fix: skip unnecessary FOR UPDATE locks in append_event when no app/user state delta

src/google/adk/sessions/database_session_service.py

@@ -531,6 +531,19 @@ async def append_event(self, session: Session, event: Event) -> Event:
     schema = self._get_schema_classes()
     is_sqlite = self.db_engine.dialect.name == _SQLITE_DIALECT
     use_row_level_locking = self._supports_row_level_locking()
+
+    # Pre-analyze which state scopes have deltas so we only acquire
+    # FOR UPDATE locks on rows that will actually be written.
+    # The result is reused later to avoid calling extract_state_delta twice.
+    state_deltas = None
+    has_app_delta, has_user_delta = False, False
+    if event.actions and event.actions.state_delta:
+      state_deltas = _session_util.extract_state_delta(
+          event.actions.state_delta
+      )
+      has_app_delta = bool(state_deltas.get("app"))
+      has_user_delta = bool(state_deltas.get("user"))
+
     async with self._with_session_lock(
         app_name=session.app_name,
         user_id=session.user_id,
@@ -554,7 +567,7 @@ async def append_event(self, session: Session, event: Event) -> Event:
             sql_session=sql_session,
             state_model=schema.StorageAppState,
             predicates=(schema.StorageAppState.app_name == session.app_name,),
-            use_row_level_locking=use_row_level_locking,
+            use_row_level_locking=use_row_level_locking and has_app_delta,
             missing_message=(
                 "App state missing for app_name="
                 f"{session.app_name!r}. Session state tables should be "
@@ -568,7 +581,7 @@ async def append_event(self, session: Session, event: Event) -> Event:
                 schema.StorageUserState.app_name == session.app_name,
                 schema.StorageUserState.user_id == session.user_id,
             ),
-            use_row_level_locking=use_row_level_locking,
+            use_row_level_locking=use_row_level_locking and has_user_delta,
             missing_message=(
                 "User state missing for app_name="
                 f"{session.app_name!r}, user_id={session.user_id!r}. "
@@ -599,11 +612,8 @@ async def append_event(self, session: Session, event: Event) -> Event:
           storage_events = [e async for e in result]
           session.events = [e.to_event() for e in storage_events]
 
-        # Extract state delta
-        if event.actions and event.actions.state_delta:
-          state_deltas = _session_util.extract_state_delta(
-              event.actions.state_delta
-          )
+        # Apply state deltas (reusing pre-analyzed result from above)
+        if state_deltas:
           app_state_delta = state_deltas["app"]
           user_state_delta = state_deltas["user"]
           session_state_delta = state_deltas["session"]

tests/unittests/sessions/test_session_service.py

@@ -1153,3 +1153,72 @@ async def test_prepare_tables_idempotent_after_creation():
     assert session.id == 's1'
   finally:
     await service.close()
+
+
+@pytest.fixture
+async def lock_spy_harness():
+  """Sets up a DatabaseSessionService with a spy on _select_required_state."""
+  service = DatabaseSessionService('sqlite+aiosqlite:///:memory:')
+  try:
+    session = await service.create_session(
+        app_name='my_app', user_id='user', session_id='s1'
+    )
+
+    calls = []
+    original_fn = database_session_service._select_required_state
+
+    async def spy(**kwargs):
+      calls.append(kwargs.get('use_row_level_locking'))
+      return await original_fn(**kwargs)
+
+    with (
+        mock.patch.object(
+            service, '_supports_row_level_locking', return_value=True
+        ),
+        mock.patch.object(
+            database_session_service,
+            '_select_required_state',
+            side_effect=spy,
+        ),
+    ):
+      yield service, session, calls
+  finally:
+    await service.close()
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    'state_delta, expected_app_lock, expected_user_lock',
+    [
+        pytest.param(
+            {'session_key': 'v1'},
+            False,
+            False,
+            id='session_only_delta',
+        ),
+        pytest.param(None, False, False, id='no_state_delta'),
+        pytest.param({'app:key1': 'v1'}, True, False, id='app_delta_only'),
+        pytest.param({'user:key1': 'v1'}, False, True, id='user_delta_only'),
+        pytest.param(
+            {'app:ak': 'av', 'user:uk': 'uv'},
+            True,
+            True,
+            id='both_app_and_user_delta',
+        ),
+    ],
+)
+async def test_append_event_conditional_for_update_locking(
+    lock_spy_harness, state_delta, expected_app_lock, expected_user_lock
+):
+  """FOR UPDATE locks should only be acquired for scopes that have deltas."""
+  service, session, calls = lock_spy_harness
+
+  kwargs = {'invocation_id': 'inv', 'author': 'user'}
+  if state_delta is not None:
+    kwargs['actions'] = EventActions(state_delta=state_delta)
+  event = Event(**kwargs)
+  await service.append_event(session, event)
+
+  assert len(calls) == 2
+  assert calls[0] is expected_app_lock
+  assert calls[1] is expected_user_lock