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

If a C-function checks a return value of another C-function of type PyObject*,
it must handle the case of unwinding. This commit fixes PyObject_Call in
abstract.c.

https://bitbucket.org/stackless-dev/stackless/issues/70
(grafted from 113b9eaa4dd219a4f249f082f6772b9e5086e1a5)

Author: Anselm Kruis

Objects/abstract.c

@@ -2048,7 +2048,7 @@ PyObject_Call(PyObject *func, PyObject *arg, PyObject *kw)
         if (!stackless)
 #endif
         Py_LeaveRecursiveCall();
-        if (result == NULL && !PyErr_Occurred())
+        if (STACKLESS_RETVAL(result) == NULL && !PyErr_Occurred())
             PyErr_SetString(
                 PyExc_SystemError,
                 "NULL result without error in PyObject_Call");

Stackless/core/stackless_impl.h

@@ -178,6 +178,9 @@ PyAPI_FUNC(PyTaskletObject *) slp_get_watchdog(PyThreadState *ts, int interrupt)
 /* an arbitrary positive number */
 #define STACKLESS_UNWINDING_MAGIC 0x7fedcba9
 
+#define STACKLESS_RETVAL(obj) \
+    (STACKLESS_UNWINDING(obj) ? Py_UnwindToken->tempval : (obj))
+
 /* macros for setting/resetting the stackless flag */
 
 #define STACKLESS_GETARG() int stackless = (stackless = slp_try_stackless, \
@@ -551,6 +554,8 @@ PyObject * slp_get_channel_callback(void);
 #define STACKLESS_RETRACT() assert(1)
 #define STACKLESS_ASSERT() assert(1)
 
+#define STACKLESS_RETVAL(obj) (obj)
+
 #define STACKLESS_DECLARE_METHOD(type, meth)
 
 #endif /* STACKLESS */