Preventing listener deletion if it has l7 policy

If L7 policy is associated to listener, the policy should be deleted
prior to listener deletion.
Trying to delete the listener with L7 policy will fail
with EntityInUse exception.
This is to preserve common neutron's API concept
to delete cascade resources' subresources only.

Change-Id: I575f898f74576613cb55d4dbba5b1b0f524dd35f
Closes-Bug: 1588372
(cherry picked from commit 58fe3494ef

neutron_lbaas/services/loadbalancer/plugin.py

@@ -803,6 +803,12 @@ class LoadBalancerPluginv2(loadbalancerv2.LoadBalancerPluginBaseV2):
         return self.db.get_listener(context, id).to_api_dict()
 
     def delete_listener(self, context, id):
+        old_listener = self.db.get_listener(context, id)
+        if old_listener.l7_policies:
+            raise loadbalancerv2.EntityInUse(
+                entity_using=models.L7Policy.NAME,
+                id=old_listener.l7_policies[0].id,
+                entity_in_use=models.Listener.NAME)
         self.db.test_and_set_status(context, models.Listener, id,
                                     constants.PENDING_DELETE)
         listener_db = self.db.get_listener(context, id)

neutron_lbaas/tests/unit/db/loadbalancer/test_db_loadbalancerv2.py

@@ -1374,6 +1374,16 @@ class LbaasListenerTests(ListenerTestBase):
             resp, body = self._get_loadbalancer_api(self.lb_id)
             self.assertEqual(0, len(body['loadbalancer']['listeners']))
 
+    def test_delete_listener_with_l7policy(self):
+        with self.listener(loadbalancer_id=self.lb_id,
+                           no_delete=True) as listener:
+            with self.l7policy(listener['listener']['id'], no_delete=True):
+                ctx = context.get_admin_context()
+                self.assertRaises(
+                    loadbalancerv2.EntityInUse,
+                    self.plugin.delete_listener,
+                    ctx, listener['listener']['id'])
+
     def test_show_listener(self):
         name = 'show_listener'
         expected_values = {'name': name,