[Libguestfs] [nbdkit PATCH] python: Let zero's may_trim parameter be optional

[Eric Blake]

On 04/06/2018 09:52 AM, Eric Blake wrote:
> In preparation for adding other optional flag arguments to the
> python bindings, start by making the existing 'may_trim' flag
> to 'zero' be optional.  That is, the plugin need not define
> the parameter if it does not make any semantic difference (ie.
> if the plugin ignores the hint and never trims); while if the
> parameter exists, we now pass it as a keyword argument rather
> than as a positional argument.  This requires the plugin to
> give the parameter a specific name, and works whether or not
> the plugin provides a default for the parameter (although we do
> now recommend that plugins provide a default value, as it makes
> plugins built against newer nbdkit more likely to run even on
> older nbdkit).  We can't see through a plugin that used
> '**kwargs', but that is less likely.
> 

So far, I've only tested under Python 2, on Fedora 27.  Before
committing, I'll have to re-test with python 3.  I already noted that:

> +/* Does a callback support the given keyword parameter? */
> +static int
> +callback_has_parameter (PyObject *fn, const char *name)
> +{
> +  int r = 0;
> +  PyObject *inspect, *pname, *spec, *args;
> +
> +  assert (script != NULL);
> +  assert (module != NULL);
> +
> +  pname = PyString_FromString ("inspect");

Needs python 2/3 protection using HAVE_PYSTRING_FROMSTRING.

> +  if (!pname)
> +    return -1;
> +  inspect = PyImport_Import (pname);
> +  Py_DECREF (pname);
> +
> +  if (!inspect)
> +    return -1;
> +
> +#if PY_MAJOR_VERSION >= 3
> +  pname = PyString_FromString ("getfullargspec");

and this should use PyUnicode_FromString.

> +#else
> +  pname = PyString_FromString ("getargspec");
> +#endif
> +  spec = PyObject_CallMethodObjArgs (inspect, pname, fn, NULL);
> +  Py_DECREF (pname);
> +  Py_DECREF (inspect);
> +  if (!spec)
> +    return -1;
> +

> @@ -530,21 +594,31 @@ py_zero (void *handle, uint32_t count, uint64_t offset, int may_trim)

>      PyTuple_SetItem (args, 2, PyLong_FromUnsignedLongLong (offset));
> -    PyTuple_SetItem (args, 3, PyBool_FromLong (may_trim));
> -    r = PyObject_CallObject (fn, args);
> +    if (zero_may_trim)
> +      kwargs = Py_BuildValue("{s:i}", "may_trim", may_trim);

I also wonder if Py_BuildValue() is any easier to use instead of
conditionals on HAVE_PYSTRING_FROMSTRING.

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

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