Stackless issue python#70: stackless specific handling of C-return values

Add two assertions to suspicious functions.

https://bitbucket.org/stackless-dev/stackless/issues/70
(grafted from bf3fa8395cd10e477bcc20b816cc659f1d5c8c0f)

Author: Anselm Kruis

Objects/abstract.c

@@ -2668,6 +2668,7 @@ PyIter_Next(PyObject *iter)
     STACKLESS_PROMOTE_METHOD(iter, tp_iternext);
     result = (*iter->ob_type->tp_iternext)(iter);
     STACKLESS_ASSERT();
+    STACKLESS_ASSERT_UNWINDING_VALUE_IS_NOT(result, NULL);
     if (result == NULL &&
         PyErr_Occurred() &&
         PyErr_ExceptionMatches(PyExc_StopIteration))

Objects/typeobject.c

@@ -4845,6 +4845,7 @@ wrap_next(PyObject *self, PyObject *args, void *wrapped)
     STACKLESS_PROMOTE_ALL();
     res = (*func)(self);
     STACKLESS_ASSERT();
+    STACKLESS_ASSERT_UNWINDING_VALUE_IS_NOT(res, NULL);
     if (res == NULL && !PyErr_Occurred())
         PyErr_SetNone(PyExc_StopIteration);
     return res;

Stackless/core/stackless_impl.h

@@ -181,6 +181,9 @@ PyAPI_FUNC(PyTaskletObject *) slp_get_watchdog(PyThreadState *ts, int interrupt)
 #define STACKLESS_RETVAL(obj) \
     (STACKLESS_UNWINDING(obj) ? Py_UnwindToken->tempval : (obj))
 
+#define STACKLESS_ASSERT_UNWINDING_VALUE_IS_NOT(obj, val) \
+    assert(!STACKLESS_UNWINDING(obj) || ((Py_UnwindToken->tempval) != (val)))
+
 /* macros for setting/resetting the stackless flag */
 
 #define STACKLESS_GETARG() int stackless = (stackless = slp_try_stackless, \
@@ -555,6 +558,7 @@ PyObject * slp_get_channel_callback(void);
 #define STACKLESS_ASSERT() assert(1)
 
 #define STACKLESS_RETVAL(obj) (obj)
+#define STACKLESS_ASSERT_UNWINDING_VALUE_IS_NOT(val) assert(1)
 
 #define STACKLESS_DECLARE_METHOD(type, meth)