NoneType trace during l3 agents are not active

[Kristina Berezovskaia]

While there are no active l3 agents we have this trace:
2015-09-09 06:07:35.935 3394 WARNING neutron.scheduler.l3_agent_scheduler [req-271baca5-bad6-4bd7-a1da-9ea7a87a8944 ] No active L3 agents
2015-09-09 06:07:35.938 3394 ERROR neutron.db.l3_agentschedulers_db [req-271baca5-bad6-4bd7-a1da-9ea7a87a8944 ] Exception encountered during router rescheduling.
2015-09-09 06:07:35.938 3394 TRACE neutron.db.l3_agentschedulers_db Traceback (most recent call last):
2015-09-09 06:07:35.938 3394 TRACE neutron.db.l3_agentschedulers_db File "/usr/lib/python2.7/dist-packages/neutron/db/l3_agentschedulers_db.py", line 121, in reschedule_routers_from_down_agents
2015-09-09 06:07:35.938 3394 TRACE neutron.db.l3_agentschedulers_db self.reschedule_router(context, binding.router_id)
2015-09-09 06:07:35.938 3394 TRACE neutron.db.l3_agentschedulers_db File "/usr/lib/python2.7/dist-packages/neutron/db/l3_agentschedulers_db.py", line 265, in reschedule_router
2015-09-09 06:07:35.938 3394 TRACE neutron.db.l3_agentschedulers_db self.schedule_router(context, router_id, candidates=candidates)
2015-09-09 06:07:35.938 3394 TRACE neutron.db.l3_agentschedulers_db File "/usr/lib/python2.7/dist-packages/neutron/db/l3_agentschedulers_db.py", line 528, in schedule_router
2015-09-09 06:07:35.938 3394 TRACE neutron.db.l3_agentschedulers_db self, context, router, candidates=candidates)
2015-09-09 06:07:35.938 3394 TRACE neutron.db.l3_agentschedulers_db File "/usr/lib/python2.7/dist-packages/neutron/scheduler/l3_agent_scheduler.py", line 358, in schedule
2015-09-09 06:07:35.938 3394 TRACE neutron.db.l3_agentschedulers_db plugin, context, router_id, candidates=candidates)
2015-09-09 06:07:35.938 3394 TRACE neutron.db.l3_agentschedulers_db File "/usr/lib/python2.7/dist-packages/neutron/scheduler/l3_agent_scheduler.py", line 239, in _schedule_router
2015-09-09 06:07:35.938 3394 TRACE neutron.db.l3_agentschedulers_db for chosen_agent in candidates:
2015-09-09 06:07:35.938 3394 TRACE neutron.db.l3_agentschedulers_db TypeError: 'NoneType' object is not iterable
2015-09-09 06:07:35.938 3394 TRACE neutron.db.l3_agentschedulers_db

Steps:
1) Create net, subnet
2) Create router with gateway to external net
3) Connect new router with new net
4) Boot vm in new net
6) Destroy one controller (on what this net is scheduled)
7) Wait some minutes
8) Check l3-agent log

iso:
VERSION:
  feature_groups:
    - mirantis
  production: "docker"
  release: "7.0"
  openstack_version: "2015.1.0-7.0"
  api: "1.0"
  build_number: "287"
  build_id: "287"
  nailgun_sha: "46a7a2177a0b7ef91422284c1c90295fee8f5c84"
  python-fuelclient_sha: "1ce8ecd8beb640f2f62f73435f4e18d1469979ac"
  fuel-agent_sha: "082a47bf014002e515001be05f99040437281a2d"
  fuel-nailgun-agent_sha: "d7027952870a35db8dc52f185bb1158cdd3d1ebd"
  astute_sha: "a717657232721a7fafc67ff5e1c696c9dbeb0b95"
  fuel-library_sha: "43224223dab8cf9627b5ecf737e60216a3fdd114"
  fuel-ostf_sha: "1f08e6e71021179b9881a824d9c999957fcc7045"
  fuelmain_sha: "6b83d6a6a75bf7bca3177fcf63b2eebbf1ad0a85"

vxlan, 3 controllers, 2 compute

[Oleg Bondarev (obondarev)]

No user impact, just traces in logs, fix is simple.

[Vitaly Sedelnik (vsedelnik)]

Won't Fix for 7.0 because of Medium importance, to be addressed in 8.0

[Sergey Belous (sbelous)]

Fixed by https://review.openstack.org/#/c/222828/2 in upstream. This patch included in stable/liberty and already synced in 8.0

[Kristina Berezovskaia (kkuznetsova)]

Verify on
VERSION:
  feature_groups:
    - mirantis
  production: "docker"
  release: "8.0"
  openstack_version: "2015.1.0-8.0"
  api: "1.0"
  build_number: "264"
  build_id: "264"
  fuel-nailgun_sha: "0e09dce510927f2cc490b898e5fe3f813bd791be"
  python-fuelclient_sha: "f033192b84263f0e699458a4274289a5198ae7e4"
  fuel-agent_sha: "660c6514caa8f5fcd482f1cc4008a6028243e009"
  fuel-nailgun-agent_sha: "a33a58d378c117c0f509b0e7badc6f0910364154"
  astute_sha: "48fd58676debcc85951db68df6d77c22daa55e52"
  fuel-library_sha: "ab7e51f345ffb7c256e0f61addcf86553d7c3867"
  fuel-ostf_sha: "23b7ae2a1a57de5a3e1861ffb7805394ca339cc2"
  fuel-mirror_sha: "6534117233a5bdc51d7d47361bc7d511e4b11e6f"
  fuelmenu_sha: "fcb15df4fd1a790b17dd78cf675c11c279040941"
  shotgun_sha: "a0bd06508067935f2ae9be2523ed0d1717b995ce"
  network-checker_sha: "a3534f8885246afb15609c54f91d3b23d599a5b1"
  fuel-upgrade_sha: "1e894e26d4e1423a9b0d66abd6a79505f4175ff6"
  fuelmain_sha: "26adf12c320936a97a9b0a84169a6e58c530e848"
(neutron+vxlan+l2, 3 controllers + 2 computes)

Repeat steps from description. There are no traces and errors in log