Fix race between create subnet and port requests

When creating an IPv6 auto-address subnet for a network, ports can be
created or updated concurrently in the same network, before the subnet
creation concludes. In such a situation, an IpAddressAlreadyAllocated
exception may be raised, because the subnet create request attempts to
allocate auto addresses to the concurrently created or updated ports,
which have been already allocated by the port requests.

This patchset adds code to the IPAM to skip the attempt to assign the
auto address to ports if they already have received them.

Change-Id: If1eb4046865f43b15ba97c52e2d0b9343dc72c19
Closes-Bug: #1655567

neutron/db/ipam_pluggable_backend.py

@@ -476,12 +476,12 @@ class IpamPluggableBackend(ipam_backend_mixin.IpamBackendMixin):
                 LOG.debug("Requesting with IP request: %s port: %s ip: %s "
                           "for subnet %s and ipam_subnet %s", ip_request,
                           port, ip, subnet, ipam_subnet)
-                ip_address = ipam_subnet.allocate(ip_request)
-                allocated = models_v2.IPAllocation(network_id=network_id,
-                                                   port_id=port['id'],
-                                                   ip_address=ip_address,
-                                                   subnet_id=subnet['id'])
                 try:
+                    ip_address = ipam_subnet.allocate(ip_request)
+                    allocated = models_v2.IPAllocation(network_id=network_id,
+                                                       port_id=port['id'],
+                                                       ip_address=ip_address,
+                                                       subnet_id=subnet['id'])
                     # Do the insertion of each IP allocation entry within
                     # the context of a nested transaction, so that the entry
                     # is rolled back independently of other entries whenever
@@ -499,6 +499,10 @@ class IpamPluggableBackend(ipam_backend_mixin.IpamBackendMixin):
                     except Exception:
                         LOG.debug("Reverting IP allocation failed for %s",
                                   ip_address)
+                except ipam_exc.IpAddressAlreadyAllocated:
+                    LOG.debug("Port %s got IPv6 auto-address in a concurrent "
+                              "create or update port request. Ignoring.",
+                              port['id'])
             return updated_ports
 
     def allocate_subnet(self, context, network, subnet, subnetpool_id):

neutron/tests/unit/db/test_db_base_plugin_v2.py

@@ -56,6 +56,7 @@ from neutron.db.models import securitygroup as sg_models
 from neutron.db import models_v2
 from neutron.db import rbac_db_models
 from neutron.db import standard_attr
+from neutron.ipam.drivers.neutrondb_ipam import driver as ipam_driver
 from neutron.ipam import exceptions as ipam_exc
 from neutron.tests import base
 from neutron.tests import tools
@@ -4342,7 +4343,8 @@ class TestSubnetsV2(NeutronDbPluginV2TestCase):
 
     def _test_create_subnet_ipv6_auto_addr_with_port_on_network(
             self, addr_mode, device_owner=DEVICE_OWNER_COMPUTE,
-            insert_db_reference_error=False, insert_port_not_found=False):
+            insert_db_reference_error=False, insert_port_not_found=False,
+            insert_address_allocated=False):
         # Create a network with one IPv4 subnet and one port
         with self.network() as network,\
             self.subnet(network=network) as v4_subnet,\
@@ -4373,15 +4375,20 @@ class TestSubnetsV2(NeutronDbPluginV2TestCase):
                 if insert_port_not_found:
                     mock_updated_port.side_effect = lib_exc.PortNotFound(
                         port_id=port['port']['id'])
+                if insert_address_allocated:
+                    mock.patch.object(
+                        ipam_driver.NeutronDbSubnet, '_verify_ip',
+                        side_effect=ipam_exc.IpAddressAlreadyAllocated(
+                            subnet_id=mock.ANY, ip=mock.ANY)).start()
                 v6_subnet = self._make_subnet(self.fmt, network, 'fe80::1',
                                               'fe80::/64', ip_version=6,
                                               ipv6_ra_mode=addr_mode,
                                               ipv6_address_mode=addr_mode)
-            if (insert_db_reference_error
+            if (insert_db_reference_error or insert_address_allocated
                 or device_owner == constants.DEVICE_OWNER_ROUTER_SNAT
                 or device_owner in constants.ROUTER_INTERFACE_OWNERS):
-                # DVR SNAT and router interfaces should not have been
-                # updated with addresses from the new auto-address subnet
+                # DVR SNAT, router interfaces and DHCP ports should not have
+                # been updated with addresses from the new auto-address subnet
                 self.assertEqual(1, len(port['port']['fixed_ips']))
             else:
                 # Confirm that the port has been updated with an address
@@ -4411,6 +4418,14 @@ class TestSubnetsV2(NeutronDbPluginV2TestCase):
             constants.IPV6_SLAAC,
             device_owner=constants.DEVICE_OWNER_DHCP)
 
+    def test_create_subnet_dhcpv6_stateless_with_ip_already_allocated(self):
+        self._test_create_subnet_ipv6_auto_addr_with_port_on_network(
+            constants.DHCPV6_STATELESS, insert_address_allocated=True)
+
+    def test_create_subnet_ipv6_slaac_with_ip_already_allocated(self):
+        self._test_create_subnet_ipv6_auto_addr_with_port_on_network(
+            constants.IPV6_SLAAC, insert_address_allocated=True)
+
     def test_create_subnet_ipv6_slaac_with_router_intf_on_network(self):
         self._test_create_subnet_ipv6_auto_addr_with_port_on_network(
             constants.IPV6_SLAAC,