dhcp: handle advertise_mtu=True when plugin does not set mtu values

DHCP agent may be used by plugins that don't set mtu value for networks.
Handle the case by not passing the DHCP option when network does not
have the value set.

Most plugins do set the value though, since it's enforced in base db
plugin class.

Closes-Bug: #1534197
Change-Id: I282b3d6b81f91eb8cea901d955cbcca6ecb2a95d

neutron/agent/linux/dhcp.py

@@ -368,7 +368,7 @@ class Dnsmasq(DhcpLocalProcess):
                 possible_leases += cidr.size
 
         if cfg.CONF.advertise_mtu:
-            mtu = self.network.mtu
+            mtu = getattr(self.network, 'mtu', 0)
             # Do not advertise unknown mtu
             if mtu > 0:
                 cmd.append('--dhcp-option-force=option:mtu,%d' % mtu)

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

@@ -1037,7 +1037,9 @@ class TestDnsmasq(TestBase):
                 possible_leases += netaddr.IPNetwork(s.cidr).size
 
         if cfg.CONF.advertise_mtu:
-            expected.append('--dhcp-option-force=option:mtu,%s' % network.mtu)
+            if hasattr(network, 'mtu'):
+                expected.append(
+                    '--dhcp-option-force=option:mtu,%s' % network.mtu)
 
         expected.append('--dhcp-lease-max=%d' % min(
             possible_leases, max_leases))
@@ -1152,6 +1154,12 @@ class TestDnsmasq(TestBase):
         self._test_spawn(['--conf-file=', '--domain=openstacklocal'],
                          network)
 
+    def test_spawn_cfg_advertise_mtu_plugin_doesnt_pass_mtu_value(self):
+        cfg.CONF.set_override('advertise_mtu', True)
+        network = FakeV4Network()
+        self._test_spawn(['--conf-file=', '--domain=openstacklocal'],
+                         network)
+
     def _test_output_init_lease_file(self, timestamp):
         expected = [
             '00:00:80:aa:bb:cc 192.168.0.2 * *',