Ansible 2.8: Regression because of new sentinel class representing None or []

[dmsimard]

Example trace from ara 1.0:

[WARNING]: Failure using method (v2_playbook_on_task_start) in callback plugin (<ansible.plugins.callback./home/dmsimard/dev/git/ansible-community/ara/ara/plugins/callback/ara_default.CallbackModule object at 0x7fb58bb36a90>): Object of type type is not
JSON serializable

Callback Exception: 
  File "/home/dmsimard/dev/git/ansible-community/ara/.tox/ansible-integration/lib/python3.7/site-packages/ansible/executor/task_queue_manager.py", line 333, in send_callback
    method(*new_args, **kwargs)
   File "/home/dmsimard/dev/git/ansible-community/ara/ara/plugins/callback/ara_default.py", line 232, in v2_playbook_on_task_start
    handler=handler,
   File "/home/dmsimard/dev/git/ansible-community/ara/ara/clients/http.py", line 104, in post
    return self._request("post", endpoint, **kwargs)
   File "/home/dmsimard/dev/git/ansible-community/ara/ara/clients/http.py", line 82, in _request
    response = func(url, **kwargs)
   File "/home/dmsimard/dev/git/ansible-community/ara/ara/clients/http.py", line 63, in post
    return self._request("post", url, data=json.dumps(payload))
   File "/usr/lib64/python3.7/json/__init__.py", line 231, in dumps
    return _default_encoder.encode(obj)
   File "/usr/lib64/python3.7/json/encoder.py", line 199, in encode
    chunks = self.iterencode(o, _one_shot=True)
   File "/usr/lib64/python3.7/json/encoder.py", line 257, in iterencode
    return _iterencode(o, 0)
   File "/usr/lib64/python3.7/json/encoder.py", line 179, in default
    raise TypeError(f'Object of type {o.__class__.__name__} '

Or screenshot from 0.x:

This is because the structure of the task object has changed.

In 2.7:

(Pdb) task._attributes
{'args': {'name': '{{ item }}', 'ansible_host': '127.0.0.1', 'ansible_connection': 'local'}, 'action': 'add_host', 'async_val': 0, 'changed_when': [], 'delay': 5, 'delegate_to': None, 'delegate_facts': None, 'failed_when': [], 'loop': ['host1', 'host2', 'host3'], 'loop_control': None, 'notify': None, 'poll': 10, 'register': None, 'retries': 3, 'until': [], 'loop_with': 'items', 'name': 'Add fake hosts in inventory', 'connection': None, 'port': None, 'remote_user': None, 'vars': {}, 'module_defaults': None, 'environment': None, 'no_log': None, 'run_once': None, 'ignore_errors': None, 'ignore_unreachable': None, 'check_mode': None, 'diff': None, 'any_errors_fatal': None, 'debugger': None, 'when': [], 'tags': [], 'become': None, 'become_method': None, 'become_user': None, 'become_flags': None}
(Pdb) task._attributes["tags"]
[]

In 2.8 (notice all the ansible.utils.sentinel.Sentinel):

(Pdb) task._attributes
{'args': {'name': '{{ item }}', 'ansible_host': '127.0.0.1', 'ansible_connection': 'local'}, 'action': 'add_host', 'async_val': <class 'ansible.utils.sentinel.Sentinel'>, 'changed_when': <class 'ansible.utils.sentinel.Sentinel'>, 'delay': <class 'ansible.utils.sentinel.Sentinel'>, 'delegate_to': <class 'ansible.utils.sentinel.Sentinel'>, 'delegate_facts': <class 'ansible.utils.sentinel.Sentinel'>, 'failed_when': <class 'ansible.utils.sentinel.Sentinel'>, 'loop': ['host1', 'host2', 'host3'], 'loop_control': <class 'ansible.utils.sentinel.Sentinel'>, 'notify': <class 'ansible.utils.sentinel.Sentinel'>, 'poll': <class 'ansible.utils.sentinel.Sentinel'>, 'register': <class 'ansible.utils.sentinel.Sentinel'>, 'retries': <class 'ansible.utils.sentinel.Sentinel'>, 'until': <class 'ansible.utils.sentinel.Sentinel'>, 'loop_with': 'items', 'name': 'Add fake hosts in inventory', 'connection': <class 'ansible.utils.sentinel.Sentinel'>, 'port': <class 'ansible.utils.sentinel.Sentinel'>, 'remote_user': <class 'ansible.utils.sentinel.Sentinel'>, 'vars': {}, 'module_defaults': <class 'ansible.utils.sentinel.Sentinel'>, 'environment': <class 'ansible.utils.sentinel.Sentinel'>, 'no_log': <class 'ansible.utils.sentinel.Sentinel'>, 'run_once': <class 'ansible.utils.sentinel.Sentinel'>, 'ignore_errors': <class 'ansible.utils.sentinel.Sentinel'>, 'ignore_unreachable': <class 'ansible.utils.sentinel.Sentinel'>, 'check_mode': <class 'ansible.utils.sentinel.Sentinel'>, 'diff': <class 'ansible.utils.sentinel.Sentinel'>, 'any_errors_fatal': <class 'ansible.utils.sentinel.Sentinel'>, 'debugger': <class 'ansible.utils.sentinel.Sentinel'>, 'when': <class 'ansible.utils.sentinel.Sentinel'>, 'tags': <class 'ansible.utils.sentinel.Sentinel'>, 'become': <class 'ansible.utils.sentinel.Sentinel'>, 'become_method': <class 'ansible.utils.sentinel.Sentinel'>, 'become_user': <class 'ansible.utils.sentinel.Sentinel'>, 'become_flags': <class 'ansible.utils.sentinel.Sentinel'>, 'collections': <class 'ansible.utils.sentinel.Sentinel'>}
(Pdb) task._attributes["tags"]
<class 'ansible.utils.sentinel.Sentinel'>

After discussing this with upstream in #ansible-devel, it turns out that this new Sentinel object is used to indicate that something has not been set (to allow a distinction between unset and set to None).

We can use something like from ansible.utils.sentinel import Sentinel; if foo is Sentinel to determine if the value is set and otherwise coerce to a value we can send to the database like None or an empty list, for example.

[dmsimard]

Fixed.
Released in 0.16.4 and 1.0b2.