alkblobs/tasks/engine-sqlite-window.md

---
id: engine-sqlite-window
name: sqlite engine-state window family — the durable single-node delete window
status: completed
depends_on: [engine-sqlite]
scope: narrow
risk: low
impact: component
level: implementation
tags: [wave-3, engines, sqlite, gc]
---

## Description

Implement the engine-state companion seam's **window family** on the
sqlite backend (`src/sqlite.rs` or its seam module): the
`EngineWindowState` impl per ADR-010 §2 **as reduced by ADR-012 §5** —
the window family only. The pin clauses and the sweep-lock body are
**dropped dead machinery** for sqlite (ADR-012 §5's table: pin clauses
**no**, sweep-lock clause **no** — the single-flighter is the
in-process lock; a sqlite fleet is a recorded non-option, ADR-010 §3).
Nothing of `EngineFleetState` may exist on this engine.

- **`ensure_gc_schema`** — creates **`gc_window` only**
  (`gc_window (key BLOB PRIMARY KEY, visible_after INTEGER)`; the
  timestamp encoding — `SystemTime` ↔ integer, sub-second precision
  for the test tunings — is a recorded design point). Never `gc_pins`;
  the embedder's liveness table is never created here (ADR-008's
  schema-ownership rule — the sweep reads it and never owns it).
  Called at store construction by the store core
  (`store-sqlite-construction` wires the call); the clause body is
  engine-side.
- **`window_stage`** — stage candidates into the durable window,
  **idempotent per `(key, deadline)`** (`INSERT OR IGNORE` semantics —
  the body's discipline; the store stages each candidate once per
  sweep, and a re-stage of the same key at a new deadline updates the
  row). Virgin store: tolerated (the table exists from construction;
  a no-tables-yet database stages lazily or errors nothing — the
  Backend trait's engine posture carries here).
- **`window_take(now)`** — take overdue candidates
  (`visible_after <= now`): the rows **leave the window table and
  return as keys** for the calling sweep to re-arbitrate and delete —
  the executor is the next sweep (ADR-012 §6.7). Empty when nothing is
  overdue; virgin store: empty. The take is one atomic step (selected
  rows removed and returned together — a row is never handed out
  twice).
- **Contract tests** per the applicability row (ADR-012 §5's table is
  the contract-test matrix): the schema creates the window table only
  (assert `gc_pins` does not exist); stage idempotence per
  `(key, deadline)` incl. the re-stage-at-new-deadline form; take
  returns exactly the overdue set, removes it, leaves future rows;
  empty/virgin shapes; the timestamp encoding's boundary
  (`visible_after == now` is overdue — `<=` per the seam doc).

The store core drives these clauses (`store-sweep-durable-window`);
this task ships the clauses and their engine-level contract tests.

## Acceptance Criteria

- [x] The sqlite backend implements `EngineWindowState` — the window
      family only; no pin-clause or sweep-lock body exists anywhere in
      the sqlite engine
- [x] `ensure_gc_schema` creates `gc_window` only; `gc_pins` is never
      created by sqlite (contract-tested)
- [x] `window_stage` is idempotent per `(key, deadline)`; a re-stage
      at a new deadline updates the row
- [x] `window_take` returns exactly the overdue set (`<= now`
      boundary), removes it atomically, leaves future rows; empty on
      none/virgin
- [x] Clause contract tests green; `cargo test` (default) green;
      `cargo clippy --all-targets -- -D warnings` and
      `cargo fmt --check` green

## References

- src/gc.rs (`EngineWindowState` — the seam contract and its clause docs)
- docs/architecture/decisions/010-backend-io-seams-and-fleet-state.md §2
- docs/architecture/decisions/012-pre-decomposition-consistency-rulings.md §5 (the applicability table; the sqlite reduction), §6.7 (the executor)
- docs/architecture/decisions/008-trait-size-probe-fleet-gc-and-large-engines.md §2 (the durable window's semantics)
- docs/architecture/decisions/005-pooled-cas-and-gc-mechanism.md (the delete-window protocol)

## Notes

- The `EngineWindowState` impl lives in `src/sqlite.rs` after the
  `Backend` impl, as `impl EngineWindowState for SqliteBackend`. The
  pin clauses and the sweep-lock body are absent by construction —
  nothing of `EngineFleetState` is implementable or referenced on this
  engine (ADR-012 §5's sqlite row: the single-flighter is the
  in-process sweep lease on `GcHome`, a sqlite fleet is a recorded
  non-option).
- **Timestamp encoding (recorded design point):** `visible_after`
  travels as a signed i64 nanosecond count from the Unix epoch —
  sub-second precision preserved for the test tunings, negatives for
  pre-epoch instants, full i64 range. Helpers `time_to_nanos` /
  `nanos_to_time` sit beside `size_u64` in `src/sqlite.rs`.
- **Staging discipline:** one `prepare_cached` upsert per row —
  `INSERT … ON CONFLICT(key) DO UPDATE SET visible_after =
  excluded.visible_after`. The row's primary key is the window
  candidate's key, so a same-deadline re-stage is idempotent (one row,
  unchanged value) and a new-deadline re-stage updates the row rather
  than duplicating it. `INSERT OR IGNORE` alone would have made the
  new-deadline update a no-op, so the upsert carries both clauses
  of the idempotence contract.
- **Take atomicity:** `DELETE FROM gc_window WHERE visible_after <= ?
  RETURNING key` — one statement on the single pooled connection, so
  selection and removal are one atomic step (a row is never handed out
  twice); `RETURNING` order is unspecified, so the returned keys are
  sorted for determinism.
- **Virgin-store tolerance split per the sqlite engine's recorded
  posture:** `window_take` is a read — a no-tables-yet database maps
  to empty via the existing `is_no_such_table` tolerance; `window_stage`
  is a write — a tableless database surfaces the engine error rather
  than silently creating schema (the constructor-vs-lazy design point:
  store construction always wires `ensure_gc_schema`, so the table
  exists whenever the engine is in store service; the lazily staged
  option was rejected as contradicting that recorded point).
- Schema timing unchanged from the pool table's recorded design point:
  the backend constructor never creates `gc_window` itself — the store
  construction call (`store-sqlite-construction`) wires
  `ensure_gc_schema`, which is idempotent and creates `gc_window`
  only. The clause-level tests use a `new_window_backend()` fixture
  that mirrors that wiring.
- The boundary contract (`visible_after <= now`, inclusive) is
  contract-tested, including the nanosecond slack row (visible at
  `now + 1ns` stays).

## Summary

The sqlite engine-state seam's window family landed: `impl
EngineWindowState for SqliteBackend` in `src/sqlite.rs` —
`ensure_gc_schema` (creates `gc_window` only, never `gc_pins`,
contract-tested through `sqlite_master`), `window_stage` (key-keyed
upsert; idempotent per `(key, deadline)`, new-deadline re-stage
updates the row), and `window_take` (`DELETE … RETURNING` — one atomic
overdue take with the `<= now` boundary, future rows left in place).
Signed-nano timestamp encoding recorded; virgin-store tolerance split
read/write per the sqlite engine's posture. Six clause contract tests
added (schema window-only, staging idempotence + deadline update,
overdue-exact take + future rows left, the `<=` boundary with
nanosecond slack, and both virgin shapes). Verification: `cargo test`
green (58 lib + 5 integration), `cargo clippy --all-targets -- -D
warnings` green, `cargo fmt --check` green.