[Libguestfs] [libnbd PATCH v3 1/5] python: Simplify passing of mutable *error to callbacks

[Eric Blake]

Instead of open-coding our code to create a PyObject wrapper of the
mutable *error to be passed to each callback, extract this into a
helper function.  We can then slightly optimize things to hang on to a
single pointer to the ctypes module, rather than looking it up on
every callback.  The generated code shrinks by more than the added
code in utils.c:

| --- python/methods.h.bak	2022-06-08 14:35:00.454786844 -0500
| +++ python/methods.h	2022-06-08 14:43:24.681596853 -0500
| @@ -35,6 +35,7 @@
|  extern PyObject *nbd_internal_py_get_aio_view (PyObject *, bool);
|  extern int nbd_internal_py_init_aio_buffer (PyObject *);
|  extern PyObject *nbd_internal_py_get_nbd_buffer_type (void);
| +extern PyObject *nbd_internal_py_wrap_errptr (int);
|
|  static inline struct nbd_handle *
|  get_handle (PyObject *obj)
| --- python/methods.c.bak	2022-06-08 14:35:00.450786838 -0500
| +++ python/methods.c	2022-06-08 14:43:24.696596877 -0500
| @@ -75,13 +75,7 @@ chunk_wrapper (void *user_data, const vo
|    PyObject *py_args, *py_ret;
|    PyObject *py_error = NULL;
|
| -  PyObject *py_error_modname = PyUnicode_FromString ("ctypes");
| -  if (!py_error_modname) { PyErr_PrintEx (0); goto out; }
| -  PyObject *py_error_mod = PyImport_Import (py_error_modname);
| -  Py_DECREF (py_error_modname);
| -  if (!py_error_mod) { PyErr_PrintEx (0); goto out; }
| -  py_error = PyObject_CallMethod (py_error_mod, "c_int", "i", *error);
| -  Py_DECREF (py_error_mod);
| +  py_error = nbd_internal_py_wrap_errptr (*error);
|    if (!py_error) { PyErr_PrintEx (0); goto out; }
|
|    py_args = Py_BuildValue ("(y#KIO)", subbuf, (int) count, offset, status,
| @@ -132,13 +126,7 @@ completion_wrapper (void *user_data, int
|    PyObject *py_args, *py_ret;
|    PyObject *py_error = NULL;
|
| -  PyObject *py_error_modname = PyUnicode_FromString ("ctypes");
| -  if (!py_error_modname) { PyErr_PrintEx (0); goto out; }
| -  PyObject *py_error_mod = PyImport_Import (py_error_modname);
| -  Py_DECREF (py_error_modname);
| -  if (!py_error_mod) { PyErr_PrintEx (0); goto out; }
| -  py_error = PyObject_CallMethod (py_error_mod, "c_int", "i", *error);
| -  Py_DECREF (py_error_mod);
| +  py_error = nbd_internal_py_wrap_errptr (*error);
|    if (!py_error) { PyErr_PrintEx (0); goto out; }
|
|    py_args = Py_BuildValue ("(O)", py_error);
| @@ -239,13 +227,7 @@ extent_wrapper (void *user_data, const c
|      if (!py_e_entries) { PyErr_PrintEx (0); goto out; }
|      PyList_SET_ITEM (py_entries, i_entries, py_e_entries);
|    }
| -  PyObject *py_error_modname = PyUnicode_FromString ("ctypes");
| -  if (!py_error_modname) { PyErr_PrintEx (0); goto out; }
| -  PyObject *py_error_mod = PyImport_Import (py_error_modname);
| -  Py_DECREF (py_error_modname);
| -  if (!py_error_mod) { PyErr_PrintEx (0); goto out; }
| -  py_error = PyObject_CallMethod (py_error_mod, "c_int", "i", *error);
| -  Py_DECREF (py_error_mod);
| +  py_error = nbd_internal_py_wrap_errptr (*error);
|    if (!py_error) { PyErr_PrintEx (0); goto out; }
|
|    py_args = Py_BuildValue ("(sKOO)", metacontext, offset, py_entries,
---
 generator/Python.ml |  9 ++-------
 python/utils.c      | 19 +++++++++++++++++++
 2 files changed, 21 insertions(+), 7 deletions(-)

diff --git a/generator/Python.ml b/generator/Python.ml
index e94f37b..6980034 100644
--- a/generator/Python.ml
+++ b/generator/Python.ml
@@ -41,6 +41,7 @@ let
 extern PyObject *nbd_internal_py_get_aio_view (PyObject *, bool);
 extern int nbd_internal_py_init_aio_buffer (PyObject *);
 extern PyObject *nbd_internal_py_get_nbd_buffer_type (void);
+extern PyObject *nbd_internal_py_wrap_errptr (int);

 static inline struct nbd_handle *
 get_handle (PyObject *obj)
@@ -184,13 +185,7 @@ let
     | CBInt _
     | CBInt64 _ -> ()
     | CBMutable (Int n) ->
-       pr "  PyObject *py_%s_modname = PyUnicode_FromString (\"ctypes\");\n" n;
-       pr "  if (!py_%s_modname) { PyErr_PrintEx (0); goto out; }\n" n;
-       pr "  PyObject *py_%s_mod = PyImport_Import (py_%s_modname);\n" n n;
-       pr "  Py_DECREF (py_%s_modname);\n" n;
-       pr "  if (!py_%s_mod) { PyErr_PrintEx (0); goto out; }\n" n;
-       pr "  py_%s = PyObject_CallMethod (py_%s_mod, \"c_int\", \"i\", *%s);\n" n n n;
-       pr "  Py_DECREF (py_%s_mod);\n" n;
+       pr "  py_%s = nbd_internal_py_wrap_errptr (*%s);\n" n n;
        pr "  if (!py_%s) { PyErr_PrintEx (0); goto out; }\n" n;
     | CBString _
     | CBUInt _
diff --git a/python/utils.c b/python/utils.c
index e0df181..cd44b90 100644
--- a/python/utils.c
+++ b/python/utils.c
@@ -173,3 +173,22 @@ nbd_internal_py_get_nbd_buffer_type (void)
   }
   return type;
 }
+
+/* Helper to package callback *error into modifiable PyObject */
+PyObject *
+nbd_internal_py_wrap_errptr (int err)
+{
+  static PyObject *py_ctypes_mod;
+
+  if (!py_ctypes_mod) {
+    PyObject *py_modname = PyUnicode_FromString ("ctypes");
+    if (!py_modname)
+      return NULL;
+    py_ctypes_mod = PyImport_Import (py_modname);
+    Py_DECREF (py_modname);
+    if (!py_ctypes_mod)
+      return NULL;
+  }
+
+  return PyObject_CallMethod (py_ctypes_mod, "c_int", "i", err);
+}
-- 
2.36.1