fix(serve): guard against NULL request_router in update_deployment_config

When AsyncioRouter.update_deployment_config() is called before the
request_router has been lazily initialised (e.g. request_router_class
is None, or the first config update arrives before any replica is
assigned), the self.request_router property returns None.

The previous code unconditionally evaluated:
    len(self.request_router.curr_replicas)
which raises AttributeError: 'NoneType' object has no attribute
'curr_replicas'.

Fix: cache the property result in a local variable and fall back to 0
when it is None.  Zero is semantically correct because no replicas are
active at that point.

Regression tests added in TestUpdateDeploymentConfigNullRouter:
- test_update_deployment_config_with_null_router_no_exception
- test_update_deployment_config_with_null_router_passes_zero_replicas
- test_update_deployment_config_with_initialized_router_passes_correct_count

Signed-off-by: chenshi5012 <chenshi5012@163.com>

Author: chenshi5012

python/ray/serve/_private/router.py

@@ -796,9 +796,15 @@ def update_deployment_config(self, deployment_config: DeploymentConfig):
                 max_backoff_s=self._max_backoff_s,
             )
 
+        # Guard against the case where request_router is None (e.g., when
+        # request_router_class is None and lazy initialization has not yet
+        # occurred). In that scenario there are no active replicas yet, so
+        # passing 0 is the correct and safe value.
+        _router = self.request_router
+        curr_num_replicas = len(_router.curr_replicas) if _router is not None else 0
         self._metrics_manager.update_deployment_config(
             deployment_config,
-            curr_num_replicas=len(self.request_router.curr_replicas),
+            curr_num_replicas=curr_num_replicas,
         )
 
     async def _resolve_request_arguments(

python/ray/serve/tests/unit/test_router.py

@@ -1864,6 +1864,120 @@ async def test_multiple_requests_each_trigger_on_request_completed(
         assert completed_ids == expected_ids
 
 
+
+@pytest.mark.asyncio
+class TestUpdateDeploymentConfigNullRouter:
+    """Regression tests for the NULL-deref fix in AsyncioRouter.update_deployment_config.
+
+    Before the fix, calling update_deployment_config() when request_router is
+    not yet initialized (i.e. self.request_router returns None) would raise an
+    AttributeError because the code unconditionally called
+    ``len(self.request_router.curr_replicas)``.
+    """
+
+    async def test_update_deployment_config_with_null_router_no_exception(self):
+        """update_deployment_config must not raise when request_router is None.
+
+        Reproduces the scenario where request_router_class is None and
+        _request_router has not been initialised yet.  The property
+        self.request_router returns None in that case, and the old code would
+        crash with:
+          AttributeError: 'NoneType' object has no attribute 'curr_replicas'
+        """
+        router = AsyncioRouter(
+            controller_handle=Mock(),
+            deployment_id=DeploymentID(name="test-deployment"),
+            handle_id="test-handle-id",
+            self_actor_id="test-node-id",
+            handle_source=DeploymentHandleSource.UNKNOWN,
+            event_loop=get_or_create_event_loop(),
+            enable_strict_max_ongoing_requests=False,
+            # Deliberately pass neither request_router nor request_router_class
+            # so that self._request_router remains None and the property
+            # returns None.
+            request_router_class=None,
+            request_router=None,
+            node_id="test-node-id",
+            availability_zone="test-az",
+            prefer_local_node_routing=False,
+            _request_router_initialized_event=asyncio.Event(),
+        )
+        # Should not raise AttributeError.
+        router.update_deployment_config(DeploymentConfig())
+
+    async def test_update_deployment_config_with_null_router_passes_zero_replicas(self):
+        """When request_router is None, curr_num_replicas forwarded to
+        MetricsManager must be 0 (not an exception)."""
+        router = AsyncioRouter(
+            controller_handle=Mock(),
+            deployment_id=DeploymentID(name="test-deployment"),
+            handle_id="test-handle-id",
+            self_actor_id="test-node-id",
+            handle_source=DeploymentHandleSource.UNKNOWN,
+            event_loop=get_or_create_event_loop(),
+            enable_strict_max_ongoing_requests=False,
+            request_router_class=None,
+            request_router=None,
+            node_id="test-node-id",
+            availability_zone="test-az",
+            prefer_local_node_routing=False,
+            _request_router_initialized_event=asyncio.Event(),
+        )
+        captured: dict = {}
+        original = router._metrics_manager.update_deployment_config
+
+        def _capture(deployment_config, curr_num_replicas):
+            captured["curr_num_replicas"] = curr_num_replicas
+            return original(deployment_config, curr_num_replicas)
+
+        router._metrics_manager.update_deployment_config = _capture
+        router.update_deployment_config(DeploymentConfig())
+
+        assert captured["curr_num_replicas"] == 0, (
+            f"Expected 0 replicas when router is None, "
+            f"got {captured['curr_num_replicas']}"
+        )
+
+    async def test_update_deployment_config_with_initialized_router_passes_correct_count(
+        self,
+    ):
+        """When request_router is initialised, curr_num_replicas reflects
+        the actual number of live replicas."""
+        fake_request_router = FakeRequestRouter(use_queue_len_cache=False)
+        r1_id = ReplicaID(
+            unique_id="test-replica-1", deployment_id=DeploymentID(name="test")
+        )
+        fake_request_router.set_replica_to_return(FakeReplica(r1_id))
+
+        router = AsyncioRouter(
+            controller_handle=Mock(),
+            deployment_id=DeploymentID(name="test-deployment"),
+            handle_id="test-handle-id",
+            self_actor_id="test-node-id",
+            handle_source=DeploymentHandleSource.UNKNOWN,
+            event_loop=get_or_create_event_loop(),
+            enable_strict_max_ongoing_requests=False,
+            request_router=fake_request_router,
+            node_id="test-node-id",
+            availability_zone="test-az",
+            prefer_local_node_routing=False,
+            _request_router_initialized_event=asyncio.Event(),
+        )
+        captured: dict = {}
+        original = router._metrics_manager.update_deployment_config
+
+        def _capture(deployment_config, curr_num_replicas):
+            captured["curr_num_replicas"] = curr_num_replicas
+            return original(deployment_config, curr_num_replicas)
+
+        router._metrics_manager.update_deployment_config = _capture
+        router.update_deployment_config(DeploymentConfig())
+
+        assert captured["curr_num_replicas"] == 1, (
+            f"Expected 1 replica, got {captured['curr_num_replicas']}"
+        )
+
+
 @pytest.mark.asyncio
 class TestCustomRequestRouterAPIs:
     """Tests for the new RequestRouter APIs introduced in this branch: