[libvirt] [GSoC] Code design for scalar and external types

[Sukrit Bhatnagar]

Hi,

I am starting this discussion thread as a continuation of my GSoC
weekly meeting with Erik and Pavel on 8th June.

I was going through src/util/virstring.c for adding cleanup macros and
saw that virStringListFree takes on char ** as an argument, and
equivalently, we declare a list of strings as char **.

For the cleanup function defined by VIR_DEFINE_AUTOPTR_FUNC, it is
required that the associated type has a name like virSomethingPtr.

It was also discussed that there are similar issues with DBus types,
but VIR_AUTOFREE can work there as we use VIR_ALLOC. I honestly don't
know much about that.

We discussed that we have two solutions:

- Create a virSomethingPtr by typedef-ing char**

As Pavel told, GLib has typedef gchar** GStrv; which is used together
with g_auto and it has g_strfreev(gchar **str_array) which is the same
as we have virStringListFree()

I have tried adding the following in src/util/virstrnig.h, and it
seems to work fine:

typedef char **virStringList;
VIR_DEFINE_AUTOPTR_FUNC(virStringList, virStringListFree)

We can use it as:
VIR_AUTOPTR(virStringList) lines = NULL;

There may be other scalar and external types where this problem
occurs, and it is not good to create a typedef for each of them, but
maybe we can make an exception for char ** and create a type for it.


- Overload VIR_AUTOFREE macro by making it variadic

As Erik told, we could make VIR_AUTOFREE a variadic macro whose
varying parameter can be the Free function name. If left blank, we use
virFree.

I went ahead with trying it and after reading some posts on
StackOverflow, I came up with this:

#define _VIR_AUTOFREE_0(type) __attribute__((cleanup(virFree))) type
#define _VIR_AUTOFREE_1(type, func) __attribute__((cleanup(func))) type

#define _VIR_AUTOFREE_OVERLOADER(_1, _2, NAME, ...) NAME
#define VIR_AUTOFREE(...) _VIR_AUTOFREE_OVERLOADER(__VA_ARGS__,
_VIR_AUTOFREE_1, _VIR_AUTOFREE_0)(__VA_ARGS__)

The required functionality is working as expected; passing only one
argument will use virFree, and passing two arguments will use the
function specified as 2nd argument. Passing more than 2 arguments will
result in an error.

The macros with _ prefix are meant to be for internal use only.
Also, @func needs to be a wrapper around virStringListFree as it will
take char ***, not just char **. We probably need to define a new
function.

Here we are specifying the Free function to use at the time of usage
of the VIR_AUTOFREE macro, which may make the code look bad:
VIR_AUTOFREE(char **, virStringListSomethingFree) lines = NULL;


Suggestions and opinions are welcome.

Thanks,
Sukrit