Raise `RuntimeError: Recursion detected in acquisition wrapper` if an object...

Raise `RuntimeError: Recursion detected in acquisition wrapper` if an object with a `__parent__` pointer points to a wrapper that in turn points to the original object.

CHANGES.txt

@@ -4,6 +4,10 @@ Changelog
 4.0 (unreleased)
 ----------------
 
+- Raise `RuntimeError: Recursion detected in acquisition wrapper` if an object
+  with a `__parent__` pointer points to a wrapper that in turn points to the
+  original object.
+
 - Prevent wrappers to be created while accessing `__parent__` on types derived
   from Explicit or Implicit base classes.
 

src/Acquisition/_Acquisition.c

@@ -545,6 +545,14 @@ Wrapper_findattr(Wrapper *self, PyObject *oname,
 	  Py_XDECREF(r); Py_XDECREF(v); Py_XDECREF(tb);
 	  r=NULL;
 	}
+      /* Deal with mixed __parent__ / aq_parent circles */
+      else if (self->container && isWrapper(self->container) &&
+               WRAPPER(self->container)->container &&
+               self == WRAPPER(WRAPPER(self->container)->container)) {
+          PyErr_SetString(PyExc_RuntimeError,
+              "Recursion detected in acquisition wrapper");
+          return NULL;
+      }
       /* normal attribute lookup */
       else if ((r=PyObject_GetAttr(self->obj,oname)))
 	{

src/Acquisition/tests.py

@@ -2383,20 +2383,17 @@ def test___parent__aq_parent_circles():
       >>> Acquisition.aq_acquire(y, 'non_existant_attr')
       Traceback (most recent call last):
       ...
-      AttributeError: non_existant_attr
+      RuntimeError: Recursion detected in acquisition wrapper
 
       >>> x.non_existant_attr
       Traceback (most recent call last):
       ...
       AttributeError: non_existant_attr
-    """
 
-    # XXX: disabled
-    """
       >>> y.non_existant_attr
       Traceback (most recent call last):
       ...
-      AttributeError: non_existant_attr
+      RuntimeError: Recursion detected in acquisition wrapper
     """
 
 def test_unwrapped_implicit_acquirer_unwraps__parent__():