Fix DetachedInstanceError for Agent instance

The stacktrace observed seems to hint to a
situation where a session is not properly
rolled back when duplicate agent entries are
found.

Since it is ok to deal with duplicates, the
addition of agents to the DB must be done
one by one, otherwise we go against the
ATOMICITY property of a transaction of all
or none. It's either that or I am barking at
the wrong tree.

This patch also added UT to cover affected
code.

Closes-bug: 1282421
Related-bug: 1282253

Change-Id: Ic5f77808cc10f885d67f5a9cf3b3836283692850

neutron/scheduler/dhcp_agent_scheduler.py

@@ -35,22 +35,25 @@ class ChanceScheduler(object):
     can be introduced later.
     """
 
-    def _schedule_bind_network(self, context, agent, network_id):
-        try:
-            binding = agentschedulers_db.NetworkDhcpAgentBinding()
-            binding.dhcp_agent = agent
-            binding.network_id = network_id
-            context.session.add(binding)
-            # try to actually write the changes and catch integrity
-            # DBDuplicateEntry
-            context.session.flush()
-        except db_exc.DBDuplicateEntry:
-            # it's totally ok, someone just did our job!
-            pass
-        LOG.debug(_('Network %(network_id)s is scheduled to be hosted by '
-                    'DHCP agent %(agent_id)s'),
-                  {'network_id': network_id,
-                   'agent_id': agent})
+    def _schedule_bind_network(self, context, agents, network_id):
+        for agent in agents:
+            context.session.begin(subtransactions=True)
+            try:
+                binding = agentschedulers_db.NetworkDhcpAgentBinding()
+                binding.dhcp_agent = agent
+                binding.network_id = network_id
+                context.session.add(binding)
+                # try to actually write the changes and catch integrity
+                # DBDuplicateEntry
+                context.session.commit()
+            except db_exc.DBDuplicateEntry:
+                # it's totally ok, someone just did our job!
+                context.session.rollback()
+                LOG.info(_('Agent %s already present'), agent)
+            LOG.debug(_('Network %(network_id)s is scheduled to be '
+                        'hosted by DHCP agent %(agent_id)s'),
+                      {'network_id': network_id,
+                       'agent_id': agent})
 
     def schedule(self, plugin, context, network):
         """Schedule the network to active DHCP agent(s).
@@ -87,8 +90,7 @@ class ChanceScheduler(object):
                 return
             n_agents = min(len(active_dhcp_agents), n_agents)
             chosen_agents = random.sample(active_dhcp_agents, n_agents)
-            for agent in chosen_agents:
-                self._schedule_bind_network(context, agent, network['id'])
+        self._schedule_bind_network(context, chosen_agents, network['id'])
         return chosen_agents
 
     def auto_schedule_networks(self, plugin, context, host):

neutron/tests/unit/test_dhcp_scheduler.py

@@ -0,0 +1,90 @@
+# Copyright 2014 OpenStack Foundation
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+# implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import mock
+
+from neutron.common import constants
+from neutron.common import topics
+from neutron import context
+from neutron.db import agents_db
+from neutron.db import agentschedulers_db
+from neutron.db import api as db
+from neutron.db import models_v2
+from neutron.openstack.common import timeutils
+from neutron.scheduler import dhcp_agent_scheduler
+from neutron.tests import base
+
+
+class DhcpSchedulerTestCase(base.BaseTestCase):
+
+    def setUp(self):
+        super(DhcpSchedulerTestCase, self).setUp()
+        db.configure_db()
+        self.ctx = context.get_admin_context()
+        self.network_id = 'foo_network_id'
+        self._save_networks([self.network_id])
+        self.addCleanup(db.clear_db)
+
+    def _get_agents(self, hosts):
+        return [
+            agents_db.Agent(
+                binary='neutron-dhcp-agent',
+                host=host,
+                topic=topics.DHCP_AGENT,
+                configurations="",
+                agent_type=constants.AGENT_TYPE_DHCP,
+                created_at=timeutils.utcnow(),
+                started_at=timeutils.utcnow(),
+                heartbeat_timestamp=timeutils.utcnow())
+            for host in hosts
+        ]
+
+    def _save_agents(self, agents):
+        for agent in agents:
+            with self.ctx.session.begin(subtransactions=True):
+                self.ctx.session.add(agent)
+
+    def _save_networks(self, networks):
+        for network_id in networks:
+            with self.ctx.session.begin(subtransactions=True):
+                self.ctx.session.add(models_v2.Network(id=network_id))
+
+    def _test__schedule_bind_network(self, agents, network_id):
+        scheduler = dhcp_agent_scheduler.ChanceScheduler()
+        scheduler._schedule_bind_network(self.ctx, agents, network_id)
+        results = (
+            self.ctx.session.query(agentschedulers_db.NetworkDhcpAgentBinding).
+            filter_by(network_id=network_id).all())
+        self.assertEqual(len(agents), len(results))
+        for result in results:
+            self.assertEqual(network_id, result.network_id)
+
+    def test__schedule_bind_network_single_agent(self):
+        agents = self._get_agents(['host-a'])
+        self._save_agents(agents)
+        self._test__schedule_bind_network(agents, self.network_id)
+
+    def test__schedule_bind_network_multi_agents(self):
+        agents = self._get_agents(['host-a', 'host-b'])
+        self._save_agents(agents)
+        self._test__schedule_bind_network(agents, self.network_id)
+
+    def test__schedule_bind_network_multi_agent_fail_one(self):
+        agents = self._get_agents(['host-a'])
+        self._save_agents(agents)
+        self._test__schedule_bind_network(agents, self.network_id)
+        with mock.patch.object(dhcp_agent_scheduler.LOG, 'info') as fake_log:
+            self._test__schedule_bind_network(agents, self.network_id)
+            self.assertEqual(1, fake_log.call_count)