DHCP: release DHCP port if not enough memory

When the DHCP agent fails to create a namespace for the DHCP
service we will release the DHCP port instead of failing silently.

This will at least give the user an indication that there is no DHCP
service. No DHCP port will exist.

Change-Id: I59af745d3991e6deb424ecd9b916b03f146c246a
Closes-bug: #1544548

neutron/agent/linux/dhcp.py

@@ -24,6 +24,7 @@ import netaddr
 from oslo_config import cfg
 from oslo_log import log as logging
 import oslo_messaging
+from oslo_utils import excutils
 from oslo_utils import uuidutils
 import six
 
@@ -1202,11 +1203,19 @@ class DeviceManager(object):
                                          namespace=network.namespace):
             LOG.debug('Reusing existing device: %s.', interface_name)
         else:
+            try:
                 self.driver.plug(network.id,
                                  port.id,
                                  interface_name,
                                  port.mac_address,
                                  namespace=network.namespace)
+            except Exception:
+                with excutils.save_and_reraise_exception():
+                    LOG.exception(_LE('Unable to plug DHCP port for '
+                                      'network %s. Releasing port.'),
+                                  network.id)
+                    self.plugin.release_dhcp_port(network.id, port.device_id)
+
             self.fill_dhcp_udp_checksums(namespace=network.namespace)
         ip_cidrs = []
         for fixed_ip in port.fixed_ips:

neutron/tests/unit/agent/dhcp/test_agent.py

@@ -1336,6 +1336,21 @@ class TestDeviceManager(base.BaseTestCase):
                           'device_id': mock.ANY}})])
         self.assertIn(fake_dhcp_port, net.ports)
 
+    def test_setup_plug_exception(self):
+        plugin = mock.Mock()
+        plugin.create_dhcp_port.return_value = fake_dhcp_port
+        self.ensure_device_is_ready.return_value = False
+        self.mock_driver.get_device_name.return_value = 'tap12345678-12'
+        dh = dhcp.DeviceManager(cfg.CONF, plugin)
+        dh._set_default_route = mock.Mock()
+        dh._cleanup_stale_devices = mock.Mock()
+        dh.driver = mock.Mock()
+        dh.driver.plug.side_effect = OSError()
+        net = copy.deepcopy(fake_network)
+        self.assertRaises(OSError, dh.setup, net)
+        plugin.release_dhcp_port.assert_called_once_with(
+            net.id, mock.ANY)
+
     def test_setup_ipv6(self):
         self._test_setup_helper(True, net=fake_network_ipv6,
                                 port=fake_ipv6_port)