Cloud update fails with "ValueError: No JSON object could be decoded"

Issue

• openstack overcloud update run --nodes Controller is is failing with:

No JSON object could be decoded
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/cliff/app.py", line 400, in run_subcommand
    result = cmd.run(parsed_args)
  File "/usr/lib/python2.7/site-packages/tripleoclient/command.py", line 25, in run
    super(Command, self).run(parsed_args)
  File "/usr/lib/python2.7/site-packages/osc_lib/command/command.py", line 41, in run
    return super(Command, self).run(parsed_args)
  File "/usr/lib/python2.7/site-packages/cliff/command.py", line 184, in run
    return_code = self.take_action(parsed_args) or 0
  File "/usr/lib/python2.7/site-packages/tripleoclient/v1/overcloud_update.py", line 167, in take_action
    verbosity=verbosity)
  File "/usr/lib/python2.7/site-packages/tripleoclient/utils.py", line 905, in run_update_ansible_action
    skip_tags=skip_tags, verbosity=verbosity)
  File "/usr/lib/python2.7/site-packages/tripleoclient/workflows/package_update.py", line 98, in update_ansible
    for payload in base.wait_for_messages(workflow_client, ws, execution):
  File "/usr/lib/python2.7/site-packages/tripleoclient/workflows/base.py", line 61, in wait_for_messages
    for payload in websocket.wait_for_messages(timeout=timeout):
  File "/usr/lib/python2.7/site-packages/tripleoclient/plugin.py", line 158, in wait_for_messages
    yield self.recv()['body']['payload']
  File "/usr/lib/python2.7/site-packages/tripleoclient/plugin.py", line 132, in recv
    return json.loads(self._ws.recv())
  File "/usr/lib64/python2.7/json/__init__.py", line 338, in loads
    return _default_decoder.decode(s)
  File "/usr/lib64/python2.7/json/decoder.py", line 366, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib64/python2.7/json/decoder.py", line 384, in raw_decode
    raise ValueError("No JSON object could be decoded")
ValueError: No JSON object could be decoded
clean_up UpdateRun: No JSON object could be decoded
clean_up UpdateRun: No JSON object could be decoded
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/osc_lib/shell.py", line 134, in run
    ret_val = super(OpenStackShell, self).run(argv)
  File "/usr/lib/python2.7/site-packages/cliff/app.py", line 279, in run
    result = self.run_subcommand(remainder)
  File "/usr/lib/python2.7/site-packages/osc_lib/shell.py", line 169, in run_subcommand
    ret_value = super(OpenStackShell, self).run_subcommand(argv)
  File "/usr/lib/python2.7/site-packages/cliff/app.py", line 400, in run_subcommand
    result = cmd.run(parsed_args)
  File "/usr/lib/python2.7/site-packages/tripleoclient/command.py", line 25, in run
    super(Command, self).run(parsed_args)
  File "/usr/lib/python2.7/site-packages/osc_lib/command/command.py", line 41, in run
    return super(Command, self).run(parsed_args)
  File "/usr/lib/python2.7/site-packages/cliff/command.py", line 184, in run
    return_code = self.take_action(parsed_args) or 0
  File "/usr/lib/python2.7/site-packages/tripleoclient/v1/overcloud_update.py", line 167, in take_action
    verbosity=verbosity)
  File "/usr/lib/python2.7/site-packages/tripleoclient/utils.py", line 905, in run_update_ansible_action
    skip_tags=skip_tags, verbosity=verbosity)
  File "/usr/lib/python2.7/site-packages/tripleoclient/workflows/package_update.py", line 98, in update_ansible
    for payload in base.wait_for_messages(workflow_client, ws, execution):
  File "/usr/lib/python2.7/site-packages/tripleoclient/workflows/base.py", line 61, in wait_for_messages
    for payload in websocket.wait_for_messages(timeout=timeout):
  File "/usr/lib/python2.7/site-packages/tripleoclient/plugin.py", line 158, in wait_for_messages
    yield self.recv()['body']['payload']
  File "/usr/lib/python2.7/site-packages/tripleoclient/plugin.py", line 132, in recv
    return json.loads(self._ws.recv())
  File "/usr/lib64/python2.7/json/__init__.py", line 338, in loads
    return _default_decoder.decode(s)
  File "/usr/lib64/python2.7/json/decoder.py", line 366, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib64/python2.7/json/decoder.py", line 384, in raw_decode
    raise ValueError("No JSON object could be decoded")
ValueError: No JSON object could be decoded

END return value: 1