When a ctypes C callback function is called, zero out the result

storage before converting the result to C data.  See the comment in
the code for details.

Provide a better context for errors when the conversion of a callback
function's result cannot be converted.

diff --git a/Modules/_ctypes/callbacks.c b/Modules/_ctypes/callbacks.c
index c091d88364a40a408340cbc93ed0cb7a0998b663..3ad6fb44e43ca0b5f58a2fde92a1cf642c7bb6d1 100644 (file)
--- a/Modules/_ctypes/callbacks.c
+++ b/Modules/_ctypes/callbacks.c
@@ -205,14 +205,24 @@ if (x == NULL) _AddTraceback(what, __FILE__, __LINE__ - 1), PyErr_Print()
 
        result = PyObject_CallObject(callable, arglist);
        CHECK("'calling callback function'", result);
+#ifdef WORDS_BIGENDIAN
+       /* See the corresponding code in callproc.c, around line 961 */
+       if (restype->type != FFI_TYPE_FLOAT && restype->size < sizeof(ffi_arg))
+               mem = (char *)mem + sizeof(ffi_arg) - restype->size;
+#endif
+       /* The code that converts 'result' into C data is not executed when
+          'callable' returns Py_None, so we zero out the memory that will
+          receive the C return data to not return random data.
+
+          Cleaner would be to call 'setfunc' anyway and complain with
+          PyErr_WriteUnraisable(), but ctypes has always accepted a Py_None
+          return value for *any* 'restype' and it would probably break too
+          much code if this is changed now.
+        */
+       memset(mem, 0, restype->size);
        if ((restype != &ffi_type_void) && result && result != Py_None) {
                PyObject *keep;
                assert(setfunc);
-#ifdef WORDS_BIGENDIAN
-               /* See the corresponding code in callproc.c, around line 961 */
-               if (restype->type != FFI_TYPE_FLOAT && restype->size < sizeof(ffi_arg))
-                       mem = (char *)mem + sizeof(ffi_arg) - restype->size;
-#endif
                keep = setfunc(mem, result, 0);
                CHECK("'converting callback result'", keep);
                /* keep is an object we have to keep alive so that the result
@@ -225,13 +235,13 @@ if (x == NULL) _AddTraceback(what, __FILE__, __LINE__ - 1), PyErr_Print()
                   itself knows how to manage the refcount of these objects.
                */
                if (keep == NULL) /* Could not convert callback result. */
-                       PyErr_WriteUnraisable(Py_None);
+                       PyErr_WriteUnraisable(callable);
                else if (keep == Py_None) /* Nothing to keep */
                        Py_DECREF(keep);
                else if (setfunc != getentry("O")->setfunc) {
                        if (-1 == PyErr_Warn(PyExc_RuntimeWarning,
                                             "memory leak in callback function."))
-                               PyErr_WriteUnraisable(Py_None);
+                               PyErr_WriteUnraisable(callable);
                }
        }
        Py_XDECREF(result);