Dont use query if network_ids is empty

A warning could be found in SQLAlchemy, if handling empty WHERE IN
clauses. This is found at neutron.db.segments_db.get_networks_segments

To avoid it, just return empty dict in such case.

Change-Id: I3cf727dede1d5909aeefbf852332818164ad3777
Closes-bug: #1586816

neutron/db/segments_db.py

@@ -87,6 +87,9 @@ def get_network_segments(session, network_id, filter_dynamic=False):
 
 
 def get_networks_segments(session, network_ids, filter_dynamic=False):
+    if not network_ids:
+        return {}
+
     with session.begin(subtransactions=True):
         query = (session.query(NetworkSegment).
                  filter(NetworkSegment.network_id.in_(network_ids)).

neutron/tests/unit/db/test_segments_db.py

@@ -0,0 +1,29 @@
+# Copyright (c) 2016 IBM Corp.
+#
+# All Rights Reserved.
+#
+#    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.db import segments_db
+from neutron.tests import base
+
+
+class TestSegmentsDb(base.BaseTestCase):
+
+    def test_get_networks_segments_with_empty_networks(self):
+        session = mock.MagicMock()
+        net_segs = segments_db.get_networks_segments(session, [])
+        self.assertFalse(session.query.called)
+        self.assertEqual({}, net_segs)