Resolve overrides on the unit's own session, not a second connection

The modelling_e2e Lambda runs on a single-connection pool (pool_size=1,
max_overflow=0) so one invocation uses one Postgres connection. But re-hydrating
a Property through PostgresUnitOfWork resolved its Landlord Overrides through a
PropertyOverridesPostgresReader built from the unit's session *factory* — which
opens a brand-new Session per call. While the unit's own read transaction was
still open (PropertyPostgresRepository.get_many had checked out the connection),
that second Session asked the pool for a second connection, found none, and timed
out after 30s:

  QueuePool limit of size 1 overflow 0 reached, connection timed out, timeout 30.00

The baseline stage (PropertyBaselineOrchestrator.run -> uow.property.get_many ->
landlord overrides) hit this on every invocation.

Read the overrides on the unit's OWN session instead. property_overrides is
committed reference data, so reading it inside the unit's transaction sees the
same rows and keeps the invocation on one connection. Extract the query/mapping
into a shared helper and add OpenSessionPropertyOverridesReader (reads on a
caller-owned, already-open session without closing it) for the unit; the
standalone PropertyOverridesPostgresReader still opens its own short session for
use outside a unit.

Regression test pins the invariant with a real pool_size=1/max_overflow=0 engine:
without the fix it reproduces the exact QueuePool timeout.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

repositories/postgres_unit_of_work.py

@@ -15,7 +15,7 @@ from repositories.product.composite_product_repository import (
     catalogue_with_off_catalogue_overrides,
 )
 from repositories.property.property_overrides_postgres_reader import (
-    PropertyOverridesPostgresReader,
+    OpenSessionPropertyOverridesReader,
 )
 from repositories.property.property_postgres_repository import (
     PropertyPostgresRepository,
@@ -46,10 +46,13 @@ class PostgresUnitOfWork(UnitOfWork):
         spatial_repo = SpatialPostgresRepository(self._session)
         # Fold Landlord Overrides onto the Effective EPC on every re-hydration
         # (ADR-0032), so what the Baseline orchestrator scores off ``uow.property``
-        # matches what the Plan was modelled from. The reader is uow-independent —
-        # ``property_overrides`` is committed reference data — so it opens its own
-        # short read session per call via the same session factory.
-        overrides_reader = PropertyOverridesPostgresReader(self._session_factory)
+        # matches what the Plan was modelled from. ``property_overrides`` is
+        # committed reference data, but the reader must read on THIS uow's session
+        # — a second session opened concurrently checks out a second connection
+        # and deadlocks the single-connection pool while this uow's transaction is
+        # open. Reading committed data inside the uow's transaction is correct and
+        # keeps the invocation on one connection.
+        overrides_reader = OpenSessionPropertyOverridesReader(self._session)
         self.property = PropertyPostgresRepository(
             self._session, epc_repo, spatial_repo, overrides_reader
         )

repositories/property/property_overrides_postgres_reader.py

@@ -2,9 +2,16 @@
 
 Read-only and uow-independent: ``property_overrides`` is committed reference
 data the ``bulk_upload_finaliser`` Lambda writes at Finalise, long before First
-Run executes — there is no transactional coupling to the ingestion run, so this
-opens its own short read session per call via the injected session factory
-(mirroring the composition root's ``lambda: Session(engine)``).
+Run executes — there is no transactional coupling to the ingestion run, so the
+standalone reader opens its own short read session per call via the injected
+session factory (mirroring the composition root's ``lambda: Session(engine)``).
+
+Inside a Unit of Work the overrides must instead be read on the UoW's *own*
+session (``OpenSessionPropertyOverridesReader``): a second session opened
+concurrently checks out a second connection, which deadlocks the modelling_e2e
+Lambda's single-connection pool while the UoW's read transaction is still open.
+Reading committed reference data inside the UoW's transaction is correct — it
+sees the same committed rows — and keeps the invocation on one connection.
 """
 
 from __future__ import annotations
@@ -21,25 +28,43 @@ from repositories.property.property_overrides_reader import (
 )
 
 
+def _resolve_overrides(session: Session, property_id: int) -> ResolvedPropertyOverrides:
+    rows = session.exec(
+        select(PropertyOverrideRow).where(
+            col(PropertyOverrideRow.property_id) == property_id
+        )
+    ).all()
+    return ResolvedPropertyOverrides(
+        rows=tuple(
+            ResolvedPropertyOverride(
+                override_component=row.override_component,
+                building_part=row.building_part,
+                override_value=row.override_value,
+            )
+            for row in rows
+        )
+    )
+
+
 class PropertyOverridesPostgresReader(PropertyOverridesReader):
+    """Opens its own short read session per call — for standalone use outside a
+    Unit of Work, where there is no shared session/connection to reuse."""
+
     def __init__(self, session_factory: Callable[[], Session]) -> None:
         self._session_factory = session_factory
 
     def overrides_for(self, property_id: int) -> ResolvedPropertyOverrides:
         with self._session_factory() as session:
-            rows = session.exec(
-                select(PropertyOverrideRow).where(
-                    col(PropertyOverrideRow.property_id) == property_id
-                )
-            ).all()
+            return _resolve_overrides(session, property_id)
 
-        return ResolvedPropertyOverrides(
-            rows=tuple(
-                ResolvedPropertyOverride(
-                    override_component=row.override_component,
-                    building_part=row.building_part,
-                    override_value=row.override_value,
-                )
-                for row in rows
-            )
-        )
+
+class OpenSessionPropertyOverridesReader(PropertyOverridesReader):
+    """Reads on a caller-owned, already-open session without closing it — for use
+    inside a Unit of Work so resolving overrides reuses the UoW's single
+    connection instead of checking out a second one."""
+
+    def __init__(self, session: Session) -> None:
+        self._session = session
+
+    def overrides_for(self, property_id: int) -> ResolvedPropertyOverrides:
+        return _resolve_overrides(self._session, property_id)

tests/repositories/test_unit_of_work.py

@@ -7,7 +7,7 @@ from typing import Any
 
 import pytest
 from sqlalchemy import Engine
-from sqlmodel import Session
+from sqlmodel import Session, create_engine
 
 from datatypes.epc.domain.epc import Epc
 from datatypes.epc.domain.epc_property_data import (
@@ -152,6 +152,62 @@ def test_unit_hydrates_a_property_with_its_landlord_overrides_folded(
     assert main.wall_insulation_type == 3
 
 
+def test_hydrating_a_property_with_overrides_stays_on_one_connection(
+    db_engine: Engine,
+) -> None:
+    """Resolving Landlord Overrides during re-hydration must read on the unit's
+    OWN session, not open a second one. The modelling_e2e Lambda runs on a
+    single-connection pool (``pool_size=1, max_overflow=0``); a second session
+    opened while the unit's read transaction is still open checks out a second
+    connection and deadlocks ("QueuePool limit of size 1 overflow 0 reached,
+    connection timed out"). This pins that the unit holds exactly one connection
+    while it hydrates a Property that has overrides.
+    """
+    # Arrange — seed a property + EPC + an override (committed reference data).
+    with PostgresUnitOfWork(_session_factory(db_engine)) as uow:
+        row = PropertyRow(portfolio_id=1, postcode="A0 0AA", address="1 St", uprn=1)
+        uow._session.add(row)  # pyright: ignore[reportPrivateUsage]
+        uow._session.flush()  # pyright: ignore[reportPrivateUsage]
+        property_id = row.id
+        assert property_id is not None
+        EpcPostgresRepository(uow._session).save(  # pyright: ignore[reportPrivateUsage]
+            _epc(), property_id=property_id
+        )
+        uow._session.add(  # pyright: ignore[reportPrivateUsage]
+            PropertyOverrideRow(
+                property_id=property_id,
+                portfolio_id=1,
+                building_part=0,
+                override_component="wall_type",
+                override_value="Solid brick, with internal insulation",
+                original_spreadsheet_description="solid brick, insulated",
+            )
+        )
+        uow.commit()
+
+    # A pool that admits exactly one connection and fails fast (not after 30s) if
+    # a second is requested — the production modelling_e2e shape.
+    single_connection = create_engine(
+        db_engine.url, pool_size=1, max_overflow=0, pool_timeout=2
+    )
+    try:
+        # Act — hydrate through the unit; this resolves the override.
+        with PostgresUnitOfWork(_session_factory(single_connection)) as uow:
+            prop = uow.property.get(property_id)
+    finally:
+        single_connection.dispose()
+
+    # Assert — reached here without a QueuePool timeout, and the override folded:
+    # cavity (4) → solid brick (3) / internal (3).
+    main = next(
+        part
+        for part in prop.effective_epc.sap_building_parts
+        if part.identifier is BuildingPartIdentifier.MAIN
+    )
+    assert main.wall_construction == 3
+    assert main.wall_insulation_type == 3
+
+
 def test_leaving_the_block_without_commit_persists_nothing(db_engine: Engine) -> None:
     # Arrange
     new_unit = lambda: PostgresUnitOfWork(_session_factory(db_engine))