[Libguestfs] [PATCH 1/2] python: Avoid crash if callback parameters cannot be built

[Richard W.M. Jones]

In the case that building the parameters to the Python event callback
fails, args was returned as NULL.  We immediately tried to call
Py_INCREF on this which crashed.  Returning NULL means the Python
function threw an exception, so print the exception and return (there
is no way to return an error here - the event is lost).

Reported-by: Yonatan Shtarkman
See: https://listman.redhat.com/archives/libguestfs/2023-February/030653.html
---
 python/handle.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/python/handle.c b/python/handle.c
index c8752baf47..f37e939e03 100644
--- a/python/handle.c
+++ b/python/handle.c
@@ -134,18 +134,21 @@ guestfs_int_py_event_callback_wrapper (guestfs_h *g,
   args = Py_BuildValue ("(Kis#O)",
                         (unsigned PY_LONG_LONG) event, event_handle,
                         buf, buf_len, py_array);
+  if (args == NULL) {
+    PyErr_PrintEx (0);
+    goto out;
+  }
+
   Py_INCREF (args);
-
   py_r = PyObject_CallObject (py_callback, args);
-
   Py_DECREF (args);
-
   if (py_r != NULL)
     Py_DECREF (py_r);
   else
     /* Callback threw an exception: print it. */
     PyErr_PrintEx (0);
 
+ out:
   PyGILState_Release (py_save);
 }
 
-- 
2.39.0