Release readiness probe database transactions

src/user_engine/adapters/postgres.py

@@ -591,7 +591,12 @@ class PostgresUserEngineStore:
                     """,
                     (LATEST_SCHEMA_VERSION,),
                 )
-                return cursor.fetchone() is not None
+                present = cursor.fetchone() is not None
+            # psycopg starts a transaction even for this readiness SELECT.
+            # End it immediately so probes cannot retain relation locks that
+            # block the next replica's idempotent schema migration.
+            self.connection.rollback()
+            return present
         except Exception:
             self.connection.rollback()
             return False

tests/test_postgres_store_adapter.py

@@ -19,10 +19,12 @@ class PostgresStoreAdapterTests(unittest.TestCase):
         )
 
     def test_ready_is_false_before_migration(self):
-        store = PostgresUserEngineStore(_FakePostgresConnection())
+        connection = _FakePostgresConnection()
+        store = PostgresUserEngineStore(connection)
 
         self.assertFalse(store.ready)
         self.assertIsNone(store.schema_version)
+        self.assertEqual(2, connection.rollback_count)
 
     def test_pending_query_keeps_failed_non_dead_letter_events_retryable(self):
         connection = _FakePostgresConnection()
@@ -49,6 +51,7 @@ class _FakePostgresConnection:
             list[dict[str, Any]],
         ] | None = None
         self.last_sql = ""
+        self.rollback_count = 0
 
     def cursor(self) -> "_FakePostgresCursor":
         return _FakePostgresCursor(self)
@@ -65,6 +68,7 @@ class _FakePostgresConnection:
         self._snapshot = None
 
     def rollback(self) -> None:
+        self.rollback_count += 1
         if self._snapshot is None:
             return
         (