ovn: support max_tunid option from NB_Global

This option is written by OVN and can be used to determine capacity of
the cluster that may depend on e.g. tunnel types enabled for
encapsulation.

The option is new in 20.09 and may be absent in older releases.  Hence
grace handling of missing option.

While this option is written by OVN regardless of encap types enabled,
it is useful only when VXLAN is enabled in the cluster. In this case,
due to limitations of the protocol header, the maximum number of
segments supported is drastically reduced from what's common when
Geneve is used.

The patch that adds VXLAN support for OVN is at:
I81c016ba9c91282d1bebb40a282077e14ce4bd6b

Change-Id: I8ddd52aa958f7dd07fee5d11913a498d6ad4f34f

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

@@ -409,7 +409,15 @@ class OVNMechanismDriver(api.MechanismDriver):
                                  const.TYPE_VXLAN,
                                  const.TYPE_VLAN])
 
+    def _get_max_tunid(self):
+        try:
+            return int(self._nb_ovn.nb_global.options.get('max_tunid'))
+        except (ValueError, TypeError):
+            # max_tunid may be absent in older OVN versions, return None
+            pass
+
     def _validate_network_segments(self, network_segments):
+        max_tunid = self._get_max_tunid()
         for network_segment in network_segments:
             network_type = network_segment['network_type']
             segmentation_id = network_segment['segmentation_id']
@@ -424,6 +432,12 @@ class OVNMechanismDriver(api.MechanismDriver):
             if not self._is_network_type_supported(network_type):
                 msg = _('Network type %s is not supported') % network_type
                 raise n_exc.InvalidInput(error_message=msg)
+            if segmentation_id and max_tunid and segmentation_id > max_tunid:
+                m = (
+                    _('Segmentation ID should be lower or equal to %d') %
+                    max_tunid
+                )
+                raise n_exc.InvalidInput(error_message=m)
 
     def create_segment_provnet_port(self, resource, event, trigger,
                                     context, segment, payload=None):

neutron/tests/unit/fake_resources.py

@@ -130,6 +130,7 @@ class FakeOvsdbNbOvnIdl(object):
         self.pg_del_ports = mock.Mock()
         self.lsp_get_up = mock.Mock()
         self.nb_global = mock.Mock()
+        self.nb_global.options.get.return_value = '100000'
         self.db_list_rows = mock.Mock()
         self.lsp_list = mock.MagicMock()
         self.db_find = mock.Mock()

neutron/tests/unit/plugins/ml2/drivers/ovn/mech_driver/test_mech_driver.py

@@ -155,6 +155,38 @@ class TestOVNMechanismDriver(test_plugin.Ml2PluginV2TestCase):
                           self.mech_driver._create_neutron_pg_drop)
         self.assertEqual(2, m_ovsdb_api.get_port_group.call_count)
 
+    def test__get_max_tunid_no_key_set(self):
+        self.mech_driver._nb_ovn.nb_global.options.get.return_value = None
+        self.assertIsNone(self.mech_driver._get_max_tunid())
+
+    def test__get_max_tunid_wrong_key_value(self):
+        self.mech_driver._nb_ovn.nb_global.options.get.return_value = '11wrong'
+        self.assertIsNone(self.mech_driver._get_max_tunid())
+
+    def test__get_max_tunid_key_set(self):
+        self.mech_driver._nb_ovn.nb_global.options.get.return_value = '100'
+        self.assertEqual(100, self.mech_driver._get_max_tunid())
+
+    def _test__validate_network_segments_id_succeed(self, val):
+        segment = {
+            "network_type": const.TYPE_VXLAN,
+            "segmentation_id": val,
+            "physical_network": "physnet1",
+        }
+        self.mech_driver._nb_ovn.nb_global.options.get.return_value = '200'
+        self.mech_driver._validate_network_segments([segment])
+
+    def test__validate_network_segments_id_below_max_limit(self):
+        self._test__validate_network_segments_id_succeed(100)
+
+    def test__validate_network_segments_id_eq_max_limit(self):
+        self._test__validate_network_segments_id_succeed(200)
+
+    def test__validate_network_segments_id_above_max_limit(self):
+        self.assertRaises(
+            n_exc.InvalidInput,
+            self._test__validate_network_segments_id_succeed, 300)
+
     @mock.patch.object(ovn_revision_numbers_db, 'bump_revision')
     def test__create_security_group(self, mock_bump):
         self.mech_driver._create_security_group(