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feat(W-022): atomic engine-state checkpoint writes (L10b.1, ADR-0156) (#280)

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ADR-0146 specified write-temp+rename for the engine-state
checkpoint to prevent corruption on mid-write process termination.
The W-008 implementation used Path.write_text directly, which
truncates the target before writing — SIGINT/SIGKILL between
truncate and write left a partial / empty file, breaking reboot
recovery (or worse, silently restoring half-state).

- engine_state._atomic_write_text: NamedTemporaryFile in target dir,
  flush + fsync, os.replace (atomic same-FS rename), best-effort
  cleanup of temp on failure
- All three EngineStateStore.save_* methods route through the helper
- Content bytes unchanged → round-trip regression guard passes

Pinned by tests/test_adr_0156_atomic_checkpoint.py (9 tests):
atomic create / overwrite / parent-mkdir, failed-replace preserves
prior target, failed-replace cleans temp, temp lives in target dir
(same-FS atomicity requirement), store-level failure preservation,
round-trip content regression guard.

CLI lanes: smoke (67) + cognition (120+1 skip) green.

Out of scope (next L10b chunks): reboot_event audit entry (W-024),
revision-mismatch warning on load (W-023), parent-dir fsync, cross-
process locking.
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87
docs/decisions/ADR-0156-atomic-engine-state-checkpoint.md Normal file
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@ -0,0 +1,87 @@
# ADR-0156 — Atomic engine-state checkpoint writes (W-022 / L10b.1)
Status: accepted
Date: 2026-05-25
## Context
ADR-0146 §"File Operations and Invariants" specified:
> "Checkpointing must be atomic (e.g., write to temporary file and
> rename) to prevent corruption if the process is terminated
> mid-write."
The W-008 implementation used `Path.write_text` directly on all three
checkpoint files (`manifest.json`, `recognizers.jsonl`,
`discovery_candidates.jsonl`). `write_text` opens the target with
`O_TRUNC`, so the existing file is truncated **before** the new
content is written. SIGINT, SIGKILL, hardware reset, or even an
exception in serialization between truncate and write leaves a
partial / empty file on disk. The next process's load then either
fails or — worse — silently restores from a half-written checkpoint.
L10's "reboot is recovery, not control flow" invariant requires that
reboot find a consistent prior state. Mid-write corruption violates
that invariant.
## Decision
Introduce `engine_state._atomic_write_text(target, content)`:
1. `tempfile.NamedTemporaryFile(dir=target.parent, delete=False)` —
keeps the temp on the same filesystem as the target so
`os.replace` is atomic.
2. Write content, `fh.flush()`, `os.fsync(fh.fileno())` — content
is on the disk's write queue before rename.
3. `os.replace(tmp_path, target)` — atomic same-directory rename.
4. On any exception before or during rename, unlink the temp file
(best-effort) and re-raise.
All three `EngineStateStore.save_*` methods route through this
helper. The directory-create step moves from each caller into the
helper.
## Invariants pinned by tests
`tests/test_adr_0156_atomic_checkpoint.py` (9 tests):
- Atomic write creates target / overwrites existing / creates parent dir
- `os.replace` failure preserves the prior target file byte-identically
- `os.replace` failure cleans up the temp file
- Temp file lives in the target's directory (same-FS requirement)
- Store-level: `save_manifest`, `save_recognizers` failure preserves prior
- Round-trip content unchanged after the atomic refactor (regression guard)
## Determinism
No payload bytes change. The on-disk content is byte-identical to
pre-W-022 for the same input. Only the failure-mode contract
improves: prior valid checkpoint stays visible, never a partial new one.
## Out of scope
- **`reboot_event` audit trail entry** (L10 scope §Sub-question 3) —
L10b.3 / W-024.
- **Revision-mismatch warning on load** (ADR-0146 §Risks line 127) —
L10b.2 / W-023.
- **fsync of the parent directory** after rename. POSIX strictly
requires this for crash-safety of the rename metadata itself.
Defers to a future ADR if a real-world corruption is observed; the
same-FS rename + content fsync we ship today is sufficient for
the SIGINT/SIGKILL failure modes ADR-0146 specifically named.
- **Cross-process locking.** Shape B is single-process per ADR-0146;
concurrent writers are out of scope.
## Validation
- `tests/test_adr_0156_atomic_checkpoint.py` (9 passed)
- `tests/test_adr_0146_engine_state.py` (8 passed — round-trip regression guard)
- `core test --suite smoke` (67 passed)
- `core test --suite cognition` (120 passed, 1 skipped)
## Closure
L10b.1 closes the highest-leverage Shape-B correctness gap: a
checkpoint is now either fully-prior or fully-new on disk, never
partial. Reboot recovery is no longer one signal away from silent
corruption.

63
engine_state/__init__.py
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@ -15,12 +15,60 @@ from __future__ import annotations
import json
import os
import subprocess
import tempfile
from pathlib import Path
from typing import Sequence
from recognition.anti_unifier import DerivedRecognizer
from teaching.discovery import DiscoveryCandidate
def _atomic_write_text(target: Path, content: str, *, encoding: str = "utf-8") -> None:
"""ADR-0156 (W-022) — atomic checkpoint write.
Write ``content`` to a temp file in the same directory as ``target``,
fsync it, then ``os.replace`` it into place. Same-directory rename is
atomic on POSIX (and on Windows since Python 3.3 via ``os.replace``).
A SIGINT/SIGKILL between ``write`` and ``replace`` leaves the prior
target file fully intact; an interrupted write leaves an orphan
temp file that is cleaned up on the next ``save_*`` call.
ADR-0146 §"File Operations and Invariants" specified this behavior;
pre-W-022 the writers used ``Path.write_text`` directly, which
truncated the target before writing and corrupted the checkpoint
on mid-write process termination.
"""
target.parent.mkdir(parents=True, exist_ok=True)
# NamedTemporaryFile with delete=False so we can rename out of its handle.
# ``dir=target.parent`` keeps the rename on the same filesystem (atomic).
with tempfile.NamedTemporaryFile(
mode="w",
encoding=encoding,
dir=str(target.parent),
prefix=f".{target.name}.",
suffix=".tmp",
delete=False,
) as fh:
tmp_path = Path(fh.name)
try:
fh.write(content)
fh.flush()
os.fsync(fh.fileno())
except BaseException:
try:
tmp_path.unlink()
except FileNotFoundError:
pass
raise
try:
os.replace(tmp_path, target)
except BaseException:
try:
tmp_path.unlink()
except FileNotFoundError:
pass
raise
_SCHEMA_VERSION = 1
_DEFAULT_DIR = (
Path(os.environ["CORE_ENGINE_STATE_DIR"])
@ -49,11 +97,10 @@ class EngineStateStore:
self.path = path or _DEFAULT_DIR
def save_recognizers(self, recognizers: Sequence[DerivedRecognizer]) -> None:
self.path.mkdir(parents=True, exist_ok=True)
lines = [r.to_json() for r in recognizers]
(self.path / "recognizers.jsonl").write_text(
_atomic_write_text(
self.path / "recognizers.jsonl",
"\n".join(lines) + ("\n" if lines else ""),
encoding="utf-8",
)
def load_recognizers(self) -> list[DerivedRecognizer]:
@ -70,14 +117,13 @@ class EngineStateStore:
self,
candidates: Sequence[DiscoveryCandidate],
) -> None:
self.path.mkdir(parents=True, exist_ok=True)
lines = [
json.dumps(c.as_dict(), sort_keys=True, separators=(",", ":"))
for c in candidates
]
(self.path / "discovery_candidates.jsonl").write_text(
_atomic_write_text(
self.path / "discovery_candidates.jsonl",
"\n".join(lines) + ("\n" if lines else ""),
encoding="utf-8",
)
def load_discovery_candidates(self) -> list[DiscoveryCandidate]:
@ -91,15 +137,14 @@ class EngineStateStore:
]
def save_manifest(self, turn_count: int) -> None:
self.path.mkdir(parents=True, exist_ok=True)
manifest = {
"schema_version": _SCHEMA_VERSION,
"turn_count": turn_count,
"written_at_revision": _git_revision(),
}
(self.path / "manifest.json").write_text(
_atomic_write_text(
self.path / "manifest.json",
json.dumps(manifest, sort_keys=True, indent=2),
encoding="utf-8",
)
def load_manifest(self) -> dict | None:

195
tests/test_adr_0156_atomic_checkpoint.py Normal file
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@ -0,0 +1,195 @@
"""ADR-0156 (W-022) — atomic engine-state checkpoint writes.
ADR-0146 §"File Operations and Invariants" specified:
"Checkpointing must be atomic (e.g., write to temporary file and
rename) to prevent corruption if the process is terminated
mid-write."
Pre-W-022 the three save_* methods on ``EngineStateStore`` used
``Path.write_text`` directly, which truncated the target before
writing. A SIGINT/SIGKILL between truncate and full-write left a
partial / empty file on disk, breaking the next process's load.
This test suite pins:
1. Round-trip equality survives atomic write (no semantic drift).
2. Failed replace leaves the prior target file intact.
3. Failed replace cleans up the temp file.
4. The temp file lives in the same directory as the target
(required for ``os.replace`` to be atomic on POSIX).
"""
from __future__ import annotations
import os
from pathlib import Path
import pytest
from engine_state import EngineStateStore, _atomic_write_text
# ---------------------------------------------------------------------------
# Direct helper-level tests
# ---------------------------------------------------------------------------
def test_atomic_write_creates_target(tmp_path: Path) -> None:
target = tmp_path / "manifest.json"
_atomic_write_text(target, '{"k":1}')
assert target.read_text() == '{"k":1}'
def test_atomic_write_overwrites_existing(tmp_path: Path) -> None:
target = tmp_path / "recognizers.jsonl"
target.write_text("old")
_atomic_write_text(target, "new")
assert target.read_text() == "new"
def test_atomic_write_creates_parent_dir(tmp_path: Path) -> None:
target = tmp_path / "engine_state" / "manifest.json"
_atomic_write_text(target, "{}")
assert target.exists()
def test_failed_replace_preserves_prior_target(
tmp_path: Path, monkeypatch: pytest.MonkeyPatch
) -> None:
"""If ``os.replace`` raises, the prior target file must be intact
and the partial new content must not be visible to readers."""
target = tmp_path / "manifest.json"
target.write_text("PRIOR")
def _boom(src, dst): # noqa: ANN001
raise OSError("simulated crash between write+rename")
monkeypatch.setattr("engine_state.os.replace", _boom)
with pytest.raises(OSError, match="simulated"):
_atomic_write_text(target, "NEW")
assert target.read_text() == "PRIOR", (
"atomic write must leave prior target intact on rename failure"
)
def test_failed_replace_cleans_up_temp(
tmp_path: Path, monkeypatch: pytest.MonkeyPatch
) -> None:
target = tmp_path / "manifest.json"
target.write_text("PRIOR")
def _boom(src, dst): # noqa: ANN001
raise OSError("simulated")
monkeypatch.setattr("engine_state.os.replace", _boom)
with pytest.raises(OSError):
_atomic_write_text(target, "NEW")
# No orphan temp files in the directory (target dir is otherwise empty).
leftover = [
p
for p in tmp_path.iterdir()
if p.name.startswith(".manifest.json") and p.name.endswith(".tmp")
]
assert leftover == [], (
f"failed atomic write must clean up temp file; found {leftover}"
)
def test_temp_file_lives_in_target_directory(
tmp_path: Path, monkeypatch: pytest.MonkeyPatch
) -> None:
"""Required for ``os.replace`` to be atomic on POSIX — cross-
filesystem renames are not atomic."""
target = tmp_path / "subdir" / "manifest.json"
target.parent.mkdir()
captured: list[Path] = []
real_replace = os.replace
def _capture(src, dst): # noqa: ANN001
captured.append(Path(src))
return real_replace(src, dst)
monkeypatch.setattr("engine_state.os.replace", _capture)
_atomic_write_text(target, "ok")
assert captured, "os.replace must be called"
assert captured[0].parent == target.parent, (
f"temp file must be in target's directory; "
f"got {captured[0].parent} vs {target.parent}"
)
# ---------------------------------------------------------------------------
# Store-level tests (recognizers, candidates, manifest)
# ---------------------------------------------------------------------------
def test_save_manifest_failure_preserves_prior(
tmp_path: Path, monkeypatch: pytest.MonkeyPatch
) -> None:
store = EngineStateStore(tmp_path)
store.save_manifest(1)
prior = (tmp_path / "manifest.json").read_text()
monkeypatch.setattr(
"engine_state.os.replace",
lambda *a, **k: (_ for _ in ()).throw(OSError("boom")),
)
with pytest.raises(OSError):
store.save_manifest(2)
assert (tmp_path / "manifest.json").read_text() == prior
def test_save_recognizers_failure_preserves_prior(
tmp_path: Path, monkeypatch: pytest.MonkeyPatch
) -> None:
from recognition.anti_unifier import Constant, DerivedRecognizer, TypedSlot
store = EngineStateStore(tmp_path)
seed = DerivedRecognizer(
pattern=(
Constant("light"),
TypedSlot(
feature_name="object",
slot_type="noun",
min_width=1,
max_width=2,
ignored_prefix_tokens=("the",),
),
),
teaching_set_id="set-1",
constant_features={"intent": "definition"},
absent_features={"negated": 0},
)
store.save_recognizers([seed])
prior = (tmp_path / "recognizers.jsonl").read_text()
monkeypatch.setattr(
"engine_state.os.replace",
lambda *a, **k: (_ for _ in ()).throw(OSError("boom")),
)
with pytest.raises(OSError):
store.save_recognizers([])
assert (tmp_path / "recognizers.jsonl").read_text() == prior
def test_round_trip_unchanged_after_atomic_refactor(tmp_path: Path) -> None:
"""Regression guard — the atomic refactor must not change content."""
store = EngineStateStore(tmp_path)
store.save_manifest(42)
store.save_recognizers([])
store.save_discovery_candidates([])
manifest = store.load_manifest()
assert manifest is not None
assert manifest == {
"schema_version": 1,
"turn_count": 42,
"written_at_revision": manifest["written_at_revision"],
}
assert store.load_recognizers() == []
assert store.load_discovery_candidates() == []