Fix DHCP Port Creation on Service Subnets

This changes the IPAM logic for service subnets to allow DHCP ports
to be created on subnets of any service type, provided that the subnet
has DHCP enabled.. This ensures that DHCP ports can still be created
automatically on 'nova:compute' service subnets, for example.

DocImpact

Change-Id: I736262f2a7d3f3d7dfdc5276e2364aca7187d18b
Closes-Bug: #1636963
(cherry picked from commit 1800ae63c2)

neutron/db/ipam_backend_mixin.py

@@ -595,12 +595,18 @@ class IpamBackendMixin(db_base_plugin_common.DbBasePluginCommon):
         return query.filter(models_v2.Subnet.network_id == network_id)
 
     def _query_filter_service_subnets(self, query, service_type):
+        Subnet = models_v2.Subnet
         ServiceType = sst_model.SubnetServiceType
         query = query.add_entity(ServiceType)
         query = query.outerjoin(ServiceType)
-        query = query.filter(or_(ServiceType.service_type.is_(None),
-                                 ServiceType.service_type == service_type))
-        return query.from_self(models_v2.Subnet)
+        query = query.filter(or_(
+            ServiceType.service_type.is_(None),
+            ServiceType.service_type == service_type,
+            # Allow DHCP ports to be created on subnets of any
+            # service type when DHCP is enabled on the subnet.
+            and_(Subnet.enable_dhcp.is_(True),
+                 service_type == const.DEVICE_OWNER_DHCP)))
+        return query.from_self(Subnet)
 
     @staticmethod
     def _query_filter_by_segment_host_mapping(query, host):

neutron/tests/unit/extensions/test_subnet_service_types.py

@@ -55,7 +55,7 @@ class SubnetServiceTypesExtensionTestCase(
               self).setUp(plugin=plugin, ext_mgr=ext_mgr)
 
     def _create_service_subnet(self, service_types=None, cidr=None,
-                               network=None):
+                               network=None, enable_dhcp=False):
         if not network:
             with self.network() as network:
                 pass
@@ -65,7 +65,8 @@ class SubnetServiceTypesExtensionTestCase(
         args = {'net_id': network['id'],
                 'tenant_id': network['tenant_id'],
                 'cidr': cidr,
-                'ip_version': self.IP_VERSION}
+                'ip_version': self.IP_VERSION,
+                'enable_dhcp': enable_dhcp}
         if service_types:
             args['service_types'] = service_types
         return self._create_subnet(self.fmt, **args)
@@ -274,6 +275,24 @@ class SubnetServiceTypesExtensionTestCase(
     def test_create_port_exhausted_subnet_no_fallback(self):
         self.test_create_port_exhausted_subnet(fallback=False)
 
+    def test_create_dhcp_port_compute_subnet(self, enable_dhcp=True):
+        with self.network() as network:
+            pass
+        res = self._create_service_subnet(['compute:nova'],
+                                          network=network,
+                                          enable_dhcp=enable_dhcp)
+        subnet = self.deserialize('json', res)['subnet']
+        network = network['network']
+        port = self._create_port(self.fmt,
+                                 net_id=network['id'],
+                                 tenant_id=network['tenant_id'],
+                                 fixed_ips=[{'subnet_id': subnet['id']}],
+                                 device_owner='network:dhcp')
+        self._assert_port_res(port, 'network:dhcp', subnet, enable_dhcp)
+
+    def test_create_dhcp_port_compute_subnet_no_dhcp(self):
+        self.test_create_dhcp_port_compute_subnet(enable_dhcp=False)
+
 
 class SubnetServiceTypesExtensionTestCasev6(
         SubnetServiceTypesExtensionTestCase):

releasenotes/notes/dhcp-for-service-subnets-06f03d902f2fb738.yaml

@@ -0,0 +1,10 @@
+---
+fixes:
+  - A special case has been added to allow the creation of DHCP ports
+    on Service Subnets that do not have the service type "network:dhcp",
+    provided that the subnet has 'enable_dhcp' set to 'True'.
+    This fixes the recurring error seen when neutron attempts to
+    automatically create a DHCP port on a dhcp-enabled subnet after the
+    subnet is created. See bug report
+    `1636963 <https://bugs.launchpad.net/neutron/+bug/1636963>`_ for
+    more details.