[Libguestfs] [nbdkit PATCH 2/2] python: Simplify calling into plugin

[Eric Blake]

On 04/06/2018 05:24 PM, Eric Blake wrote:
> PyObject_CallObject is powerful, but awkward - we have to wrap
> all arguments into a temporary tuple before using it.
> 
> Let python do more of the work by using PyObject_CallFunction
> anywhere that all arguments can be described by a Py_BuildValue()
> format string, instead of creating one-shot arguments ourselves.
> In fact, for our py_config(), this makes it easier to not worry
> about python 2 String vs. python 3 Unicode.
> 
> Similarly, PyObject_CallFunctionObjArgs is nicer when we
> already have PyObjects for all parameters (including in py_open(),
> where we can't use a Py_BuildValue() string for converting a
> C int into Py_True or Py_False, but can easily avoid the tuple
> wrapper).
> 
> py_zero() is not converted, as it will be changed differently
> to make 'may_trim' an optional callback parameter.
> 
> Signed-off-by: Eric Blake <eblake at redhat.com>
> ---
>  plugins/python/python.c | 97 ++++++++-----------------------------------------
>  1 file changed, 15 insertions(+), 82 deletions(-)
> 

> @@ -258,17 +257,8 @@ py_config (const char *key, const char *value)
>      /* Other parameters are passed to the Python .config callback. */
>      PyErr_Clear ();
> 
> -    args = PyTuple_New (2);

In fact, this patch is a corner-case bug fix.  Here, and in all sorts of
similar places, we are failing to check for args == NULL (unlikely
except on OOM conditions, but still a valid error condition, where we
would presumably have a nice Python error object to report failure with)...

> -#ifdef HAVE_PYSTRING_FROMSTRING
> -    PyTuple_SetItem (args, 0, PyString_FromString (key));

...and instead passing NULL on to a function that would crash.  We are
likewise not checking for errors here (although such errors should never
be possible).

> -    PyTuple_SetItem (args, 1, PyString_FromString (value));
> -#else
> -    PyTuple_SetItem (args, 0, PyUnicode_FromString (key));
> -    PyTuple_SetItem (args, 1, PyUnicode_FromString (value));
> -#endif
> -    r = PyObject_CallObject (fn, args);
> +    r = PyObject_CallFunction (fn, "ss", key, value);

The replacement code is not only shorter, it has fewer places to worry
about corner-case boilerplate error checking.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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