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

Mon Jun 11 10:59:11 UTC 2018

On Mon, Jun 11, 2018 at 12:53:47PM +0200, Erik Skultety wrote:
>On Mon, Jun 11, 2018 at 12:43:59PM +0200, Martin Kletzander wrote:
>> On Mon, Jun 11, 2018 at 12:12:16PM +0200, Pavel Hrdina wrote:
>> > On Sat, Jun 09, 2018 at 11:12:29PM +0200, Martin Kletzander wrote:
>> > > On Sat, Jun 09, 2018 at 10:06:55PM +0530, Sukrit Bhatnagar wrote:
>> > > > 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.
>> >
>> > I don't like this solution for several reasons, first of all
>> > VIR_AUTOFREE should be as simple as calling virFree().  If we decide for
>> > this or similar solution, we should create a new macro, not overload
>> > this one.
>> >
>> > In order to use this we would have to create another free function for
>> > each specific type anyway because the function passed to attribute
>> > cleanup takes a pointer to the actual type so as Sukrit mentioned
>> > virStringListFree would not be good enough and there would have to be
>> > some wrapper for it.
>> >
>> > > Just my two cents, but I like the second variant more.  For few reasons:
>> > >
>> > > - If we typedef char ** to something, then all users of that function will need
>> > >  to cast it back to char ** since they will be accessing the underlying strings
>> > >  (char *), even if there is a macro or a function for that, it seems the code
>> > >  will be less readable.
>> > >
>> > > - We are using a trick similar to the second variant in tests/virmock.h,
>> > >  although for a different purpose.  See VIR_MOCK_COUNT_ARGS
>> > >
>> > > - With the first approach we're going to have to create unnecessary types and
>> > >  possibly lot of them.
>> >
>> > Yes, for each type we would have to create a new typePtr and that is not
>> > nice so we might need to revise the design of VIR_AUTOPTR to take 'type'
>> > instead of 'typePtr' and as GLib is doing and create the special typedef
>> > only inside the macro and only for this purpose.
>> >
>> > Another issue that we need to take into account is that the external
>> > free functions might not be 'NULL' safe which we need to somehow ensure
>> > if they will be used with attribute cleanup.
>> >
>> > The original design is probably wrong and was heavily counting on the
>> > existing libvirt implementation.  We might need to update the design
>> > to make it the similar as GLib:
>> >
>> > #define VIR_AUTOPTR_FUNC_NAME(type) virAutoPtr##type
>> > #define VIR_AUTOPTR_TYPE_NAME(type) ##typeAutoPtr
>> >
>> > #define VIR_AUTOPTR(type) \
>> >    __attribute__((cleanup(VIR_AUTOPTR_FUNC_NAME(type)))) VIR_AUTOPTR_TYPE_NAME(type)
>> >
>> > #define VIR_DEFINE_AUTOPTR_FUNC(type, func) \
>> >    typedef type *VIR_AUTOPTR_TYPE_NAME(type); \
>> >    static inline void VIR_AUTOPTR_FUNC_NAME(type)(type **_ptr) \
>> >    { \
>> >        if (*_ptr) \
>> >            (func)(*_ptr); \
>> >    }
>> >
>>
>> I haven't followed all the discussions, but won't this be a problem
>> Sukrit is talking about that we need to fix?  For `char **` the above
>> will just not work.
>>
>> Just making sure we all understand each other.
>
>In the very last sentence of Pavel's last reply he said that char ** would have
>to be special-cased regardless ;).
>

Maybe that's why I shouldn't mix in such conversations.  Because I'm blind :)

Anyway if that is special-cased then we still need to make a special function
for that, so it feels contradictory to the rest of the message.  With Sukrit's
variant 2 it wouldn't have to be.  And it would still be as easy to use for
other types as Pavel wants it to be.

But don't get blocked on me ;)
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