Sync router fails with db exception

While investigating https://bugs.launchpad.net/neutron/+bug/1210664, salvatore-orlando discovered that a db exception was being raised during router syncing:

Traceback (most recent call last):
  File "/opt/stack/neutron/neutron/openstack/common/rpc/amqp.py", line 424, in _process_data
    **args)
  File "/opt/stack/neutron/neutron/common/rpc.py", line 44, in dispatch
    neutron_ctxt, version, method, namespace, **kwargs)
  File "/opt/stack/neutron/neutron/openstack/common/rpc/dispatcher.py", line 172, in dispatch
    result = getattr(proxyobj, method)(ctxt, **kwargs)
  File "/opt/stack/neutron/neutron/db/l3_rpc_base.py", line 47, in sync_routers
    plugin.auto_schedule_routers(context, host, router_ids)
  File "/opt/stack/neutron/neutron/db/agentschedulers_db.py", line 303, in auto_schedule_routers
    self, context, host, router_ids)
  File "/opt/stack/neutron/neutron/scheduler/l3_agent_scheduler.py", line 113, in auto_schedule_routers
    context.session.add(binding)
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/session.py", line 456, in __exit__
    self.commit()
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/session.py", line 368, in commit
    self._prepare_impl()
  File "/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/session.py", line 347, in _prepare_impl
    self.session.flush()
  File "/opt/stack/neutron/neutron/openstack/common/db/sqlalchemy/session.py", line 542, in _wrap
    raise exception.DBError(e)
DBError: (IntegrityError) (1452, 'Cannot add or update a child row: a foreign key constraint fails (`ovs_neutron`.`routerl3agentbindings`, CONSTRAINT `routerl3agentbindings_ibfk_1` FOREIGN KEY (`router_id`) REFERENCES `routers` (`id`) ON DELETE CASCADE)') 'INSERT INTO routerl3agentbindings (id, router_id, l3_agent_id) VALUES (%s, %s, %s)' ('2df68c3d-f3c9-43d2-bf45-e2e57e84b054', 'c4502f1f-a093-4c7c-b161-929b6342509b', '85d6b60f-f3ff-4437-8f5a-af165087f3ea')

This can be reproduced by running the quantum smoke test in tempest (nosetests tempest/scenario/test_network_basic_ops.py). The smoke test passes - the exception only occurs at test cleanup. It may be that the router syncing code is working with stale state after router deletion.