Add debug logs to OVSInterfaceDriver.plug_new

DVR lifecycle functional tests fail frequently with exceptions
like this: "Network interface * not found in namespace *"
The reproduction locally is hard if possible, so by adding
extra log messages I hope we can have more clue where to find
the possible root cause of this issue.

Change-Id: I2c6acf3c29f3d1e7574240376f7bcb46c98579c6
Related-Bug: #1995031

neutron/agent/linux/interface.py

@@ -401,6 +401,9 @@ class OVSInterfaceDriver(LinuxInterfaceDriver):
             root_dev.disable_ipv6()
         else:
             ns_dev = ip.device(device_name)
+        if not ns_dev:
+            LOG.warning("Device %s is not ready in namespace %s!",
+                        device_name, namespace)
 
         internal = not self.conf.ovs_use_veth
         self._ovs_add_port(bridge, tap_name, port_id, mac_address,
@@ -429,6 +432,13 @@ class OVSInterfaceDriver(LinuxInterfaceDriver):
                 with excutils.save_and_reraise_exception():
                     ovs = ovs_lib.OVSBridge(bridge)
                     ovs.delete_port(tap_name)
+            except Exception as exc:
+                LOG.warning("Failed to plug interface %s to bridge %s in "
+                            "namespace %s due to unknown reason: %s",
+                            device_name, bridge, namespace, str(exc))
+                with excutils.save_and_reraise_exception():
+                    ovs = ovs_lib.OVSBridge(bridge)
+                    ovs.delete_port(tap_name)
 
         # NOTE(ihrachys): the order here is significant: we must set MTU after
         # the device is moved into a namespace, otherwise OVS bridge does not

neutron/tests/unit/agent/linux/test_interface.py

@@ -461,6 +461,7 @@ class TestOVSInterfaceDriver(TestBase):
             expected = [
                 mock.call(),
                 mock.call().device('tap0'),
+                mock.ANY,
                 mock.call().device().link.set_address('aa:bb:cc:dd:ee:ff'),
                 mock.call().device().link.set_address('aa:bb:cc:dd:ee:ff')]
             if namespace: