fix(decorators): async lock double-check L2 hits record get telemetry (LAB-3769)

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Maintainer note — this PR also carries a dependency fix. anyio was
constrained to >=4.14.2 here rather than in a separate PR, at the
maintainer's request, because it was the only thing keeping this branch red.

Dependency: anyio >= 4.14.2 and h2 >= 4.4.1

anyio 4.11.0 carries GHSA-82r6-8w77-94w6
/ CVE-2026-63374 (IDNA-2003 hostname encoding lets a hijacked connection to an
internationalised domain pass TLS certificate validation — CVSS 9.3),
GHSA-5p39-cfhj-2xmp /
CVE-2026-64847 (undrained process-pool stderr pipe deadlocks the worker), and
GHSA-3w57-8xmc-8v26 /
CVE-2026-63349. All three are fixed in 4.14.2.

The first attempt at this fix did not work, and is worth calling out. The floor
was added to [tool.uv] constraint-dependencies, matching the entries already in
that block. That table is uv-local — it constrains this repository's lockfile and is
never emitted as Requires-Dist, so pip install cachekit ignored it completely.
Verified against the built wheel: METADATA listed nine Requires-Dist entries and
neither anyio nor h2 was among them. CI passed regardless, because both audit
gates run pip-audit against the already-constrained venv — a green board was false
assurance, not evidence.

Both floors now live in [project] dependencies, where they are emitted as
Requires-Dist and actually bind a downstream install. The wheel built from this
branch ships Requires-Dist: anyio>=4.14.2 and Requires-Dist: h2>=4.4.1.

Two related corrections:

• h2 >= 4.4.1 (GHSA-6hr6-w5qg-qmwg,
  a request-smuggling primitive) had the identical problem — it is where the anyio
  entry was copied from — so it moves too.
• Both rationale comments claimed the dependency "reaches users through the
  cachekit[http2] extra". There is no such extra (they are data, json,
  memcached); httpx[http2] is an unconditional runtime dependency, so exposure is
  every install, not an opt-in subset. Corrected at both entries.

The constraint table now holds only genuinely dev-only transitives and documents why,
so the next floor does not repeat this. Re-resolving also drops sniffio (vendored
into anyio from 4.14) and moves typing-extensions to 4.16.0; nothing in src/
or tests/ imports sniffio, and both remaining importers guard with except ImportError. pip-audit reports no known vulnerabilities.

Merged main — conflict resolution

main picked up #294, which widened the L2 hit tuple to
(found, value, envelope, size_bytes) at the same three async sites this
branch touches, so both double-check hit returns conflicted. Two decisions
worth a reviewer's eye:

• size_bytes is still derived from the envelope, not read from the tuple's
  new slot. The handler reports len() of whatever the backend returned,
  which is a character count for a str envelope; fix(decorators): async get hits record serializer/size/hit like the sync path (LAB-3765) #297 made this label
  byte-denominated on purpose. Changing the source inside a merge would be an
  unreviewed behaviour change, so the .encode("utf-8") stays.
• _record_l2_hit_async returns nothing, and is best-effort. perf(decorators): backfill L1 on sync L2 hits; size stats by envelope length (LAB-348) #294 moved envelope
  encoding inside _l1_backfill_from_l2, and both async readers annotate that slot
  bytes, so the re-encode the return value saved never fired — while the return
  type varied with collect_stats. It is gone. The helper body is now wrapped the
  same way _l1_backfill_from_l2 is: every call site sits inside an except Exception that falls through to a recompute, so a throwing metrics collector could
  convert a hit already in hand into a full recompute during exactly the stampede the
  lock exists to absorb. Telemetry never costs a served hit.

Both double-check branches were re-verified by mutation after the merge:
deleting the record call from either one fails that parametrised case alone.

---

Summary

This PR fixes a telemetry gap where async L2 cache hits arriving through the distributed lock's double-check path were not recorded, making thundering-herd hits invisible to cache_operations_total and cache_info() L2 statistics (LAB-3769).

Problem

The uncontended async L2 hit path correctly recorded get/serializer="rust"/hit=True telemetry (from LAB-3765), but the two post-lock _l2_double_check hit returns did not. This meant that when another worker filled the cache while this request waited on the distributed lock — precisely the traffic the lock exists to absorb — the resulting hit was never counted in operation totals or L2 stats.

Changes

• Extracted a shared _record_l2_hit_async helper in wrapper.py that consolidates the L2 hit telemetry logic (operation context, success recording, record_cache_operation, and record_l2_hit). It returns the UTF-8-encoded envelope so callers can pass it directly to _l1_backfill_from_l2 without re-encoding.

• Refactored the uncontended L2 hit site to use the new helper, replacing the inline telemetry recording.

• Added telemetry recording to both double-check hit paths inside the lock. Each now measures the double-check read duration and calls _record_l2_hit_async before backfilling L1 and returning, ensuring contended hits report the same labels as uncontended ones.

Testing

• Added tests/unit/test_async_l2_double_check_hit_labels.py, which reproduces contention by patching the backend's get to miss once (forcing the wrapper into the lock path) then hit on the double-check read. It verifies exactly one get operation is recorded with serializer="rust", hit=True, and the correct byte size.

---

Summary

This PR fixes a telemetry gap in the async cache decorator's double-check lock path, ensuring L2 cache hits discovered during double-check reads properly record get telemetry.

Problem

When an async cached function experiences cache contention, the wrapper falls through the pre-lock check into the lock path. A double-check read then occurs (standing in for "another request filled the cache while we waited"). This double-check hit can be reached through two distinct control-flow branches:

1. Lock-acquired branch — the lock was successfully obtained
2. Lock-timeout branch — the lock acquisition timed out

Both branches record telemetry via the same _record_l2_hit_async helper, but the test coverage previously only exercised the lock-acquired case (the mock lock was always granted).

Changes

• Parametrized the test over both lock outcomes (lock-acquired and lock-timeout) so both double-check hit return paths are covered.
• Updated _LockableByteStore to accept a lock_acquired flag, allowing the mock to yield either True or False from acquire_lock, thereby steering the test through the desired control-flow branch.
• Added duration assertions verifying that duration_ms is recorded as a non-negative float. This guards against a regression where the double-check read's timing window could be dropped — a bug that would otherwise still pass the existing label-only assertions.

Impact

Ensures consistent, complete telemetry (including timing) for L2 cache hits regardless of which lock outcome path the async wrapper takes during contention.

---

Summary

This PR fixes telemetry recording for async L2 cache hits that arrive via the lock's double-check path, and standardizes the size_bytes telemetry label to be consistently byte-denominated across all cache hit sites (LAB-3769).

Changes

Async L2 hit telemetry (LAB-3769)

• Refactored _record_l2_hit_async to no longer return an envelope, decoupling telemetry recording from envelope encoding. Both post-lock double-check hit sites now record get telemetry consistently, ensuring thundering-herd hits served via the lock's double-check are no longer invisible to cache_operations_total / cache_info().

Consistent byte-denominated size_bytes

• The size_bytes label is now derived from the encoded envelope rather than the handler's raw len() (which reports character count for str envelopes). This keeps the metric byte-denominated across every hit site, matching the bytes stored by L1 backfill.
• Handler cache-hit tuples were extended to carry an explicit size_bytes slot: async now unpacks (found, result, cached_data, size_bytes) and the sync path unpacks the same 4-tuple, replacing ad-hoc len(str(...).encode(...)) computations.

Sync path L1 backfill (LAB-348 / LAB-557)

• Added L1 backfill from L2 envelopes on the sync hit path, honoring the stale-exclusion and remaining-freshness bound invariants (previously only done on the async path). The sync path now captures the freshness value (fresh_for) from freshness-capable L2 reads to bound the L1 lifetime.

Robustness of _l1_backfill_from_l2 (LAB-348)

• Made L1 backfill best-effort by catching TypeError from L1Cache.put (raised on a non-bytes envelope from an out-of-contract backend), logging a warning and skipping rather than allowing the exception to demote a served hit into a recompute. Other exceptions still propagate as genuine L1 bugs. The function was also converted to an early-return guard clause for clarity.

---

Summary

This PR ensures that async L2 cache hits arriving through the lock's double-check path correctly record get telemetry, addressing a gap where thundering-herd hits could be invisible to cache_operations_total / cache_info().

Changes

Telemetry recording for double-check L2 hits (LAB-3769)

The _record_l2_hit_async helper is now consistently used across all three L2 hit sites in the async wrapper:

• The uncontended read path
• Both post-lock double-check hit paths (where another request filled the cache while the current request waited on the lock)

Previously the double-check hit paths already called _record_l2_hit_async, but this change refactors the helper to return the computed envelope so callers can reuse it.

Avoiding duplicate UTF-8 encoding

_record_l2_hit_async now returns the UTF-8-encoded envelope when collect_stats computed one (otherwise it returns the original cached_data unchanged). Callers capture this return value and pass it directly to _l1_backfill_from_l2, avoiding encoding the same string payload twice per hit.

The updated docstring clarifies that:

• size_bytes is derived from the envelope (byte-denominated), not from the handler's size_bytes slot which reports a character count for string envelopes (LAB-348, LAB-3765).
• Since _l1_backfill_from_l2 encodes defensively itself, passing the original payload back would still be correct — the reuse is purely an efficiency improvement.

Impact

• Async L2 double-check cache hits are now properly reflected in cache operation telemetry and stats.
• Reduces redundant UTF-8 encoding work on the hit path when statistics collection is enabled.

---

Summary

This PR fixes telemetry error handling in the async lock double-check path when recording L2 cache hits.

Problem

In _record_l2_hit_async, telemetry was previously wrapped in a broad except Exception that logged at WARNING level. However, any unexpected error not caught here would propagate to the caller's except Exception block, which logs at DEBUG under "Double-check cache failed after lock acquisition" and then recomputes the value. This meant a buggy telemetry collector could silently turn a cache hit — the very hit the lock exists to protect — into a full recompute under contention, once per contender.

Changes

src/cachekit/decorators/wrapper.py

• Split the single except Exception handler into two distinct clauses:
  • except (ValueError, TypeError): the collector's documented/expected refusals (e.g., duplicated timeseries, mismatched label sets, non-numeric observations), logged at WARNING as before.
  • except Exception: unexpected errors indicating a bug in the telemetry stack, now logged at ERROR with the exception type named — making the failure loud rather than allowing the caller to demote the hit into a recompute.
• Added explanatory comments documenting why the unexpected case is deliberately caught (to prevent misattributed DEBUG logging and per-contender recomputes) rather than narrowly scoped.

tests/unit/test_async_l2_double_check_hit_labels.py

• Added a parametrized test verifying that a throwing metrics collector never costs the served hit. It covers both handler clauses — a collector refusal (ValueError) and an unexpected bug (AttributeError) — asserting the hit is still served from the double-check read and the function body is not executed a second time.

Impact

Telemetry failures — expected or unexpected — no longer cause cache hits under lock contention to be demoted into recomputes, preserving the stampede-protection guarantee the lock provides.

---

Summary

This PR fixes a telemetry bug in the async cache decorator where L2 cache hits served from the double-check read path could fail to be recorded in local statistics when an external telemetry collector rejected the operation.

Problem

In the _record_l2_hit_async helper, the local stat recording (_stats.record_l2_hit()) was placed after calls to the external telemetry collector. If the collector raised one of its documented refusals (e.g., duplicated timeseries, mismatched label set, non-numeric observation), the exception would prevent the local L2 hit counter from being updated—even though the cache hit had already been served to the caller.

This resulted in served hits going missing from cache_info().l2_hits and skewing the average L2 latency, effectively hiding hits that were actually delivered.

Fix

The local stat recording is now performed first, before any interaction with the external collector. Since recording the local stat is pure arithmetic under a lock and cannot realistically fail, this ordering guarantees that a served hit is always counted locally, regardless of whether the external collector refuses the operation.

Test Changes

The test in test_async_l2_double_check_hit_labels.py was updated to verify the fix:

• It now captures the l2_hits count before the operation and asserts the delta rather than an absolute value (since counters are keyed by module/qualname and shared across parametrized runs).
• It adds an assertion that cache_info().l2_hits is still incremented by 1 even when the external collector (record_cache_operation) raises an exception, confirming that a collector refusal no longer costs the local L2 stat.

Summary by CodeRabbit

• Bug Fixes
  • Improved cache hit tracking for asynchronous operations, including requests completed after lock contention.
  • Cache operation metrics now consistently include payload size and response timing information.
  • Telemetry errors no longer trigger unnecessary recomputation of cached results.
• Tests
  • Added coverage for asynchronous cache hits following both successful lock acquisition and lock timeouts.
• Chores
  • Updated required runtime components to address transitive security vulnerabilities.

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Walkthrough

Async L2-hit telemetry now uses one helper for primary and lock double-check paths. The helper records operation success, optional UTF-8 envelope size, and L2 statistics. Tests cover both lock outcomes and telemetry failures. The project also adds mandatory anyio and h2 constraints.

Changes

Async L2 telemetry

Layer / File(s)	Summary
Shared async L2-hit telemetry src/cachekit/decorators/wrapper.py	Adds _record_l2_hit_async for operation success, optional UTF-8 envelope sizing, and L2 statistics recording. The primary async hit path uses the helper.
Double-check hit integration src/cachekit/decorators/wrapper.py, tests/unit/test_async_l2_double_check_hit_labels.py	The lock-acquisition and lock-timeout paths measure read duration and use the shared helper. Tests verify hit labels, payload size, duration, and that telemetry failures do not trigger recomputation.
Mandatory dependency constraints pyproject.toml	Adds anyio>=4.14.2 and h2>=4.4.1 to mandatory dependencies. Removes the duplicate uv-only h2 constraint.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix

Merge Risk: 🔵 Low · up to f6181

When telemetry collection fails, callers still receive the cached value, but reported L2-hit counts and latency become inaccurate. This is bounded observability drift and should be corrected before merge if these statistics are relied upon.

🚥 Pre-merge checks | ✅ 3 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description gives detailed context about the telemetry fix, dependency changes, testing, and security risks. However, it does not follow the required template and omits the required Type of Change…	Rework the description using the repository template. Add the required headings and complete the applicable checkboxes, including the dependency security checks, test status, backward-compatibility statement, and any additional reviewer not…
Docstring Coverage	⚠️ Warning	Docstring coverage is 30.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 20 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main change: recording telemetry for async lock double-check L2 hits. It is concise and specific.
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.

Full details: Description check

Explanation

The description gives detailed context about the telemetry fix, dependency changes, testing, and security risks. However, it does not follow the required template and omits the required Type of Change, Security Checklist, Backward Compatibility, and Additional Notes sections.

Resolution

Rework the description using the repository template. Add the required headings and complete the applicable checkboxes, including the dependency security checks, test status, backward-compatibility statement, and any additional reviewer notes.

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Inline comments:
In `@tests/unit/test_async_l2_double_check_hit_labels.py`:
- Around line 1-102: Add coverage for the lock-timeout double-check hit in
test_lock_timeout_cache_populated_during_wait, where acquire_lock yields False.
Capture the wrapper’s recorded get operation like
test_async_l2_double_check_hit_records_get and assert exactly one L2 hit has
serializer "rust", hit=True, the expected envelope size, and a recorded read
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Codecov Report

❌ Patch coverage is 94.73684% with 1 line 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	94.73%	0 Missing and 1 partial ⚠️

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Inline comments:
In `@src/cachekit/decorators/wrapper.py`:
- Around line 847-855: Move _stats.record_l2_hit(get_duration_ms) before the
failure-prone telemetry and collector operations in the L2 hit path, or place it
in a separate protected block, so every served cached hit updates L2 hit count
and latency even when record_cache_operation() raises.

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