Do not raise an error deleting neighbour entry

Ever since https://review.openstack.org/#/c/418609/
merged, changing to use the pyroute2 library for managing
neighbour entries, there has been an up-tick in DVR
error messages - "DVR: Failed updating arp entry" is
happening trying to delete a non-existent entry.

Since the error is not fatal, just catch and ignore it.

Change-Id: If6c4715b85c0bd65f5f58600ce5572fa60651510

neutron/privileged/agent/linux/ip_lib.py

@@ -16,6 +16,7 @@ import socket
 import pyroute2
 from pyroute2.netlink import rtnl
 from pyroute2.netlink.rtnl import ndmsg
+from pyroute2 import NetlinkError
 
 from neutron._i18n import _
 from neutron import privileged
@@ -132,13 +133,19 @@ def delete_neigh_entry(ip_version, ip_address, mac_address, device, namespace,
     :param namespace: The name of the namespace in which to delete the entry
     """
     family = _IP_VERSION_FAMILY_MAP[ip_version]
-    _run_iproute('delete',
+    try:
-                 device,
+        _run_iproute('delete',
-                 namespace,
+                     device,
-                 dst=ip_address,
+                     namespace,
-                 lladdr=mac_address,
+                     dst=ip_address,
-                 family=family,
+                     lladdr=mac_address,
-                 **kwargs)
+                     family=family,
+                     **kwargs)
+    except NetlinkError as e:
+        # trying to delete a non-existent entry shouldn't raise an error
+        if e.code == errno.ENOENT:
+            return
+        raise
 
 
 @privileged.default.entrypoint

neutron/tests/functional/agent/linux/test_ip_lib.py

@@ -261,6 +261,16 @@ class IpLibTestCase(IpLibTestFramework):
             ip_lib.dump_neigh_entries(4, device="nosuchdevice",
                                       namespace=attr.namespace)
 
+    def test_delete_neigh_entries(self):
+        attr = self.generate_device_details(
+            ip_cidrs=["%s/24" % TEST_IP, "fd00::1/64"]
+        )
+        mac_address = utils.get_random_mac('fa:16:3e:00:00:00'.split(':'))
+        device = self.manage_device(attr)
+
+        # trying to delete a non-existent entry shouldn't raise an error
+        device.neigh.delete(TEST_IP_NEIGH, mac_address)
+
     def _check_for_device_name(self, ip, name, should_exist):
         exist = any(d for d in ip.get_devices() if d.name == name)
         self.assertEqual(should_exist, exist)

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

@@ -20,12 +20,14 @@ import netaddr
 from neutron_lib import exceptions
 import pyroute2
 from pyroute2.netlink.rtnl import ndmsg
+from pyroute2 import NetlinkError
 import testtools
 
 from neutron.agent.common import utils  # noqa
 from neutron.agent.linux import ip_lib
 from neutron.common import exceptions as n_exc
 from neutron import privileged
+from neutron.privileged.agent.linux import ip_lib as priv_lib
 from neutron.tests import base
 
 NETNS_SAMPLE = [
@@ -1642,6 +1644,12 @@ class TestIpNeighCommand(TestIPCmdBase):
             family=2,
             ifindex=1)
 
+    @mock.patch.object(priv_lib, '_run_iproute')
+    def test_delete_entry_not_exist(self, mock_run_iproute):
+        # trying to delete a non-existent entry shouldn't raise an error
+        mock_run_iproute.side_effect = NetlinkError(errno.ENOENT, None)
+        self.neigh_cmd.delete('192.168.45.100', 'cc:dd:ee:ff:ab:cd')
+
     @mock.patch.object(pyroute2, 'NetNS')
     def test_dump_entries(self, mock_netns):
         mock_netns_instance = mock_netns.return_value