Fix for an obscure bug introduced by revs 46806 and 46808, with a test.

The problem of checking too eagerly for recursive calls is the
following: if a RuntimeError is caused by recursion, and if code needs
to normalize it immediately (as in the 2nd test), then
PyErr_NormalizeException() needs a call to the RuntimeError class to
instantiate it, and this hits the recursion limit again...  causing
PyErr_NormalizeException() to never finish.

Moved this particular recursion check to slot_tp_call(), which is not
involved in instantiating built-in exceptions.

Backport candidate.

Lib/test/test_exceptions.py

@@ -305,6 +305,18 @@ class ExceptionTests(unittest.TestCase):
         x = DerivedException(fancy_arg=42)
         self.assertEquals(x.fancy_arg, 42)
 
+    def testInfiniteRecursion(self):
+        def f():
+            return f()
+        self.assertRaises(RuntimeError, f)
+
+        def g():
+            try:
+                return g()
+            except ValueError:
+                return -1
+        self.assertRaises(RuntimeError, g)
+
 def test_main():
     run_unittest(ExceptionTests)
 

Objects/abstract.c

@@ -1796,17 +1796,7 @@ PyObject_Call(PyObject *func, PyObject *arg, PyObject *kw)
         ternaryfunc call;
 
 	if ((call = func->ob_type->tp_call) != NULL) {
-		PyObject *result = NULL;
+		PyObject *result = (*call)(func, arg, kw);
-		/* slot_tp_call() will be called and ends up calling
-		   PyObject_Call() if the object returned for __call__ has
-		   __call__ itself defined upon it.  This can be an infinite
-		   recursion if you set __call__ in a class to an instance of
-		   it. */
-		if (Py_EnterRecursiveCall(" in __call__")) {
-		    return NULL;
-		}
-		result = (*call)(func, arg, kw);
-		Py_LeaveRecursiveCall();
 		if (result == NULL && !PyErr_Occurred())
 			PyErr_SetString(
 				PyExc_SystemError,

Objects/typeobject.c

@@ -4590,7 +4590,16 @@ slot_tp_call(PyObject *self, PyObject *args, PyObject *kwds)
 
 	if (meth == NULL)
 		return NULL;
+
+	/* PyObject_Call() will end up calling slot_tp_call() again if
+	   the object returned for __call__ has __call__ itself defined
+	   upon it.  This can be an infinite recursion if you set
+	   __call__ in a class to an instance of it. */
+	if (Py_EnterRecursiveCall(" in __call__"))
+		return NULL;
 	res = PyObject_Call(meth, args, kwds);
+	Py_LeaveRecursiveCall();
+
 	Py_DECREF(meth);
 	return res;
 }