alkstore/tasks/mem-engine-locks.md

id, name, status, depends_on, scope, risk, impact, level, tags

id	name	status	depends_on	scope	risk	impact	level	tags
mem-engine-locks	Mem engine — named locks (in-memory TTL registry)	completed	mem-engine-seam-tx	narrow	low	component	implementation	wave-6 mem-engine locks

Description

The named-locks mechanism over an in-memory TTL registry (engine-mem.md §Mapping the contract, locks row):

• store.try_lock(name, owner, ttl) — acquire or fail (Option<Lock> ; no-work is a value); validation at entry (empty → InvalidName, reserved → ReservedName); duration guard at entry — ttl <= 0 rejects with opaque Err (ADR-023 §2, durations kind), never a silent immediately-expired lease.
• Lock handle (ADR-019): renew(ttl) (absolute reset; renews past expiry refused) and release. Registry entries carry owner + deadline; expiry is a clock check at try/renew boundaries (tokio time; no ambient revoker) — after TTL expiry exclusion lapses silently (the ADR-008 §7 row), re-acquirable; the holder may still release.
• Single-instance contention semantics: two try_locks on the same name within the registry are serialized by the guarded-state discipline — first wins, second gets None cleanly (the SQLite busy-path question has no mem analog; loser is a value, never an error). Not part of the tx seam (no lock_tx — contract-verbatim: a lock held across a business tx is held by explicit acquisition; the overlay neither sees nor stages locks).

Acceptance Criteria

• try_lock/renew/release over the TTL registry; entry validation (names + duration guard) — pinned
• TTL expiry: silent lapse, re-acquisition, renew-past-expiry refused — pinned (clock-controlled tests via the state core's clock accessor)
• Concurrent-contention shape: loser is None, winner's lease intact — pinned
• cargo check --target wasm32-unknown-unknown -p alkstore-mem clean; workspace gates green

References

• docs/architecture/core-contract.md §named locks
• docs/architecture/decisions/008-contract-v1-pinning.md §7; 019-mechanism-handle-surfaces.md; 023 §2
• docs/architecture/engine-mem.md §Mapping the contract (locks row)
• tasks/pg-engine-locks.md (the structural twin's task of record — the duration guards land identically)

Notes

Filled by implementation agent, 2026-10-10.

1. The implementation already landed in the seam task — mem-engine-seam-tx Note 1 (the all-or-nothing trait-impl disposition, the same one the notify-listen and queues twins recorded): the MemLock registry mechanism in alkstore-mem/src/lock.rs was implemented with the seam's whole-engine scope; this task found its mechanism implemented.
2. Full coverage reconciliation — zero new tests needed: every shape the description and the three acceptance criteria enumerate was already pinned by the seam suite in lock.rs: the contention shape (first wins, loser None as a clean value, same-owner re-grant without a second lease row — contention_grants_once_and_ the_loser_reads_none), the duration guard (ttl <= 0 → opaque Database Err on both try_lock and renew, ADR-023 §2's durations kind — duration_guards_reject_non_positive_ttls), the TTL expiry row via the frozen clock (in-TTL refusal, silent lapse, re-acquisition, old-holder renew refused — expiry_lapses_silently_ and_reacquires), renew-past-expiry refused + the new holder's clean life (renew_past_expiry_is_refused), the release shapes (held-true, past-expiry-still-the-holder's, re-acquired-elsewhere no-op — release_shapes), entry validation (empty/blank/reserved name, empty owner — entry_validation), and the locks-are-not-tx-scoped posture (the overlay neither sees nor stages locks — locks_are_not_tx_scoped). Note also the seam task's recorded semantis delta (seam Notes 7): mem pins renew-past-expiry refused, where the SQLite substrate would revive a not-yet-purged lapsed row — the stricter, contract-honest arm.
3. No production code changed and no test changed — the task is a disposition: the surface was verified correct as-found.

Summary

Filled on completion, 2026-10-10.

Completed the mem engine's locks task as a pure reconciliation: the TTL-registry mechanism landed with the seam task's whole-engine scope, and the reconciliation against the description's pin list found all three acceptance criteria's shapes already unit-pinned in alkstore-mem/src/lock.rs's seven tests (contention/loser-None, duration guards on both entry kinds, clock-controlled silent lapse + re-acquisition + renew-past-expiry refused, release shapes, entry validation, no-lock_tx). No code and no tests changed; acceptance criteria satisfied as-found. Verified: cargo test -p alkstore-mem green, workspace cargo test green, cargo clippy --all-targets -- -D warnings clean, cargo fmt --check clean, cargo check --target wasm32-unknown-unknown -p alkstore-mem clean.