Don't provide MTU value in metadata service if DHCP is enabled

For networks with subnets with enabled DHCP service don't provide
mtu value in the metadata. That way cloud-init will not configure it
"statically" in e.g. netplan's config file and guest OS will use MTU
value provided by the DHCP service.


Closes-Bug: #1899487
Change-Id: Ib775c2210349b72b3dc033554ac6d8b35b8d2d79

nova/tests/unit/virt/test_netutils.py

@@ -17,6 +17,17 @@ from nova.virt import netutils
 
 
 class TestNetUtilsTestCase(test.NoDBTestCase):
+
+    def _get_fake_instance_nw_info(self, num_networks, dhcp_server, mtu):
+        network_info = fake_network.fake_get_instance_nw_info(self,
+                                                              num_networks)
+        for vif in network_info:
+            for subnet in vif['network']['subnets']:
+                subnet['meta']['dhcp_server'] = dhcp_server
+            vif['network']['meta']['mtu'] = mtu
+
+        return network_info
+
     def test_get_cached_vifs_with_vlan_no_nw_info(self):
         # Make sure that an empty dictionary will be returned when
         # nw_info is None
@@ -39,3 +50,15 @@ class TestNetUtilsTestCase(test.NoDBTestCase):
         expected = {'fa:16:3e:d1:28:e4': '2145'}
         self.assertEqual(expected,
                          netutils.get_cached_vifs_with_vlan(network_info))
+
+    def test__get_link_mtu(self):
+        network_info_dhcp = self._get_fake_instance_nw_info(
+            1, '192.168.0.100', 9000)
+        network_info_no_dhcp = self._get_fake_instance_nw_info(
+            1, None, 9000)
+
+        for vif in network_info_dhcp:
+            self.assertIsNone(netutils._get_link_mtu(vif))
+
+        for vif in network_info_no_dhcp:
+            self.assertEqual(9000, netutils._get_link_mtu(vif))

nova/virt/netutils.py

@@ -263,12 +263,19 @@ def _get_eth_link(vif, ifc_num):
         'id': link_id,
         'vif_id': vif['id'],
         'type': nic_type,
-        'mtu': vif['network']['meta'].get('mtu'),
+        'mtu': _get_link_mtu(vif),
         'ethernet_mac_address': vif.get('address'),
     }
     return link
 
 
+def _get_link_mtu(vif):
+    for subnet in vif['network']['subnets']:
+        if subnet['meta'].get('dhcp_server'):
+            return None
+    return vif['network']['meta'].get('mtu')
+
+
 def _get_nets(vif, subnet, version, net_num, link_id):
     """Get networks for the given VIF and subnet
 

releasenotes/notes/Do-not-send-mtu-value-in-metadata-for-networks-with-enabled-dhcp-641506f2a13b540f.yaml

@@ -0,0 +1,5 @@
+---
+other:
+  - |
+    For networks which have any subnets with enabled DHCP, MTU value is not send
+    in the metadata. In such case MTU is configured through the DHCP server.