fix(drain): fail closed on missing or unproven acknowledgement evidence (#661)

The acks_or_timeout check treated an absent `acks` key as proof that no
session needed to acknowledge: `drain_state.get("acks") or {}` collapsed
absent, None, and empty into the same value, and the resulting empty mapping
satisfied `no_sessions_to_ack`. The impact report's counts.sessions_live_other
was never consulted, so absence of evidence was read as evidence of absence.

Reproduced at head 1cbbde0089: with
sessions_live_other = 3 and the acknowledgement key absent, acks_or_timeout
passed with detail "no other live sessions required to acknowledge", the proof
minted clean, and gate_apply_restart returned verdict allow — a restart
authorized against three live sessions with zero acknowledgement evidence, and
the resulting artifact carried a valid signature.

Whether acknowledgement is required is now derived from the impact report,
never from the shape of the drain state:

- _live_session_count() reads counts.sessions_live_other and returns None for a
  missing, malformed, negative, or bool value, so an unreadable report fails
  closed instead of reading as "nobody was live".
- Absent, None, non-mapping, empty, partially-covering, and unparseable or
  stale acknowledgement data all fail closed while live sessions require
  acknowledgement.
- _is_acknowledged() no longer coerces with str(); only an explicit
  "ack"/"acked"/"acknowledged" string counts, so None, timestamps, and
  "pending"/"stale" markers are never read as an acknowledgement.
- Present-but-unacknowledged entries fail closed even when the report claims
  zero live sessions: that contradiction is not safe to resolve in favour of
  the restart.
- ack_timeout_policy_applied stays strict (`value is True`), so an absent, null,
  or non-boolean value cannot open the gate on its own.

The genuine no-other-live-sessions case still passes, now justified by the
report proving sessions_live_other == 0 rather than by the absence of data.

Adds AcknowledgementFailClosedTests: 14 cases / 26 subtests covering missing,
null, empty, malformed, stale, partial-coverage, and unproven-count inputs,
the valid-acknowledgement and zero-live-session paths, timeout-policy
strictness, and that a failed check blocks proof.clean, verification, and the
restart gate.

Restart-surface suite: 132 passed, 38 subtests (branch baseline 118 passed,
12 subtests; +14 new tests, no regressions).

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01VEaP3TohHLFWkp3Z2mmuZw

tests/test_drain_proof.py

@@ -379,5 +379,218 @@ class SecretHygieneTests(unittest.TestCase):
         self.assertNotIn(SECRET.decode(), blob)
 
 
+def _drained_report_with_live_sessions(count: int) -> dict:
+    """Report with ``count`` other live sessions but nothing in flight.
+
+    Every other checklist item passes against this report, so a failure
+    isolates the acknowledgement check rather than tripping on mutations.
+    """
+
+    sessions = [
+        {
+            "session_id": "prgs-controller-1-req",
+            "role": "controller",
+            "profile": "prgs-controller",
+            "pid": _live_pid(),
+            "status": "active",
+            "last_heartbeat_at": NOW.isoformat(),
+        }
+    ]
+    for index in range(count):
+        sessions.append(
+            {
+                "session_id": f"prgs-author-{index}",
+                "role": "author",
+                "profile": "prgs-author",
+                "pid": _live_pid(),
+                "status": "active",
+                "last_heartbeat_at": NOW.isoformat(),
+            }
+        )
+    report = rc.evaluate_restart_impact(
+        {"sessions": sessions, "leases": [], "inventory_complete": True},
+        now=NOW,
+        requesting_session_id="prgs-controller-1-req",
+    )
+    return report.as_dict()
+
+
+class AcknowledgementFailClosedTests(unittest.TestCase):
+    """Acknowledgement evidence must fail closed unless explicitly verified.
+
+    Regression cover for the reviewed fail-open on PR #882: an absent ``acks``
+    key collapsed to ``{}`` and was read as "no other live sessions required to
+    acknowledge", so a proof minted clean and the restart gate allowed while the
+    impact report still showed other live sessions.
+    """
+
+    def _state(self, **overrides) -> dict:
+        state = _clean_drain_state()
+        state.pop("acks", None)
+        state["ack_timeout_policy_applied"] = False
+        state.update(overrides)
+        return state
+
+    def _acks_check(self, proof) -> dp.DrainCheck:
+        return next(c for c in proof.checks if c.name == dp.CHECK_ACKS_OR_TIMEOUT)
+
+    def _build(self, report: dict, state: dict):
+        return dp.build_drain_proof(
+            impact_report=report, drain_state=state, now=NOW, secret=SECRET
+        )
+
+    def assertAcksFailClosed(self, report: dict, state: dict) -> None:
+        proof = self._build(report, state)
+        self.assertFalse(self._acks_check(proof).passed)
+        self.assertIn(dp.CHECK_ACKS_OR_TIMEOUT, proof.failed_checks)
+        self.assertFalse(proof.clean)
+
+    # --- missing / null / empty / malformed ------------------------------
+
+    def test_missing_acks_key_with_live_sessions_fails_closed(self):
+        """The exact reviewed defect: absent key, three other live sessions."""
+        report = _drained_report_with_live_sessions(3)
+        self.assertEqual(report["counts"]["sessions_live_other"], 3)
+        state = self._state()
+        self.assertNotIn("acks", state)
+        proof = self._build(report, state)
+        check = self._acks_check(proof)
+        self.assertFalse(check.passed)
+        self.assertNotIn("no other live sessions", check.detail)
+        self.assertIn("fail closed", check.detail)
+        self.assertFalse(proof.clean)
+        self.assertEqual(proof.failed_checks, [dp.CHECK_ACKS_OR_TIMEOUT])
+
+    def test_none_acks_with_live_sessions_fails_closed(self):
+        self.assertAcksFailClosed(
+            _drained_report_with_live_sessions(2), self._state(acks=None)
+        )
+
+    def test_empty_acks_with_live_sessions_fails_closed(self):
+        self.assertAcksFailClosed(
+            _drained_report_with_live_sessions(1), self._state(acks={})
+        )
+
+    def test_malformed_acks_fail_closed(self):
+        for malformed in ([], "ack", 7, ("ack",), True):
+            with self.subTest(malformed=malformed):
+                self.assertAcksFailClosed(
+                    _drained_report_with_live_sessions(1),
+                    self._state(acks=malformed),
+                )
+
+    # --- stale / unproven values -----------------------------------------
+
+    def test_stale_or_unproven_ack_values_fail_closed(self):
+        for value in ("pending", "stale", "unknown", "", None, True, 1, NOW):
+            with self.subTest(value=value):
+                self.assertAcksFailClosed(
+                    _drained_report_with_live_sessions(1),
+                    self._state(acks={"prgs-author-0": value}),
+                )
+
+    def test_partial_coverage_fails_closed(self):
+        """Fewer acknowledgements than the report's live-session count."""
+        self.assertAcksFailClosed(
+            _drained_report_with_live_sessions(3),
+            self._state(acks={"prgs-author-0": "ack"}),
+        )
+
+    def test_one_unacked_entry_among_many_fails_closed(self):
+        self.assertAcksFailClosed(
+            _drained_report_with_live_sessions(2),
+            self._state(acks={"prgs-author-0": "ack", "prgs-author-1": "pending"}),
+        )
+
+    def test_unproven_live_session_count_fails_closed(self):
+        """A missing or malformed count cannot prove nobody had to acknowledge."""
+        malformed_counts = (
+            None,
+            {},
+            {"sessions_live_other": None},
+            {"sessions_live_other": "3"},
+            {"sessions_live_other": -1},
+            {"sessions_live_other": True},
+        )
+        for counts in malformed_counts:
+            with self.subTest(counts=counts):
+                report = _drained_report_with_live_sessions(0)
+                if counts is None:
+                    report.pop("counts", None)
+                else:
+                    report["counts"] = counts
+                self.assertAcksFailClosed(report, self._state())
+
+    # --- valid evidence still passes -------------------------------------
+
+    def test_complete_valid_acks_pass(self):
+        report = _drained_report_with_live_sessions(2)
+        state = self._state(
+            acks={"prgs-author-0": "ack", "prgs-author-1": "acknowledged"}
+        )
+        proof = self._build(report, state)
+        self.assertTrue(self._acks_check(proof).passed)
+        self.assertTrue(proof.clean)
+        self.assertEqual(proof.failed_checks, [])
+
+    def test_no_other_live_sessions_still_passes(self):
+        """Intended behavior retained: zero live sessions needs no acks."""
+        report = _drained_report_with_live_sessions(0)
+        self.assertEqual(report["counts"]["sessions_live_other"], 0)
+        proof = self._build(report, self._state())
+        check = self._acks_check(proof)
+        self.assertTrue(check.passed)
+        self.assertIn("sessions_live_other=0", check.detail)
+        self.assertTrue(proof.clean)
+
+    # --- timeout policy cannot become a second fail-open ------------------
+
+    def test_unproven_timeout_policy_cannot_open_the_gate(self):
+        for value in (None, "true", "yes", 1, "True", [], {}):
+            with self.subTest(value=value):
+                self.assertAcksFailClosed(
+                    _drained_report_with_live_sessions(2),
+                    self._state(ack_timeout_policy_applied=value),
+                )
+
+    def test_explicit_timeout_policy_permits(self):
+        proof = self._build(
+            _drained_report_with_live_sessions(2),
+            self._state(ack_timeout_policy_applied=True),
+        )
+        check = self._acks_check(proof)
+        self.assertTrue(check.passed)
+        self.assertIn("timeout policy", check.detail)
+        self.assertTrue(proof.clean)
+
+    # --- the gate itself must deny ---------------------------------------
+
+    def test_failed_ack_check_denies_the_restart_gate(self):
+        report = _drained_report_with_live_sessions(3)
+        proof = self._build(report, self._state())
+        self.assertFalse(proof.clean)
+        decision = dp.gate_apply_restart(
+            proof=proof.as_dict(),
+            now=NOW,
+            secret=SECRET,
+            expected_impact_fingerprint=dp.impact_fingerprint(report),
+        )
+        self.assertFalse(decision.allow)
+        self.assertEqual(decision.verdict, dp.GATE_DENY)
+        self.assertIsNotNone(decision.incident)
+
+    def test_unclean_ack_proof_fails_verification(self):
+        report = _drained_report_with_live_sessions(3)
+        proof = self._build(report, self._state())
+        result = dp.verify_drain_proof(
+            proof.as_dict(),
+            now=NOW,
+            secret=SECRET,
+            expected_impact_fingerprint=dp.impact_fingerprint(report),
+        )
+        self.assertFalse(result.valid)
+        self.assertFalse(result.clean)
+
+
 if __name__ == "__main__":
     unittest.main()