alkstore/tasks/sqlite-engine-locks.md

---
id: sqlite-engine-locks
name: SQLite engine — named locks (`try_lock`, `Lock` handle, duration guards)
status: completed
depends_on: [sqlite-engine-seam-tx]
scope: narrow
risk: low
impact: component
level: implementation
tags: [wave-3, sqlite-engine]
---

## Description

Implement `Store::try_lock` and the `Lock` trait over the substrate's
lock ops (`lock_acquire`, `lock_release`, `lock_renew`):

- `try_lock(name, owner, ttl)` — validated entry point (name:
  shared-namespace rules; owner: non-empty only); `None` = someone
  holds it (no-work is a value). Returns the boxed `Lock` handle.
- **Duration guard (ADR-023 §2)**: `ttl <= 0` is a rejection —
  `Err`, opaque `Database` shape — enforced at the trait-impl entry,
  before the substrate. The substrate's `lock_renew` already guards
  `ttl_s <= 0` (its message preserved in the source chain where it
  fires); `lock_acquire` does not — the engine guards both call sites
  uniformly so the rule is contract-side, not substrate-side (the pg
  engine implements the same rule with no inherited guard at all).
- `Lock::renew(ttl)` — same duration guard; a new full TTL window
  from now; `false` = lost it (expired and re-acquired elsewhere).
- `Lock::release(self)` — consuming; `true` = was held; `false` =
  already expired/re-acquired (no-op, not an error).
- Exclusion lapses silently at TTL expiry (no revocation event) —
  the substrate's expired-row delete on next acquire is the
  mechanism; pin the lapse-then-reacquire shape in tests.

## Acceptance Criteria

- [x] `try_lock` grants exclusively (second acquirer gets `None`);
      release makes it acquirable again
- [x] Duration guards: `try_lock`/`renew` with `ttl <= 0` return
      opaque `Err` (`Database`) — never a granted lock, never a
      panic; substrate guard message preserved in the source chain on
      `renew` — pinned per ADR-023 §2's backlog row
- [x] `renew` extends from now (full window, not additive); foreign
      owner cannot renew; `false` on lapsed/re-acquired
- [x] `release` removes only the owner's row; `false` = no-op shape
- [x] TTL lapse: after expiry, another owner acquires (silent lapse)
- [x] `cargo test -p alkstore-sqlite`, clippy `-D warnings`, fmt clean

## References

- docs/architecture/core-contract.md (locks section, numeric-domain rules)
- docs/architecture/decisions/008-contract-v1-pinning.md §7
- docs/architecture/decisions/019-mechanism-handle-surfaces.md §1
- docs/architecture/decisions/023-fourth-review-round.md §2
- alkstore-sqlite/src/substrate/ops.rs (lock_* fns)

## Notes

> Decisions of record the description didn't pin:

- **Same-owner re-acquire returns `Some`, not `None`** — the
  substrate's `lock_acquire` is opportunistic-expiry-delete +
  `INSERT OR IGNORE` + read-back: the same owner's re-acquire keeps
  the original row (TTL *not* refreshed) and reports granted. Pinned
  as the inherited shape (engine-sqlite.md's mapping row: "re-acquire
  does not refresh TTL — inherited deliberately"; the research
  register's D-4 kept `lock_acquire` verbatim). The handle returned
  is a fresh boxed `Lock`; `renew` remains the only refresh path.
- **Post-lapse release returns `true` while the stale row survives,
  `false` only once the row is gone/foreign** — the substrate's
  `lock_release` is a blind `(name, owner)` delete and expired rows
  persist until the next acquire's opportunistic delete (no general
  lock-expiry sweep, upstream-inherited). So immediately after a
  lapse the stale holder's release deletes its own stale row
  (`true`); after a foreign re-acquire (or any row-less state) it is
  the no-op `false`. Tests pin both arms; the contract text's "true =
  was held" reads as row-scoped, which the substrate defines.
- **The duration-guard detail rides the source chain, not
  `Display`** — the engine guard's message
  ("ttl must be a positive duration (seconds), got {ttl}") is the
  `#[source]` inner error (ADR-008 §5's chain discipline; the
  `Database` display is `database error` opaque). The substrate's
  direct-caller message `ttl_s must be positive` is asserted verbatim
  at the substrate call site (A-2 above).
- **Closed-store posture**: `try_lock`/`renew`/`release` check the
  engine's closed flag (or the writer slot's unavailability) and fail
  closed with opaque `Database` — the engine-wide shape every wave-3
  mechanism carries; release preserves the substrate row delete as
  the truth-teller rather than caching a handle-local "held" flag.
- `store/lock_tests.rs` follows the wave-3 bounded-wait posture for
  TTL expiry (second-precision `unixepoch()`; no tight-race sleeps);
  the short-window lapse steps stamp short expiries directly through
  a separate connection (the queue tests' honest cross-connection
  probe shape) instead of sleeping out multi-second windows.

## Summary

> `alkstore-sqlite/src/lock.rs`: the acquire path (`try_lock` —
> shared-namespace + non-empty owner validation, the ADR-023 §2
> duration guard on `ttl <= 0` engine-side before the substrate, the
> closed-store check, then `lock_acquire` through the writer slot,
> `Ok(None)` = held elsewhere) and `SqliteLockHandle` (`Lock` impl:
> `renew` with the same duration guard over `lock_renew`'s full-window
> update; consuming `release(self: Box<Self>)` over `lock_release`'s
> owner-scoped delete). `store.rs`'s `try_lock` stub replaced with the
> wiring. Six acceptance tests in `store/lock_tests.rs` pin the
> exclusion/loser shapes, both duration guards + the substrate's
> verbatim message, full-window renew and stale-holder refusals,
> owner-scoped release with both boolean arms, the silent
> lapse-then-reacquire shape, and the cross-instance re-acquire (the
> verification backlog's SQLite re-acquisition pin at engine level;
> the contract-suite row remains for the suite itself). Verified:
> `cargo build`, `cargo test` (workspace; 177 + 25 + 3 pass),
> `cargo clippy --all-targets -- -D warnings`, `cargo fmt --check`
> — all clean.