Comment 1 for bug 1565176

I ran some tests and it seems a recursion results in infinite recursive calls that end with run time error.

Here's a sample selected log:
RuntimeError: maximum recursion depth exceeded^M
Traceback (most recent call last):^M
^M
  File "/usr/local/lib/python2.7/dist-packages/oslo_messaging/rpc/server.py", line 155, in _process_incoming^M
    res = self.dispatcher.dispatch(message)^M
^M
  File "/usr/local/lib/python2.7/dist-packages/oslo_messaging/rpc/dispatcher.py", line 222, in dispatch^M
    return self._do_dispatch(endpoint, method, ctxt, args)^M
^M
  File "/usr/local/lib/python2.7/dist-packages/oslo_messaging/rpc/dispatcher.py", line 192, in _do_dispatch^M
    result = func(ctxt, **new_args)^M
^M
  File "/opt/stack/congress/congress/policy_engines/agnostic.py", line 22
20, in simulate^M
    delta, trace, as_list)
    File "/opt/stack/congress/congress/policy_engines/agnostic.py", line 820, in simulate^M
    action_theory, delta, trace, as_list)^M
^M
  File "/opt/stack/congress/congress/policy_engines/agnostic.py", line 1247, in _simulate_string^M
    delta, trace)^M
^M
  File "/opt/stack/congress/congress/policy_engines/agnostic.py", line 1296, in _simulate_obj^M
    result = set(th_object.select(query))^M
^M
  File "/opt/stack/congress/congress/datalog/topdown.py", line 129, in select^M
    find_all=find_all)^M
^M
  File "/opt/stack/congress/congress/datalog/topdown.py", line 230, in top_down_evaluation^M
     save=None)^M
^M
  File "/opt/stack/congress/congress/datalog/topdown.py", line 257, in top_down_abduction^M
    self._top_down_eval(context, caller)^M
^M
  File "/opt/stack/congress/congress/datalog/topdown.py", line 332, in _top_down_eval^M
    return self._top_down_truth(context, caller)^M
^M
  File "/opt/stack/congress/congress/datalog/topdown.py", line 435, in _top_down_truth^M
    return self._top_down_includes(context, caller)^M
^M
  File "/opt/stack/congress/congress/datalog/topdown.py", line 439, in _top_down_includes^M
    is_true = self._top_down_th(context, caller)^M
    File "/opt/stack/congress/congress/datalog/topdown.py", line 473, in _top_down_th^M
    if self._top_down_eval(new_context, caller):^M
^M
  File "/opt/stack/congress/congress/datalog/topdown.py", line 332, in _top_down_eval^M
    return self._top_down_truth(context, caller)^M
^M
  File "/opt/stack/congress/congress/datalog/topdown.py", line 435, in _top_down_truth^M
    return self._top_down_includes(context, caller)^M
^M
  File "/opt/stack/congress/congress/datalog/topdown.py", line 439, in _top_down_includes^M

There are at least three options for addressing this error:
1. Perform static analysis of the query+policy rules to know ahead of time it'd lead to infinite recursive calls. Reject query.
2. Check the call stack to terminate infinite recursive call early.
3. Catch the RuntimeError when recursive call limit reached, then give a meaningful error message to user.

Given that the problem occurs infrequently, I think 3 is acceptable.

---

Note that there may be more complex variants of this problem. For example:

$ openstack congress policy rule create alpha 'a(x) :- b(x)'
+---------+--------------------------------------+
| Field | Value |
+---------+--------------------------------------+
| comment | |
| id | 36b90aee-a05a-45fd-9270-403289330f0d |
| name | None |
| rule | a(x) :- |
| | b(x) |
+---------+--------------------------------------+
user@ubuntu:~$ openstack congress policy simulate alpha 'b(x)' 'b+(x) :- a(x)' action
Internal server error (HTTP 500) (Request-ID: req-3714139c-ae7d-4767-8d70-93791905133f)
user@ubuntu:~$ openstack congress policy simulate alpha 'b(1)' 'b+(1) :- a(1)' action
Internal server error (HTTP 500) (Request-ID: req-d64b1fba-0840-461c-9d51-6a2d9095b723)