Add abort functionality and handle exception in cleanup (#150)

* Try-except cleanup Prolog
* Feature: Add abort method to Prolog class

pyswip/core.py

@@ -1367,5 +1367,9 @@ def cleanupProlog():
         # TODO Prolog documentation says cleanup with code 0 may be interrupted
         # If the program has come to an end the prolog system should not
         # interfere with that. Therefore we may want to use 1 instead of 0.
-        PL_cleanup(int(_hook.exit_code or 0))
+        try:
+            exit_code = int(_hook.exit_code or 0)
+        except ValueError:
+            exit_code = 0
+        PL_cleanup(exit_code)
         _isCleaned = True

pyswip/prolog.py

@@ -80,16 +80,19 @@ class Prolog:
 
     # We keep track of open queries to avoid nested queries.
     _queryIsOpen = False
+    _queryWrapper = None
 
     class _QueryWrapper(object):
 
         def __init__(self):
+            self._swipl_qid = None
+            self._swipl_fid = None
             if Prolog._queryIsOpen:
                 raise NestedQueryError("The last query was not closed")
 
         def __call__(self, query, maxresult, catcherrors, normalize):
             Prolog._init_prolog_thread()
-            swipl_fid = PL_open_foreign_frame()
+            self._swipl_fid = PL_open_foreign_frame()
 
             swipl_head = PL_new_term_ref()
             swipl_args = PL_new_term_refs(2)
@@ -143,6 +146,11 @@ def _init_prolog_thread(cls):
         elif pengine_id == -2:
             print("{WARN} Single-threaded swipl build, beware!")
 
+    @classmethod
+    def abort(cls):
+        # The clean_up method is called to make sure that no query is still running
+        cls._queryWrapper.clean_up()
+
     @classmethod
     def asserta(cls, assertion, catcherrors=False):
         next(cls.query(assertion.join(["asserta((", "))."]), catcherrors=catcherrors))
@@ -183,7 +191,8 @@ def query(cls, query, maxresult=-1, catcherrors=True, normalize=True):
         >>> print sorted(prolog.query("father(michael,X)"))
         [{'X': 'gina'}, {'X': 'john'}]
         """
-        return cls._QueryWrapper()(query, maxresult, catcherrors, normalize)
+        cls._queryWrapper = cls._QueryWrapper()
+        return cls._queryWrapper(query, maxresult, catcherrors, normalize)
 
 
 def normalize_values(values):

tests/test_prolog.py

@@ -111,3 +111,38 @@ def test_prolog_read_file(self):
         prolog = pl.Prolog()
         prolog.consult("tests/test_read.pl")
         list(prolog.query('read_file("tests/test_read.pl", S)'))
+
+    def test_abort_query(self):
+        """
+        SWI-Prolog cannot have nested queries called by the foreign function
+        interface, that is, if we open a query and are getting results from it,
+        we cannot open another query before closing that one.
+
+        However, the interface allows to interrupt a query via the abort method.
+        This test makes sure that this feature works correctly.
+        """
+        p = pl.Prolog()
+
+        # Add something to the base
+        p.assertz("father(john,mich)")
+        p.assertz("father(john,gina)")
+        p.assertz("mother(jane,mich)")
+
+        somequery = "father(john, Y)"
+        otherquery = "mother(jane, X)"
+
+
+        # This should not throw an exception
+        for q in p.query(somequery):
+            p.abort()
+            for j in p.query(otherquery):
+                # This should not throw an error, because we aborted the previous query
+                pass
+
+        # But this one should
+        with self.assertRaises(pl.NestedQueryError):
+            for q in p.query(somequery):
+                for j in p.query(otherquery):
+                    # This should throw an error, because I opened the second
+                    # query
+                    pass