[Libguestfs] [libnbd PATCH 2/2] python: Fix memory leak on callback error

[Eric Blake]

Commit 89af010d ("python: Fix more memory leaks", v1.5.2) tried to
patch some python memory leaks on callback error paths.  But it missed
an obvious one: we are mistakenly calling Py_INCREF on the result of
Py_BuildValue, which results in an object that is over-referenced and
thus never gets freed.  Worse but less likely, if Py_BuildValue fails
due to no memory, Py_INCREF(NULL) causes a crash.

I tested this by watching the memory usage of:
$ nbdkit --no-sr memory 1
$ nbdsh -u nbd://localhost
nbd> def f(a, b, c, d, e):
...   pass
...
nbd> def leak():
...   for i in range(0, 100000):
...    try:
...     h.block_status(1, 0, f)
...    except:
...     pass
...

then repeatedly using leak().

Fixes: 936488d4 ("python: Implement Callback properly.", v0.1)
---

Hmm, I'm still seeing a leak even with this applied, so I'm missing
yet another problem.

 generator/Python.ml | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/generator/Python.ml b/generator/Python.ml
index 49281bf..cb6c6b8 100644
--- a/generator/Python.ml
+++ b/generator/Python.ml
@@ -1,6 +1,6 @@
 (* hey emacs, this is OCaml code: -*- tuareg -*- *)
 (* nbd client library in userspace: Python bindings
- * Copyright (C) 2013-2020 Red Hat Inc.
+ * Copyright (C) 2013-2021 Red Hat Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
@@ -216,7 +216,7 @@ let print_python_closure_wrapper { cbname; cbargs } =
     | CBArrayAndLen _ | CBMutable _ -> assert false
   ) cbargs;
   pr ");\n";
-  pr "  Py_INCREF (py_args);\n";
+  pr "  if (!py_args) { PyErr_PrintEx (0); return -1; }\n";
   pr "\n";
   pr "  py_save = PyGILState_Ensure ();\n";
   pr "  py_ret = PyObject_CallObject (data->fn, py_args);\n";
-- 
2.33.1