fix(webui): compensate stray allocations and keep preview side-effect free

Addresses both blockers from the PR #902 review (review 589) for issue #643.

B1 — an allocator-created assignment could be orphaned and reported as no
mutation.

apply_request re-previews, then re-runs the allocator with apply=True. The CAS
fingerprint hashes only {kind, number} plus exclusions, so a competing lease
taken on the requested unit inside the window leaves the fingerprint identical:
the pin passes, the selection loop skips the now-claimed unit and commits an
assignment on the *next* one, and _selection_matches then fails on egress. The
old code returned mutation_performed False with that lease still committed and
owned by a synthetic session nothing heartbeats. The window contains a second
full load_queue_snapshot(), so it is seconds wide, and foreign sessions acting
on this repo concurrently are an observed condition.

The egress mismatch now releases the assignment the allocator created before
refusing. When the release succeeds the refusal reports mutation_performed
False and a compensation record; when it fails the response carries
mutation_performed True, an explicit orphaned_assignment, and a
gitea_release_workflow_lease reclaim action, because claiming nothing changed
while a lease is live is the defect rather than a report of it.

The same state was reachable through _run_allocator's bare except Exception:
allocate_next_work only catches InvalidWorkKindError, LeaseRequiredError and
ControlPlaneError, so anything raised after assign_and_lease committed arrived
as "no result" with a durable lease. A None result on the apply path now sweeps
and releases whatever this flow's session owns. That sweep is only possible
because of the B2 fix below — the session id is now stable across the flow, so
the lease is findable.

B2 — the "read-only" preview wrote to the control-plane DB.

allocate_next_work called db.upsert_session and db.expire_stale_leases
unconditionally, before the apply branch was consulted, and default_allocator
minted a fresh webui-request-<hex> per call. Every preview therefore appended a
never-reused session row and mutated global lease state while the payload said
dry_run True / mutation_performed False, driven by an operator refreshing a
form. One apply wrote two rows and bound the lease to the second, which is why
B1's orphan had no reclaimable owner.

allocate_next_work gains a keyword-only side_effect_free flag, default False so
every existing caller is byte-for-byte unchanged. Under the flag both writes are
suppressed and expired leases are instead filtered out of the claim map in
memory, which reaches the same selection the sweep would have produced without
persisting anything; a claim whose expiry cannot be parsed is kept, since an
unreadable expiry is not evidence that work is free. side_effect_free with
apply=True fails closed rather than silently reserving. request_service mints
one session id per request flow and threads it through both the dry-run and the
apply, and the dry-run now routes through the side-effect-free path.

Coverage.

test_allocator_drift_on_apply_is_not_read_as_an_assignment asserted the defect —
it built the orphan state and then required mutation_performed to be False, which
a leak satisfies. It now requires the compensating release. Added: release-failure
surfacing a reclaim action, an assignment with no lease id, the post-commit
exception route, and session-id identity across the flow. The two areas the review
named as having zero coverage now have it: default_allocator past its two
fail-closed early returns (side_effect_free routing, session-id pass-through and
minting, scope and fingerprint propagation) and default_claims_source (scoped
read, and an unreadable substrate denying rather than reading as "nothing
claimed"). allocator_service gains side-effect-free tests against a real
temp-file DB plus the expiry-filter unit tests.

Every new guard was mutation-tested by reverting it one at a time: B1
compensation removed → 3 failures; post-commit sweep removed → 1; session id
re-minted per call → 2; side_effect_free ignored → 3; in-memory expiry filter
removed → 1.

Verification: WEBUI_TEST_OFFLINE=1 python -m pytest tests/ -q from this
branches/ worktree gives 23 failed, 5263 passed, 6 skipped, 899 subtests. The
sorted FAILED set is identical to the reviewer's clean-master baseline at
2f4dec83 (23 failed, 5190 passed) — no new, changed, or disappeared failure —
and 21 tests were added over the reviewed head's 5242. Zero conflict markers;
py_compile passes; git diff --check clean.

Refs #643, PR #902

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01V6xFqovhbArPv61j9KCGkL

tests/test_allocator_service.py

@@ -7,6 +7,7 @@ import tempfile
 import threading
 import unittest
 from concurrent.futures import ThreadPoolExecutor, as_completed
+from datetime import datetime, timezone
 
 from allocator_service import (
     OUTCOME_ASSIGNED,
@@ -15,6 +16,7 @@ from allocator_service import (
     OUTCOME_PREVIEW,
     OUTCOME_WAIT,
     WorkCandidate,
+    _drop_expired_claims,
     allocate_next_work,
     candidate_from_dict,
     classify_skip,
@@ -362,5 +364,161 @@ class AllocatorServiceTest(unittest.TestCase):
         self.assertIn("unavailable", res["reasons"][0].lower())
 
 
+class SideEffectFreeAllocationTest(unittest.TestCase):
+    """``side_effect_free`` dry runs write nothing to the control plane (#643).
+
+    A plain ``apply=False`` still called ``upsert_session`` and
+    ``expire_stale_leases`` before the apply branch was consulted, so a caller
+    advertising a read-only preview mutated on every call — one unreferenced
+    session row per preview, plus a global lease sweep.
+    """
+
+    def setUp(self) -> None:
+        self._tmp = tempfile.TemporaryDirectory()
+        self.db = ControlPlaneDB(os.path.join(self._tmp.name, "cp.sqlite3"))
+
+    def tearDown(self) -> None:
+        self._tmp.cleanup()
+
+    def _alloc(self, **kwargs):
+        defaults = dict(
+            db=self.db,
+            session_id="s-preview",
+            role="author",
+            remote="prgs",
+            org="org",
+            repo="repo",
+            candidates=[
+                WorkCandidate(kind="issue", number=643, labels=("status:ready",))
+            ],
+            apply=False,
+            profile_name="prgs-author",
+            username="jcwalker3",
+        )
+        defaults.update(kwargs)
+        return allocate_next_work(**defaults)
+
+    def _session_ids(self) -> set[str]:
+        return {str(r.get("session_id")) for r in self.db.list_sessions()}
+
+    def test_side_effect_free_preview_writes_no_session_row(self):
+        before = self._session_ids()
+        result = self._alloc(side_effect_free=True)
+        self.assertEqual(result["outcome"], OUTCOME_PREVIEW)
+        self.assertEqual(self._session_ids(), before)
+        self.assertNotIn("s-preview", self._session_ids())
+
+    def test_plain_dry_run_still_registers_a_session(self):
+        # The default is unchanged for every existing caller.
+        self._alloc()
+        self.assertIn("s-preview", self._session_ids())
+
+    def test_repeated_previews_do_not_accumulate_rows(self):
+        for index in range(5):
+            self._alloc(side_effect_free=True, session_id=f"s-{index}")
+        self.assertEqual(self._session_ids(), set())
+
+    def test_side_effect_free_does_not_sweep_stale_leases(self):
+        self.db.upsert_session(session_id="owner", role="author", pid=1)
+        assigned = self.db.assign_and_lease(
+            session_id="owner",
+            role="author",
+            remote="prgs",
+            org="org",
+            repo="repo",
+            kind="issue",
+            number=999,
+            lease_ttl_seconds=-60,  # already expired
+        )
+        self.assertEqual(assigned.outcome, "assigned")
+
+        self._alloc(side_effect_free=True)
+
+        # The expired row is still 'active' in the DB: nothing swept it.
+        statuses = {
+            r["lease_id"]: r["status"]
+            for r in self.db.list_leases(
+                remote="prgs", org="org", repo="repo",
+                statuses=("active", "expired"),
+            )
+        }
+        self.assertEqual(statuses.get(assigned.lease_id), "active")
+
+    def test_expired_claims_are_filtered_in_memory_so_work_stays_selectable(self):
+        """The read-only mirror of the sweep: expired claims must not block."""
+        self.db.upsert_session(session_id="owner", role="author", pid=1)
+        self.db.assign_and_lease(
+            session_id="owner",
+            role="author",
+            remote="prgs",
+            org="org",
+            repo="repo",
+            kind="issue",
+            number=643,
+            lease_ttl_seconds=-60,  # expired: must not withhold #643
+        )
+        result = self._alloc(side_effect_free=True)
+        self.assertEqual(result["outcome"], OUTCOME_PREVIEW)
+        self.assertEqual(result["selected"]["number"], 643)
+
+    def test_a_live_claim_still_withholds_the_work(self):
+        self.db.upsert_session(session_id="owner", role="author", pid=1)
+        self.db.assign_and_lease(
+            session_id="owner",
+            role="author",
+            remote="prgs",
+            org="org",
+            repo="repo",
+            kind="issue",
+            number=643,
+            lease_ttl_seconds=3600,
+        )
+        result = self._alloc(side_effect_free=True)
+        self.assertNotEqual(result["outcome"], OUTCOME_ASSIGNED)
+        self.assertNotEqual((result.get("selected") or {}).get("number"), 643)
+
+    def test_side_effect_free_with_apply_fails_closed(self):
+        result = self._alloc(side_effect_free=True, apply=True)
+        self.assertFalse(result["success"])
+        self.assertEqual(result["outcome"], OUTCOME_NO_SAFE)
+        self.assertIsNone(result["assignment"])
+        self.assertIn("incompatible with apply", result["reasons"][0])
+        # And it reserved nothing.
+        self.assertEqual(
+            self.db.list_leases(remote="prgs", org="org", repo="repo"), []
+        )
+
+
+class DropExpiredClaimsTest(unittest.TestCase):
+    """The in-memory expiry filter behind side-effect-free previews (#643)."""
+
+    def test_unparseable_expiry_is_kept_rather_than_assumed_free(self):
+        claims = {
+            ("issue", 1): {"lease_id": "l1", "expires_at": "not-a-date"},
+            ("issue", 2): {"lease_id": "l2"},
+            ("issue", 3): {"lease_id": "l3", "expires_at": None},
+        }
+        self.assertEqual(_drop_expired_claims(claims), claims)
+
+    def test_expired_dropped_and_future_kept(self):
+        now = datetime(2026, 7, 25, 12, 0, tzinfo=timezone.utc)
+        claims = {
+            ("issue", 1): {"expires_at": "2026-07-25T11:59:59+00:00"},
+            ("issue", 2): {"expires_at": "2026-07-25T12:00:01+00:00"},
+            ("issue", 3): {"expires_at": "2026-07-25T12:00:00+00:00"},  # boundary
+        }
+        kept = _drop_expired_claims(claims, now=now)
+        self.assertEqual(set(kept), {("issue", 2)})
+
+    def test_naive_and_zulu_timestamps_are_treated_as_utc(self):
+        now = datetime(2026, 7, 25, 12, 0, tzinfo=timezone.utc)
+        claims = {
+            ("issue", 1): {"expires_at": "2026-07-25T11:00:00"},  # naive, past
+            ("issue", 2): {"expires_at": "2026-07-25T13:00:00Z"},  # zulu, future
+        }
+        kept = _drop_expired_claims(claims, now=now)
+        self.assertEqual(set(kept), {("issue", 2)})
+
+
 if __name__ == "__main__":
     unittest.main()