Catch duplicate errors scheduling SNAT service

Catch DBDuplicateEntry errors when scheduling SNAT to
a service node to prevent harmless tracebacks in the log.
These can occur if scheduling occurs concurrently,
so they are safe to ignore.

Closes-Bug: #1381958
Change-Id: If242b04b372609f640f3ce88f4245c17a45bf69d

neutron/db/l3_dvrscheduler_db.py

@@ -15,6 +15,7 @@
 
 import random
 
+from oslo.db import exception as db_exc
 import sqlalchemy as sa
 from sqlalchemy import orm
 from sqlalchemy.orm import exc
@@ -26,7 +27,7 @@ from neutron.db import agents_db
 from neutron.db import l3_agentschedulers_db as l3agent_sch_db
 from neutron.db import model_base
 from neutron.db import models_v2
-from neutron.i18n import _LW
+from neutron.i18n import _LI, _LW
 from neutron.openstack.common import log as logging
 from neutron.plugins.ml2 import db as ml2_db
 
@@ -299,7 +300,11 @@ class L3_DVRsch_db_mixin(l3agent_sch_db.L3AgentSchedulerDbMixin):
         snat_candidates = self.get_snat_candidates(sync_router,
                                                    active_l3_agents)
         if snat_candidates:
+            try:
                 chosen_agent = self.bind_snat_servicenode(
                     context, router_id, snat_candidates)
+            except db_exc.DBDuplicateEntry:
+                LOG.info(_LI("SNAT already bound to a service node."))
+                return
             self.bind_dvr_router_servicenode(
                 context, router_id, chosen_agent)

neutron/tests/unit/test_l3_schedulers.py

@@ -19,6 +19,7 @@ import uuid
 
 import mock
 from oslo.config import cfg
+from oslo.db import exception as db_exc
 from oslo.utils import importutils
 from oslo.utils import timeutils
 from sqlalchemy.orm import query
@@ -970,6 +971,19 @@ class L3DvrSchedulerTestCase(testlib_api.SqlTestCase,
         }
         return agent, router
 
+    def test_schedule_snat_router_duplicate_entry(self):
+        self._prepare_schedule_snat_tests()
+        with contextlib.nested(
+            mock.patch.object(self.dut, 'get_l3_agents'),
+            mock.patch.object(self.dut, 'get_snat_candidates'),
+            mock.patch.object(self.dut, 'bind_snat_servicenode',
+                              side_effect=db_exc.DBDuplicateEntry()),
+            mock.patch.object(self.dut, 'bind_dvr_router_servicenode')
+        ) as (mock_gl3, mock_snat_canidates, mock_bind_snat, mock_bind_dvr):
+            self.dut.schedule_snat_router(self.adminContext, 'foo', 'bar')
+        self.assertTrue(mock_bind_snat.called)
+        self.assertFalse(mock_bind_dvr.called)
+
     def test_schedule_router_unbind_snat_servicenode_negativetest(self):
         router = {
             'id': 'foo_router_id',