[OVN] Handle RouterNotFound exception in set_gateway_mtu

When handling multiple router update/delete petitions, if they are
executed at the same time, there is a chance that the set_gateway_mtu
tries to update a no-longer-existing router, triggering a
RouterNotFound error from the _get_router_ports function. Now, if there
is not a router, the _get_router_ports function will return an empty
list instead of raising the error.

Closes-Bug: #1961967
Signed-off-by: Elvira García <egarciar@redhat.com>
Change-Id: Ic54214ca4965274cd58c1d3a278d8f3ffcda0a64

neutron/plugins/ml2/drivers/ovn/mech_driver/ovsdb/ovn_client.py

@@ -1242,6 +1242,8 @@ class OVNClient(object):
         self._transaction(commands, txn=txn)
 
     def _get_router_ports(self, context, router_id, get_gw_port=False):
+        # _get_router() will raise a RouterNotFound error if there's no router
+        # with the router_id
         router_db = self._l3_plugin._get_router(context, router_id)
         if get_gw_port:
             return [p.port for p in router_db.attached_ports]
@@ -1489,15 +1491,18 @@ class OVNClient(object):
         is_gw_port = const.DEVICE_OWNER_ROUTER_GW == port.get(
             'device_owner')
         if is_gw_port and ovn_conf.is_ovn_emit_need_to_frag_enabled():
-            network_ids = set([port['network_id'] for port in
-                            self._get_router_ports(admin_context,
-                                port['device_id'])])
-            for net in self._plugin.get_networks(admin_context,
-                                        filters={'id': network_ids}):
-                if net['mtu'] > network['mtu']:
-                    options[ovn_const.OVN_ROUTER_PORT_GW_MTU_OPTION] = str(
-                            network['mtu'])
-                    break
+            try:
+                network_ids = set([port['network_id'] for port in
+                    self._get_router_ports(admin_context, port['device_id'])])
+                for net in self._plugin.get_networks(admin_context,
+                                            filters={'id': network_ids}):
+                    if net['mtu'] > network['mtu']:
+                        options[ovn_const.OVN_ROUTER_PORT_GW_MTU_OPTION] = str(
+                                network['mtu'])
+                        break
+            except l3_exc.RouterNotFound:
+                # Don't add any mtu info if the router no longer exists
+                pass
         return options
 
     def _create_lrouter_port(self, context, router, port, txn=None):

neutron/tests/unit/services/ovn_l3/test_plugin.py

@@ -1620,6 +1620,52 @@ class TestOVNL3RouterPlugin(test_mech_driver.Ml2PluginV2TestCase):
             mock.ANY, self.fake_router_port,
             ovn_const.TYPE_ROUTER_PORTS)
 
+    @mock.patch('neutron.plugins.ml2.plugin.Ml2Plugin.get_network')
+    @mock.patch('neutron.plugins.ml2.plugin.Ml2Plugin.get_networks')
+    @mock.patch('neutron.plugins.ml2.drivers.ovn.mech_driver.ovsdb.'
+                'ovn_client.OVNClient._get_router_ports')
+    @mock.patch('neutron.db.l3_db.L3_NAT_dbonly_mixin.add_router_interface')
+    def test_add_router_interface_need_to_frag_enabled_then_remove(
+            self, ari, grps, gns, gn):
+        config.cfg.CONF.set_override(
+            'ovn_emit_need_to_frag', True, group='ovn')
+        router_id = 'router-id'
+        interface_info = {'port_id': 'router-port-id',
+                'network_id': 'priv-net'}
+        ari.return_value = self.fake_router_interface_info
+        # If we remove the router halfway the return value of
+        # _get_routers_ports will be []
+        grps.side_effect = l3_exc.RouterNotFound(router_id=router_id)
+        self.get_router.return_value = self.fake_router_with_ext_gw
+        network_attrs = {'id': 'prov-net', 'mtu': 1200}
+        prov_net = fake_resources.FakeNetwork.create_one_network(
+                attrs=network_attrs).info()
+        self.fake_router_port['device_owner'] = (
+            constants.DEVICE_OWNER_ROUTER_GW)
+        gn.return_value = prov_net
+        gns.return_value = [self.fake_network]
+
+        self.l3_inst.add_router_interface(self.context, router_id,
+                                          interface_info)
+
+        # Make sure that the "gateway_mtu" option was set to the router port
+        fake_router_port_assert = self.fake_router_port_assert
+        fake_router_port_assert['gateway_chassis'] = mock.ANY
+        fake_router_port_assert['options'] = {}
+
+        self.l3_inst._nb_ovn.add_lrouter_port.assert_called_once_with(
+            **fake_router_port_assert)
+        # Since if_exists = True it will safely return
+        self.l3_inst._nb_ovn.lrp_set_options(
+            name='lrp-router-port-id', if_exists=True,
+            options=fake_router_port_assert)
+        # If no if_exists is provided, it is defaulted to true, so this
+        # operation is safe.
+        self.l3_inst._nb_ovn.set_lrouter_port_in_lswitch_port.\
+            assert_called_once_with(
+                'router-port-id', 'lrp-router-port-id', is_gw_port=True,
+                lsp_address=ovn_const.DEFAULT_ADDR_FOR_LSP_WITH_PEER)
+
     def _test_get_router_availability_zones(self, azs, expected):
         lr = fake_resources.FakeOvsdbRow.create_one_ovsdb_row(
             attrs={'id': 'fake-router', 'external_ids': {