Sync (daemon-thread) refreshes are never pruned. They take and give back a slot exactly as before.

On L1, the per-key check runs in ObjectCache.get_with_swr before any acquire. A stranded L1 key whose only traffic is its own hits therefore recovers at the next refresh attempt on another key of that function, or at its hard TTL as today. Pruning on the hit path would put a lock on every hit for that rare case.

Tests

TestL1OnlySWRStoppedLoop (tests/unit/test_l1_only_swr.py) and TestAsyncSWRStoppedLoop (tests/unit/test_swr_decorator.py), each for loops left stopped and loops closed without cancelling:
- With the cap at 1 and the hold long, a stale hit on key 1 starts 0 refreshes. With the hold expired it starts 1. A stale hit on the stranded key 0 then starts one too, which shows the prune released key 0. The closed-loop cases also collect the dropped task and check that closing its coroutine raises nothing.
- With the cap at 2 and three loops paused mid-refresh, resuming them runs 2 refreshes, not 6. This mirrors test_refresh_pruned_from_stopped_loop_does_not_resume.
- A pruned refresh that resumes ends cancelled and frees nothing. A newer refresh keeps the only slot and the key. On L1 the resumed task calls neither fail_refresh nor complete_refresh.
- test_pruned_revalidation_does_not_store_its_late_result: a pruned L2 revalidation whose function swallows the cancellation returns its late value but stores nothing, and the newer value stays. It fails without the holds check: three stores instead of two.
On main's scheduler, 6 of the first 8 new tests fail. The two resume tests pass trivially there, because main never prunes; they guard the cancel. With the cancel removed, both resume tests and both ownership tests fail. With release always claiming ownership, the L1 ownership test fails.
The existing sync-path tests pass unchanged: test_async_refreshes_capped_at_32_distinct_stale_keys, test_slot_exhaustion_skips_revalidation, test_thread_start_failure_releases_slot and test_forked_child_revalidates_key_stuck_inflight_in_parent. Only the TestSWRForkIsolation class docstring was reworded, from "semaphore" to "pool".
Local results: ruff check, ruff format --check and basedpyright src/ are clean. tests/unit (4362 passed), tests/critical (228 passed), the src doctests, tests/docs and --markdown-docs README.md docs/ all pass. The new tests passed 20 of 20 repeated runs.

Docs

docs/configuration.md: the "Stale-While-Revalidate (stale_ttl)" rules and the L1-only SWR bullets now state the 32-per-function cap and what happens to a refresh stranded on a stopped loop. They use the same terms as docs/backends/cachekitio.md's TTL refresh paragraph.
docs/free-threading.md: the SWR row of the concurrency audit now describes the pool lock in place of the semaphore and set.

Type of Change

Bug fix (non-breaking)

…7295) The L1-only and L2 async SWR schedulers gave a refresh slot back only when the refresh task finished. A task left pending on a loop that stopped without closing (sync code calling loop.run_until_complete) never finishes, so it held its slot and its key for good; L2 also leaked on a loop closed without cancelling its tasks, because the scheduler held the task strongly. Once every slot was held this way, stale hits from other loops stopped refreshing until the hard TTL. Both pools now use _RefreshPool, which applies the TTL refresh scheduler's rule: the cap is hard on threads and running loops, and best-effort on stopped ones. A task whose loop is not running keeps its slot and key for 30 s from admission; the next acquire then releases both (L1: cancel_refresh on the entry; L2: the key is the holder's) and cancels the task on its own loop. The pool holds the task refs that the two task sets held before. Release checks ownership: a pruned task that runs again gets False and touches neither the slot nor the key, because an L1 refresh's version is the entry generation, which a newer refresh of the same entry shares. L2's per-key in-flight check now runs under the pool lock, so its check-then-add race is gone.

coderabbitai · 2026-10-03T21:15:26Z

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Summary by CodeRabbit

Bug Fixes
- Stalled asynchronous cache refreshes are cancelled after 30 seconds, helping prevent them from blocking later refreshes indefinitely.
- Older refreshes can no longer overwrite results from a newer refresh after resuming.
- L1-only cached values remain available while a refresh is stalled; after the cache expires, the next request recomputes the value.
Documentation
- Documented a limit of 32 concurrent refreshes per decorated function, along with the behaviour of stalled refreshes.

Walkthrough

SWR refresh tracking now uses per-function pools with bounded slots and pruning for tasks stranded on stopped event loops. CachekitIO and L1-only refreshes check slot ownership before updating cache state. Documentation and tests cover limits, pruning, cancellation, and late task results.

Changes

SWR refresh lifecycle

Layer / File(s)	Summary
Refresh pool lifecycle `src/cachekit/decorators/wrapper.py`, `docs/free-threading.md`	`_RefreshPool` tracks slots under a lock and prunes completed tasks or tasks on stopped loops after a hold period. Pruned tasks are cancelled on their own loops.
CachekitIO SWR integration `src/cachekit/decorators/wrapper.py`, `tests/unit/test_swr_decorator.py`, `docs/configuration.md`	CachekitIO SWR uses keyed pool slots for revalidations and checks slot ownership before storing async results. Tests cover stopped and closed loops, pruning, replacement refreshes, and late results. Documentation describes the limit and stopped-loop behaviour.
L1-only SWR integration `src/cachekit/decorators/wrapper.py`, `tests/unit/test_l1_only_swr.py`, `docs/configuration.md`	L1-only SWR links pool slots to `ObjectCache` refresh cancellation and updates cache state only while a task holds its slot. Tests cover pruning and refresh-state protection. Documentation describes the limit and stopped-loop behaviour.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant SWR_decorator
  participant RefreshPool
  participant Event_loop
  participant Revalidation_task
  participant Cache
  SWR_decorator->>RefreshPool: acquire keyed slot
  SWR_decorator->>Revalidation_task: schedule task with slot
  RefreshPool->>Event_loop: schedule cancellation after stopped-loop hold
  Event_loop->>Revalidation_task: deliver cancellation when loop runs
  Revalidation_task->>RefreshPool: check slot ownership
  Revalidation_task->>Cache: store result only while slot is held

Merge Risk: 🔵 Low · up to 1ab81

The change limits SWR refreshes per function and releases refreshes stuck on stopped event loops. No production defect was found. One test file depends on helpers defined in another test file, so moving those helpers into a shared module would make the test suite easier to maintain.

Security Architecture Review

Security architecture risk: 🔵 Low · up to 1ab81

The change improves recovery from stranded refreshes and protects newer local refresh state. A bounded consistency risk remains: an external cache write already dispatched by an abandoned refresh can finish after a newer write. No new attacker-facing entrypoint or verified security vulnerability was established.

Retained concerns

Low · reliability · inferred: Stopped-loop pruning releases an L2 key before an already-dispatched backend write necessarily terminates. A newer refresh can therefore be admitted while the abandoned write can still overwrite its result. This is a documented recovery trade-off affecting cache-state integrity and failure containment, not an established authorization bypass or cross-tenant vulnerability.

Security review details

Security Blast Radius

inferred — Capacity pressure is shared across keys using a decorated function's pool. The late-write outcome concerns the same backend cache key; the examined recovery path does not establish cross-tenant access, increased credentials, or broader service authority.

Security Findings and Attack Paths

inferred — The supported adverse sequence is an old backend write remaining in flight, its loop stopping, another admission pruning its holder, and a newer refresh writing first. No request-controlled mechanism for stopping the loop or security-sensitive consumer of the resulting cache value was established, so this is a consistency concern rather than a verified exploit.

Trust Boundaries and Controls

observed — Slot identity prevents pruned L1 async work from settling another refresh's marker. Generation checks further protect replaced or evicted entries. L2's ownership check blocks late recomputation results before dispatch, while the distributed lease remains time-limited and separate from the write operation.

Resilience and Maintainability Implications

observed — Stranded tasks no longer permanently consume admission capacity once a later admission prunes them. Recovery deliberately trades strict termination of abandoned work for continued refresh availability; already-dispatched writes remain outside local cancellation guarantees.

Hardening Proposals

proposed — If cached values govern security-sensitive decisions requiring monotonic freshness, consider backend-enforced generation or fencing checks and an interrupted-write ordering test before relying on local slot ownership as a consistency guarantee.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 52.17% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 46 functions across 3 files. (2 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly identifies the main change: releasing SWR refresh slots stranded on stopped event loops.
Description check	✅ Passed	The description explains the problem, the change, its motivation, testing, documentation updates and non-breaking bug-fix type. It does not use the template’s Description and Motivation headings or in…

Full details: Docstring Coverage

Explanation

Docstring coverage is 52.17% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 46 functions across 3 files. (2 skipped: 2 unsupported.)

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### [1/2] src/cachekit/decorators/wrapper.py:1484
Issue identified during code review:
Overbroad exception handler in src/cachekit/decorators/wrapper.py: `except BaseException:` catches `KeyboardInterrupt`, `SystemExit`, and `asyncio.CancelledError` along with the errors the wrapper expects. When a task running the wrapped function is cancelled or the process receives Ctrl+C, this handler intercepts the signal instead of letting it propagate. Fix: catch only the expected exceptions (e.g., `KeyError`, `ValueError`, `TimeoutError`) and let all others bubble up.

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codecov · 2026-10-03T21:19:28Z

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❌ Patch coverage is 89.33333% with 8 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/cachekit/decorators/wrapper.py	89.33%	3 Missing and 5 partials ⚠️

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…B-7295) Cancelling a revalidation pruned from a stopped loop only stops it if the decorated function lets CancelledError through. One that swallows it and returns went on to store its result, overwriting the value a newer revalidation of the key had already stored. The revalidation now checks that it still holds its slot after the recompute and before the store; past that check only cachekit code runs, which lets cancellation through.

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Review comments at @tests/unit/test_swr_decorator.py:
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27Bslash6 added 2 commits October 4, 2026 08:33

test(swr): type the stopped-loop hit helper's coroutine (LAB-7295)

1ab81d8

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fix(swr): release refresh slots stranded on stopped event loops (LAB-7295) - #488

fix(swr): release refresh slots stranded on stopped event loops (LAB-7295)#488
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Summary by CodeRabbit

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Sequence Diagram(s)

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