abstract_get_bases(): Clarify exactly what the return values and

states can be for this function, and ensure that only AttributeErrors
are masked.  Any other exception raised via the equivalent of
getattr(cls, '__bases__') should be propagated up.

abstract_issubclass(): If abstract_get_bases() returns NULL, we must
call PyErr_Occurred() to see if an exception is being propagated, and
return -1 or 0 as appropriate.  This is the specific fix for a problem
whereby if getattr(derived, '__bases__') raised an exception, an
"undetected error" would occur (under a debug build).  This nasty
situation was uncovered when writing a security proxy extension type
for the Zope3 project, where the security proxy raised a Forbidden
exception on getattr of __bases__.

PyObject_IsInstance(), PyObject_IsSubclass(): After both calls to
abstract_get_bases(), where we're setting the TypeError if the return
value is NULL, we must first check to see if an exception occurred,
and /not/ mask an existing exception.

Neil Schemenauer should double check that these changes don't break
his ExtensionClass examples (there aren't any test cases for those
examples and abstract_get_bases() was added by him in response to
problems with ExtensionClass).  Neil, please add test cases if
possible!

I belive this is a bug fix candidate for Python 2.2.2.

diff --git a/Objects/abstract.c b/Objects/abstract.c
index 622e8191d813d78c9bf4bed816bf4f2a87deed2c..d10d9d4b0ca2217d71969d38f780e7cac4bcc361 100644 (file)
--- a/Objects/abstract.c
+++ b/Objects/abstract.c
@@ -1861,6 +1861,32 @@ PyObject_CallFunctionObjArgs(PyObject *callable, ...)
 
 /* isinstance(), issubclass() */
 
+/* abstract_get_bases() has logically 4 return states, with a sort of 0th
+ * state that will almost never happen.
+ *
+ * 0. creating the __bases__ static string could get a MemoryError
+ * 1. getattr(cls, '__bases__') could raise an AttributeError
+ * 2. getattr(cls, '__bases__') could raise some other exception
+ * 3. getattr(cls, '__bases__') could return a tuple
+ * 4. getattr(cls, '__bases__') could return something other than a tuple
+ *
+ * Only state #3 is a non-error state and only it returns a non-NULL object
+ * (it returns the retrieved tuple).
+ *
+ * Any raised AttributeErrors are masked by clearing the exception and
+ * returning NULL.  If an object other than a tuple comes out of __bases__,
+ * then again, the return value is NULL.  So yes, these two situations
+ * produce exactly the same results: NULL is returned and no error is set.
+ *
+ * If some exception other than AttributeError is raised, then NULL is also
+ * returned, but the exception is not cleared.  That's because we want the
+ * exception to be propagated along.
+ *
+ * Callers are expected to test for PyErr_Occurred() when the return value
+ * is NULL to decide whether a valid exception should be propagated or not.
+ * When there's no exception to propagate, it's customary for the caller to
+ * set a TypeError.
+ */
 static PyObject *
 abstract_get_bases(PyObject *cls)
 {
@@ -1872,13 +1898,16 @@ abstract_get_bases(PyObject *cls)
                if (__bases__ == NULL)
                        return NULL;
        }
-
        bases = PyObject_GetAttr(cls, __bases__);
-       if (bases == NULL || !PyTuple_Check(bases)) {
-               Py_XDECREF(bases);
+       if (bases == NULL) {
+               if (PyErr_ExceptionMatches(PyExc_AttributeError))
+                       PyErr_Clear();
+               return NULL;
+       }
+       if (!PyTuple_Check(bases)) {
+               Py_DECREF(bases);
                return NULL;
        }
-
        return bases;
 }
 
@@ -1895,9 +1924,11 @@ abstract_issubclass(PyObject *derived, PyObject *cls)
                return 1;
 
        bases = abstract_get_bases(derived);
-       if (bases == NULL)
+       if (bases == NULL) {
+               if (PyErr_Occurred())
+                       return -1;
                return 0;
-
+       }
        n = PyTuple_GET_SIZE(bases);
        for (i = 0; i < n; i++) {
                r = abstract_issubclass(PyTuple_GET_ITEM(bases, i), cls);
@@ -1942,7 +1973,9 @@ PyObject_IsInstance(PyObject *inst, PyObject *cls)
        else {
                PyObject *cls_bases = abstract_get_bases(cls);
                if (cls_bases == NULL) {
-                       PyErr_SetString(PyExc_TypeError,
+                       /* Do not mask errors. */
+                       if (!PyErr_Occurred())
+                               PyErr_SetString(PyExc_TypeError,
                                "isinstance() arg 2 must be a class or type");
                        return -1;
                }
@@ -1977,7 +2010,9 @@ PyObject_IsSubclass(PyObject *derived, PyObject *cls)
 
                derived_bases = abstract_get_bases(derived);
                if (derived_bases == NULL) {
-                       PyErr_SetString(PyExc_TypeError,
+                       /* Do not mask errors */
+                       if (!PyErr_Occurred())
+                               PyErr_SetString(PyExc_TypeError,
                                        "issubclass() arg 1 must be a class");
                        return -1;
                }
@@ -1985,7 +2020,9 @@ PyObject_IsSubclass(PyObject *derived, PyObject *cls)
 
                cls_bases = abstract_get_bases(cls);
                if (cls_bases == NULL) {
-                       PyErr_SetString(PyExc_TypeError,
+                       /* Do not mask errors */
+                       if (!PyErr_Occurred())
+                               PyErr_SetString(PyExc_TypeError,
                                        "issubclass() arg 2 must be a class");
                        return -1;
                }