I hit a bug which looks like the same (or at least connected to this one).

While trying to run a full python3 Sahara job (https://review.openstack.org/#/c/576929/) I hit this (see

2018-07-17 18:26:16.262 18749 INFO sahara.main [-] Sahara engine started
2018-07-17 18:26:16.262 18749 DEBUG sahara.service.periodic [-] Starting periodic tasks with initial delay 57 seconds setup /opt/stack/sahara/sahara/service/periodic.py:253
2018-07-17 18:26:16.269 18749 CRITICAL sahara [-] Unhandled error: TypeError: can't concat bytes to str
2018-07-17 18:26:16.269 18749 ERROR sahara Traceback (most recent call last):
2018-07-17 18:26:16.269 18749 ERROR sahara   File "/usr/local/bin/sahara-engine", line 10, in <module>
2018-07-17 18:26:16.269 18749 ERROR sahara     sys.exit(main())
2018-07-17 18:26:16.269 18749 ERROR sahara   File "/opt/stack/sahara/sahara/cli/sahara_engine.py", line 49, in main
2018-07-17 18:26:16.269 18749 ERROR sahara     server.setup_sahara_engine()
2018-07-17 18:26:16.269 18749 ERROR sahara   File "/opt/stack/sahara/sahara/main.py", line 109, in setup_sahara_engine
2018-07-17 18:26:16.269 18749 ERROR sahara     periodic.setup()
2018-07-17 18:26:16.269 18749 ERROR sahara   File "/opt/stack/sahara/sahara/service/periodic.py", line 262, in setup
2018-07-17 18:26:16.269 18749 ERROR sahara     pt = _make_periodic_tasks()
2018-07-17 18:26:16.269 18749 ERROR sahara   File "/opt/stack/sahara/sahara/service/periodic.py", line 137, in _make_periodic_tasks
2018-07-17 18:26:16.269 18749 ERROR sahara     class SaharaPeriodicTasks(periodic_task.PeriodicTasks):
2018-07-17 18:26:16.269 18749 ERROR sahara   File "/opt/stack/sahara/sahara/service/periodic.py", line 139, in SaharaPeriodicTasks
2018-07-17 18:26:16.269 18749 ERROR sahara     CONF.periodic_coordinator_backend_url, 'sahara-periodic-tasks')
2018-07-17 18:26:16.269 18749 ERROR sahara   File "/opt/stack/sahara/sahara/service/coordinator.py", line 98, in __init__
2018-07-17 18:26:16.269 18749 ERROR sahara     super(HashRing, self).__init__(backend_url)
2018-07-17 18:26:16.269 18749 ERROR sahara   File "/opt/stack/sahara/sahara/service/coordinator.py", line 51, in __init__
2018-07-17 18:26:16.269 18749 ERROR sahara     self.coordinator.start()
2018-07-17 18:26:16.269 18749 ERROR sahara   File "/usr/local/lib/python3.5/dist-packages/tooz/coordination.py", line 687, in start
2018-07-17 18:26:16.269 18749 ERROR sahara     super(CoordinationDriverWithExecutor, self).start(start_heart)
2018-07-17 18:26:16.269 18749 ERROR sahara   File "/usr/local/lib/python3.5/dist-packages/tooz/coordination.py", line 423, in start
2018-07-17 18:26:16.269 18749 ERROR sahara     self._start()
2018-07-17 18:26:16.269 18749 ERROR sahara   File "/usr/local/lib/python3.5/dist-packages/tooz/drivers/memcached.py", line 71, in wrapper
2018-07-17 18:26:16.269 18749 ERROR sahara     return func(*args, **kwargs)
2018-07-17 18:26:16.269 18749 ERROR sahara   File "/usr/local/lib/python3.5/dist-packages/tooz/drivers/memcached.py", line 277, in _start
2018-07-17 18:26:16.269 18749 ERROR sahara     self.heartbeat()
2018-07-17 18:26:16.269 18749 ERROR sahara   File "/usr/local/lib/python3.5/dist-packages/tooz/drivers/memcached.py", line 71, in wrapper
2018-07-17 18:26:16.269 18749 ERROR sahara     return func(*args, **kwargs)
2018-07-17 18:26:16.269 18749 ERROR sahara   File "/usr/local/lib/python3.5/dist-packages/tooz/drivers/memcached.py", line 479, in heartbeat
2018-07-17 18:26:16.269 18749 ERROR sahara     self.client.set(self._encode_member_id(self._member_id),
2018-07-17 18:26:16.269 18749 ERROR sahara   File "/usr/local/lib/python3.5/dist-packages/tooz/drivers/memcached.py", line 291, in _encode_member_id
2018-07-17 18:26:16.269 18749 ERROR sahara     return self.MEMBER_PREFIX + member_id
2018-07-17 18:26:16.269 18749 ERROR sahara TypeError: can't concat bytes to str
2018-07-17 18:26:16.269 18749 ERROR sahara 

(see http://logs.openstack.org/29/576929/7/experimental/sahara-tests-scenario-py3/01fd285/controller/logs/screen-sahara-eng.txt.gz )

Discussed a bit on IRC few weeks ago: http://eavesdrop.openstack.org/irclogs/%23openstack-oslo/%23openstack-oslo.2018-06-21.log.html#t2018-06-21T22:22:29

Did anyone thought a bit more about this lately?