paramiko.channel.py _wait_for_event hangs

paramiko===2.7.2

Log traceback:

Traceback (most recent call last):
File "/opt/stack/tempest/.tox/tempest/lib/python3.8/site-packages/neutron_tempest_plugin/scenario/test_port_forwardings.py", line 160, in test_port_forwarding_editing_and_deleting_tcp_rule
utils.wait_until_true(
File "/opt/stack/tempest/.tox/tempest/lib/python3.8/site-packages/neutron_tempest_plugin/common/utils.py", line 79, in wait_until_true
while not predicate():
File "/opt/stack/tempest/.tox/tempest/lib/python3.8/site-packages/neutron_tempest_plugin/scenario/test_port_forwardings.py", line 153, in no_fip_pf_connectivity
return not fip_pf_connectivity(6)
File "/opt/stack/tempest/.tox/tempest/lib/python3.8/site-packages/neutron_tempest_plugin/scenario/test_port_forwardings.py", line 146, in fip_pf_connectivity
self.check_servers_hostnames(
File "/opt/stack/tempest/.tox/tempest/lib/python3.8/site-packages/neutron_tempest_plugin/scenario/base.py", line 537, in check_servers_hostnames
ssh_client.get_hostname())
File "/opt/stack/tempest/.tox/tempest/lib/python3.8/site-packages/neutron_tempest_plugin/common/ssh.py", line 292, in get_hostname
return self.exec_command('hostname')
File "/opt/stack/tempest/.tox/tempest/lib/python3.8/site-packages/tenacity/init.py", line 333, in wrapped_f
return self(f, *args, **kw)
File "/opt/stack/tempest/.tox/tempest/lib/python3.8/site-packages/tenacity/init.py", line 423, in call
do = self.iter(retry_state=retry_state)
File "/opt/stack/tempest/.tox/tempest/lib/python3.8/site-packages/tenacity/init.py", line 360, in iter
return fut.result()
File "/usr/lib/python3.8/concurrent/futures/_base.py", line 432, in result
return self.__get_result()
File "/usr/lib/python3.8/concurrent/futures/_base.py", line 388, in __get_result
raise self._exception
File "/opt/stack/tempest/.tox/tempest/lib/python3.8/site-packages/tenacity/init.py", line 426, in call
result = fn(*args, **kwargs)
File "/opt/stack/tempest/.tox/tempest/lib/python3.8/site-packages/neutron_tempest_plugin/common/ssh.py", line 171, in exec_command
return super(Client, self).exec_command(cmd=cmd, encoding=encoding)
File "/opt/stack/tempest/tempest/lib/common/ssh.py", line 162, in exec_command
channel.exec_command(cmd)
File "/opt/stack/tempest/.tox/tempest/lib/python3.8/site-packages/paramiko/channel.py", line 72, in _check
return func(self, *args, **kwds)
File "/opt/stack/tempest/.tox/tempest/lib/python3.8/site-packages/paramiko/channel.py", line 257, in exec_command
self._wait_for_event()
File "/opt/stack/tempest/.tox/tempest/lib/python3.8/site-packages/paramiko/channel.py", line 1219, in _wait_for_event
self.event.wait()
File "/usr/lib/python3.8/threading.py", line 558, in wait
signaled = self._cond.wait(timeout)
File "/usr/lib/python3.8/threading.py", line 302, in wait
waiter.acquire()
File "/opt/stack/tempest/.tox/tempest/lib/python3.8/site-packages/fixtures/_fixtures/timeout.py", line 52, in signal_handler
raise TimeoutException()
fixtures._fixtures.timeout.TimeoutException

Full log output:
https://04a9f9fdd9afdf12de4e-f889a65b4dfb1f628c8309e9eb44b225.ssl.cf2.rackcdn.com/787304/5/check/neutron-tempest-plugin-scenario-openvswitch-iptables_hybrid/bb9e9a9/testr_results.html

Start with paramiko/channel.py, reading Channel.exec_command and _wait_for_event alongside the provided traceback and linked full log. Determine why the reported exec_command path waits indefinitely; done means the hang is resolved or clearly bounded and the behavior has regression coverage.