Do not assume the existence of a trunk bridge since os-vif may have deleted it

Since https://review.opendev.org/c/openstack/neutron/+/837780 and
https://review.opendev.org/c/openstack/os-vif/+/841499 it is possible
that nova/os-vif already deleted the trunk bridge before we get to
handle_trunk_remove() -> unwire_subports_for_trunk() -> _update_trunk_metadata().

In the past we assumed that nobody else deleted the trunk bridge before
us and because of that when os-vif started deleting the trunk bridge we
started producing error messages like:

"Failed to store metadata for trunk ...: Cannot find Bridge with name=tbr-..." and
"Error executing command (ListPortsCommand): ovsdbapp.backend.ovs_idl.idlutils.RowNotFound: Cannot find Bridge with name=tbr-...:

With this patch we no longer assume that the trunk bridge exists in
_update_trunk_metadata().

Change-Id: I2b9202ddf653c0755c2aacb6b88ce8c983bb29dc
Closes-Bug: #2095000
Related-Bug: #1869244
Related-Bug: #1997025

neutron/services/trunk/drivers/openvswitch/agent/ovsdb_handler.py

@@ -450,8 +450,13 @@ class OVSDBHandler:
         :param subport_ids: subports affecting the metadata.
         :param wire: if True subport_ids are added, otherwise removed.
         """
-        trunk_bridge = trunk_bridge or ovs_lib.OVSBridge(
-            utils.gen_trunk_br_name(trunk_id))
+        bridge_name = utils.gen_trunk_br_name(trunk_id)
+        trunk_bridge = trunk_bridge or ovs_lib.OVSBridge(bridge_name)
+        if not trunk_bridge.bridge_exists(bridge_name):
+            LOG.info(
+                "Could not update trunk metadata in a non-existent bridge "
+                "(likely already deleted by nova/os-vif): %s", bridge_name)
+            return
         port = port or self._get_parent_port(trunk_bridge)
         _port_id, _trunk_id, old_subports = self._get_trunk_metadata(port)
         if wire:

neutron/tests/unit/services/trunk/drivers/openvswitch/agent/test_ovsdb_handler.py

@@ -336,3 +336,15 @@ class TestOVSDBHandler(base.BaseTestCase):
         expected_subport_ids = '["foo_subport_1"]'
         self._test__update_trunk_metadata_wire_flag(
             mock_br, False, external_ids, subport_ids, expected_subport_ids)
+
+    @mock.patch('neutron.agent.common.ovs_lib.OVSBridge')
+    def test__update_trunk_metadata_no_tbr_no_raise(self, br):
+        mock_br = br.return_value
+        mock_br.configure_mock(**{'bridge_exists.return_value': False})
+        try:
+            self.ovsdb_handler._update_trunk_metadata(
+                mock_br, None, self.trunk_id, ['foo_subport_1'])
+        except RuntimeError:
+            self.fail(
+                "_update_trunk_metadata() should not raise RuntimeError "
+                "when the trunk bridge does not exist")