Fix KeyError failure in _sync_subnet_dhcp_options()

If the netron-ovn-db-sync-util is run while neutron-server
is active (which is not recommended), it can randomly fail
if there are active API calls in flight to create networks
and/or subnets.

Skip the subnet and log a warning if detected.

Closes-bug: #2045811
Change-Id: Ic5d9608277dd5c4881b3e4b494e1864be0bed1b4
(cherry picked from commit e4323e1)

Author: brianphaley

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

@@ -870,10 +870,20 @@ def _sync_subnet_dhcp_options(self, ctx, db_networks,
             LOG.warning('DHCP options for subnet %s present in '
                         'Neutron but out of sync with OVN NB DB', subnet_id)
             if self.mode == SYNC_MODE_REPAIR:
+                # If neutron-server is running we could race and find a
+                # subnet without a cached network, just skip it to avoid
+                # a KeyError below.
+                network_id = utils.ovn_name(subnet['network_id'])
+                if network_id not in db_networks:
+                    LOG.warning('Network %s for subnet %s not found in OVN NB '
+                                'DB network cache, possible race condition, '
+                                'please check that neutron-server is stopped! '
+                                'Skipping subnet.', network_id, subnet_id)
+                    continue
                 try:
                     LOG.warning('Adding/Updating DHCP options for subnet %s '
                                 'in OVN NB DB', subnet_id)
-                    network = db_networks[utils.ovn_name(subnet['network_id'])]
+                    network = db_networks[network_id]
                     # _ovn_client._add_subnet_dhcp_options doesn't create
                     # a new row in DHCP_Options if the row already exists.
                     # See commands.AddDHCPOptionsCommand.

neutron/tests/unit/plugins/ml2/drivers/ovn/mech_driver/ovsdb/test_ovn_db_sync.py

@@ -104,6 +104,17 @@ def setUp(self):
                          'gateway_ip': '20.0.0.1',
                          'dns_nameservers': [],
                          'host_routes': [],
+                         'ip_version': 4},
+                        # A subnet without a known network should be skipped,
+                        # see bug #2045811
+                        {'id': 'notfound',
+                         'network_id': 'notfound',
+                         'enable_dhcp': True,
+                         'cidr': '30.0.0.0/24',
+                         'tenant_id': 'tenant1',
+                         'gateway_ip': '30.0.0.1',
+                         'dns_nameservers': [],
+                         'host_routes': [],
                          'ip_version': 4}]
 
         self.security_groups = [