ip_lib: support creating Linux dummy interface

This is for use by a DHCP agent interface driver in the
networking-calico project.  networking-calico connects VMs without
using bridging, so it needs an unbridged DHCP port interface with
which it can associate the DHCP subnet prefix, and the Linux dummy
interface (in conjunction with use of Dnsmasq's --bridge-interface
feature) is suitable for that purpose.

Partial-Bug: #1486649
Change-Id: I5485c187bc44bac3c2942c4fc9e5a502912a81a2

neutron/agent/linux/ip_lib.py

@@ -152,6 +152,11 @@ class IPWrapper(SubProcessBase):
         """Delete a virtual interface between two namespaces."""
         self._as_root([], 'link', ('del', name))
 
+    def add_dummy(self, name):
+        """Create a Linux dummy interface with the given name."""
+        self._as_root([], 'link', ('add', name, 'type', 'dummy'))
+        return IPDevice(name, namespace=self.namespace)
+
     def ensure_namespace(self, name):
         if not self.netns.exists(name):
             ip = self.netns.add(name)

neutron/tests/functional/agent/linux/test_ip_lib.py

@@ -24,6 +24,7 @@ from neutron.agent.common import config
 from neutron.agent.linux import interface
 from neutron.agent.linux import ip_lib
 from neutron.common import utils
+from neutron.tests.common import net_helpers
 from neutron.tests.functional.agent.linux import base
 from neutron.tests.functional import base as functional_base
 
@@ -164,3 +165,16 @@ class IpLibTestCase(IpLibTestFramework):
 
         routes = ip_lib.get_routing_table(4, namespace=attr.namespace)
         self.assertEqual(expected_routes, routes)
+
+    def _check_for_device_name(self, ip, name, should_exist):
+        exist = any(d for d in ip.get_devices() if d.name == name)
+        self.assertEqual(should_exist, exist)
+
+    def test_dummy_exists(self):
+        namespace = self.useFixture(net_helpers.NamespaceFixture())
+        dev_name = base.get_rand_name()
+        device = namespace.ip_wrapper.add_dummy(dev_name)
+        self.addCleanup(self._safe_delete_device, device)
+        self._check_for_device_name(namespace.ip_wrapper, dev_name, True)
+        device.link.delete()
+        self._check_for_device_name(namespace.ip_wrapper, dev_name, False)

neutron/tests/unit/agent/linux/test_ip_lib.py

@@ -313,6 +313,14 @@ class TestIpWrapper(base.BaseTestCase):
                                              run_as_root=True, namespace=None,
                                              log_fail_as_error=True)
 
+    def test_add_dummy(self):
+        ip_lib.IPWrapper().add_dummy('dummy0')
+        self.execute.assert_called_once_with([], 'link',
+                                             ('add', 'dummy0',
+                                              'type', 'dummy'),
+                                             run_as_root=True, namespace=None,
+                                             log_fail_as_error=True)
+
     def test_get_device(self):
         dev = ip_lib.IPWrapper(namespace='ns').device('eth0')
         self.assertEqual(dev.namespace, 'ns')