checkPermission: align behavior with objects raising in __getattr__

The observed problem was a behavior different between C and python
implementation on python 3, happening with Zope python script. When the
context can not be accessed by the current user, Zope binds a
`Shared.DC.Scripts.Bindings.UnauthorizedBinding`, a class that raises an
Unauthorized error when the context is actually accessed, in order to
postpone the Unauthorized if something is actually accessed. This class
does implements this by raising Unauthorized in __getattr__.

The python implementation of `checkPermission` uses `hasattr` and
`hasattr` has changed between python2 and python3, on python2 it was
ignoring all exceptions, including potential Unauthorized errors and
just returning False, but on python3 these errors are raised.
This change of behavior of python causes checkPermission to behave
differently: when using python implementation on python2 or when using
C implementation, such Unauthorized errors were gracefully handled and
caused checkPermission to return False, but on python3 checkPermission
raises.

This change make this scenario behave the same between python2, python3
and C implementation: Unauthorized errors raised in __getattr__ are
supported. The code is also micro-simplified by doing only one getattr
instead of hasattr and then getattr.

CHANGES.rst

@@ -6,6 +6,9 @@ For changes before version 3.0, see ``HISTORY.rst``.
 7.1 (unreleased)
 ----------------
 
+- Make Python implementation behave same as C implementation when using
+  ``checkPermission`` with objects raising ``Unauthorized`` in ``__getattr__``.
+
 
 7.0 (2024-05-30)
 ----------------

src/AccessControl/ImplPython.py

@@ -31,6 +31,7 @@
 from Acquisition import aq_parent
 from ExtensionClass import Base
 from zope.interface import implementer
+from zExceptions import Unauthorized as zExceptions_Unauthorized
 
 PURE_PYTHON = int(os.environ.get('PURE_PYTHON', '0'))
 if PURE_PYTHON:
@@ -71,8 +72,11 @@ def rolesForPermissionOn(perm, object, default=_default_roles, n=None):
     r = None
 
     while True:
-        if hasattr(object, n):
+        try:
             roles = getattr(object, n)
+        except (AttributeError, zExceptions_Unauthorized):
+            pass
+        else:
             if roles is None:
                 if _embed_permission_in_roles:
                     return (('Anonymous',), n)

src/AccessControl/cAccessControl.c

@@ -1847,13 +1847,16 @@ c_rolesForPermissionOn(PyObject *perm, PyObject *object,
   Py_INCREF(r);
 
   /*
-    while 1:
+    while True:
   */
   while (1)
     {
       /*
-        if hasattr(object, n):
+        try:
             roles = getattr(object, n)
+        except (AttributeError, zExceptions_Unauthorized):
+            pass
+        else:
       */
       PyObject *roles = PyObject_GetAttr(object, n);
       if (roles != NULL)

src/AccessControl/tests/testZopeSecurityPolicy.py

@@ -157,6 +157,15 @@ class PartlyProtectedSimpleItem3 (PartlyProtectedSimpleItem1):
     __roles__ = sysadmin_roles
 
 
+class DynamicallyUnauthorized(SimpleItemish):
+    # This class raises an Unauthorized on attribute access,
+    # similar to Zope's Shared.DC.Scripts.Bindings.UnauthorizedBinding
+    __ac_local_roles__ = {}
+
+    def __getattr__(self, name):
+        raise Unauthorized('Not authorized to access: %s' % name)
+
+
 class SimpleClass:
     attr = 1
 
@@ -173,6 +182,7 @@ def setUp(self):
         a.item1 = PartlyProtectedSimpleItem1()
         a.item2 = PartlyProtectedSimpleItem2()
         a.item3 = PartlyProtectedSimpleItem3()
+        a.d_item = DynamicallyUnauthorized()
         uf = UserFolder()
         a.acl_users = uf
         self.uf = a.acl_users
@@ -351,6 +361,11 @@ def test_checkPermission_proxy_role_scope(self):
                                                     r_subitem,
                                                     context))
 
+    def test_checkPermission_dynamically_unauthorized(self):
+        d_item = self.a.d_item
+        context = self.context
+        self.assertFalse(self.policy.checkPermission('View', d_item, context))
+
     def testUnicodeRolesForPermission(self):
         r_item = self.a.r_item
         context = self.context