[Fullstack] Double check that agent is dead when it should be dead

In some fullstack tests it is expected that agent is DOWN in the Neutron
DB. It could happen sometimes that in almost the same time test's client
was doing GET /v2.0/agents/{agent_id} call and got result with
"alive=False" and in other thread rpc worker was processing heartbeat
from the agent so it was revived just after API request was finished.
That was causing test failures in some cases.
This patch adds second API call to get agent again after 2 seconds if it
was already marked as DEAD, just to make sure that it is really dead ;)

Closes-Bug: #2045757
Change-Id: I1c20c90b8abd760f3a53b24024f19ef2bd189b5a

neutron/tests/fullstack/base.py

@@ -16,6 +16,7 @@ from concurrent import futures
 import itertools
 import os
 import random
+import time
 
 import netaddr
 from neutron_lib.tests import tools
@@ -101,6 +102,17 @@ class BaseFullStackTestCase(testlib_api.MySQLTestCaseMixin,
     def _wait_until_agent_down(self, agent_id):
         def _agent_down():
             agent = self.client.show_agent(agent_id)['agent']
+            if not agent.get('alive'):
+                # NOTE(slaweq): to avoid race between heartbeat written in the
+                # database and response to this API call, lets make sure that
+                # agent is really dead. See bug
+                # https://bugs.launchpad.net/neutron/+bug/2045757
+                # for details.
+                # 2 seconds delay should be more than enough to make sure that
+                # all pending heartbeats are already written in the Neutron
+                # database
+                time.sleep(2)
+                agent = self.client.show_agent(agent_id)['agent']
             return not agent.get('alive')
 
         common_utils.wait_until_true(_agent_down)