Try to enable dnsmasq process several times

Sometimes, during restart of dnsmasq process it may happend that
after process is killed, start attempt is made too fast, before
old process really unbind from IP address on which it was listening.
That causes an issue with starting dnsmasq process again.

In patch [1] disable() method was changed that it can wait
until process is really not active (no pid for it) but that didn't
solve the problem with starting a new dnsmasq process completely and
sometimes it still happens, at least in functional tests.

So now, enable() method is changed so that it will try to enable
dnsmasq process for 1 minute, until it will really be spawned properly.

[1] https://review.openstack.org/#/c/634390/

Change-Id: I18d73b787fa3ab8803e12d5e5eb2bb7109205aba
Closes-Bug: #1811126
(cherry picked from commit 157e09e6af)

neutron/agent/linux/dhcp.py

@@ -217,13 +217,26 @@ class DhcpLocalProcess(DhcpBase):
 
     def enable(self):
         """Enables DHCP for this network by spawning a local process."""
-        if self.active:
-            self.restart()
-        elif self._enable_dhcp():
-            fileutils.ensure_tree(self.network_conf_dir, mode=0o755)
-            interface_name = self.device_manager.setup(self.network)
-            self.interface_name = interface_name
-            self.spawn_process()
+        try:
+            common_utils.wait_until_true(self._enable)
+        except common_utils.WaitTimeout:
+            LOG.error("Failed to start DHCP process for network %s",
+                      self.network.id)
+
+    def _enable(self):
+        try:
+            if self.active:
+                self.restart()
+            elif self._enable_dhcp():
+                fileutils.ensure_tree(self.network_conf_dir, mode=0o755)
+                interface_name = self.device_manager.setup(self.network)
+                self.interface_name = interface_name
+                self.spawn_process()
+            return True
+        except exceptions.ProcessExecutionError as error:
+            LOG.debug("Spawning DHCP process for network %s failed; "
+                      "Error: %s", self.network.id, error)
+            return False
 
     def _get_process_manager(self, cmd_callback=None):
         return external_process.ProcessManager(
@@ -237,10 +250,9 @@ class DhcpLocalProcess(DhcpBase):
     def disable(self, retain_port=False, block=False):
         """Disable DHCP for this network by killing the local process."""
         self.process_monitor.unregister(self.network.id, DNSMASQ_SERVICE_NAME)
-        pm = self._get_process_manager()
-        pm.disable()
+        self._get_process_manager().disable()
         if block:
-            common_utils.wait_until_true(lambda: not pm.active)
+            common_utils.wait_until_true(lambda: not self.active)
         if not retain_port:
             self._destroy_namespace_and_port()
         self._remove_config_files()

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

@@ -19,6 +19,7 @@ import mock
 import netaddr
 from neutron_lib.api.definitions import extra_dhcp_opt as edo_ext
 from neutron_lib import constants
+from neutron_lib import exceptions
 from oslo_config import cfg
 import oslo_messaging
 from oslo_utils import fileutils
@@ -1134,23 +1135,31 @@ class TestDhcpLocalProcess(TestBase):
     @mock.patch.object(fileutils, 'ensure_tree')
     def test_enable(self, ensure_dir):
         attrs_to_mock = dict(
-            (a, mock.DEFAULT) for a in ['active', 'interface_name']
+            (a, mock.DEFAULT) for a in
+            ['active', 'interface_name', 'spawn_process']
         )
 
         with mock.patch.multiple(LocalChild, **attrs_to_mock) as mocks:
             mocks['active'].__get__ = mock.Mock(return_value=False)
             mocks['interface_name'].__set__ = mock.Mock()
+            mocks['spawn_process'].side_effect = [
+                exceptions.ProcessExecutionError(
+                    returncode=2, message="Test dnsmasq start failed"),
+                None]
             lp = LocalChild(self.conf,
                             FakeDualNetwork())
+
             lp.enable()
 
             self.mock_mgr.assert_has_calls(
                 [mock.call(self.conf, None),
                  mock.call().setup(mock.ANY)])
-            self.assertEqual(lp.called, ['spawn'])
-            self.assertTrue(mocks['interface_name'].__set__.called)
-            ensure_dir.assert_called_with(
-                '/dhcp/cccccccc-cccc-cccc-cccc-cccccccccccc', mode=0o755)
+            self.assertEqual(2, mocks['interface_name'].__set__.call_count)
+            ensure_dir.assert_has_calls([
+                mock.call(
+                    '/dhcp/cccccccc-cccc-cccc-cccc-cccccccccccc', mode=0o755),
+                mock.call(
+                    '/dhcp/cccccccc-cccc-cccc-cccc-cccccccccccc', mode=0o755)])
 
     def _assert_disabled(self, lp):
         self.assertTrue(lp.process_monitor.unregister.called)