Document strange behavior with parent pointer cycles involving an intermediate...

Document strange behavior with parent pointer cycles involving an intermediate object, but show that these don't lead to infinite recursion

src/Acquisition/tests.py

@@ -2427,6 +2427,45 @@ class TestParent(unittest.TestCase):
         self.assertRaises(AttributeError, Acquisition.aq_acquire,
             y, 'non_existant_attr')
 
+    def test_parent_parent_parent_circles(self):
+        class Impl(Acquisition.Implicit):
+            hello = 'world'
+        class Impl2(Acquisition.Implicit):
+            hello = 'world'
+        class Impl3(Acquisition.Implicit):
+            hello = 'world2'
+            only = 'here'
+
+        a = Impl()
+        b = Impl2()
+        c = Impl3()
+        a.__parent__ = b
+        b.__parent__ = c
+        c.__parent__ = a
+
+        # This is not quite what you'd expect, an AQ circle with an
+        # intermediate object gives strange results
+        self.assertTrue(a.__parent__.__parent__ is a)
+        self.assertTrue(a.__parent__.__parent__.__parent__.aq_base is b)
+        self.assertTrue(b.__parent__.__parent__ is b)
+        self.assertTrue(c.__parent__.__parent__ is c)
+
+        self.assertEqual(Acquisition.aq_acquire(a, 'hello'), 'world')
+        self.assertEqual(Acquisition.aq_acquire(b, 'hello'), 'world')
+        self.assertEqual(Acquisition.aq_acquire(c, 'hello'), 'world2')
+
+        self.assertRaises(AttributeError, Acquisition.aq_acquire,
+            a, 'only')
+        self.assertEqual(Acquisition.aq_acquire(b, 'only'), 'here')
+        self.assertEqual(Acquisition.aq_acquire(c, 'only'), 'here')
+
+        self.assertRaises(AttributeError, Acquisition.aq_acquire,
+            a, 'non_existant_attr')
+        self.assertRaises(AttributeError, Acquisition.aq_acquire,
+            b, 'non_existant_attr')
+        self.assertRaises(AttributeError, Acquisition.aq_acquire,
+            c, 'non_existant_attr')
+
 
 class TestUnicode(unittest.TestCase):