[libvirt] [PATCH 8/9] virobject: Check if @parent is the first member in class

[Michal Privoznik]

On 04/16/2018 02:30 PM, Erik Skultety wrote:
> On Fri, Apr 13, 2018 at 04:47:15PM +0200, Michal Privoznik wrote:
>> Our virObject code relies heavily on the fact that the first
>> member of the class struct is type of virObject (or some
>> derivation of if). Let's check for that.
> 
> If a class is missing 'parent' memeber, it's a bug in the definition of the
> struct/class, therefore there should be a static assertion rather than a
> runtime check.

If a class is missing parent then you'd hit compile time error because
offsetof() is trying to get offset of a non-existent member.

> 
>>
>> Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
>> ---
>>  src/util/virobject.c | 31 +++++++++++++++++++++----------
>>  src/util/virobject.h |  5 ++++-
>>  2 files changed, 25 insertions(+), 11 deletions(-)
>>
>> diff --git a/src/util/virobject.c b/src/util/virobject.c
>> index c5a98d21cc..e184f5349e 100644
>> --- a/src/util/virobject.c
>> +++ b/src/util/virobject.c
>> @@ -77,6 +77,7 @@ virObjectOnceInit(void)
>>  {
>>      if (!(virObjectClass = virClassNew(NULL,
>>                                         "virObject",
>> +                                       0,
>>                                         sizeof(virObject),
>>                                         NULL)))
> 
> Also, I don't like this extra parameter, which really shouldn't be needed; you
> created a macro which hides this parameter, but that doesn't mean that
> design-wise it makes sense to have it there, think of it as a constructor, you
> don't pass a constructor an offset of the class' member, because it shouldn't
> have need for it, but you do, solely for the purpose of checking whether we have
> a particular member in place.
> So, to start a discussion about this (I also think Dan posted something related
> to this recently, but I don't seem to be able to find it in the archives - do I
> even archive?!!!), I came up with my first compile-time hack ever, it seems to
> work like expected, but I'd like to hear your opinions both the macro itself
> and the approach we're going to take, so here's my replacement patch:
> 
> diff --git a/src/util/virobject.h b/src/util/virobject.h
> index 92dd51239..2a973d401 100644
> --- a/src/util/virobject.h
> +++ b/src/util/virobject.h
> @@ -75,8 +75,12 @@ virClassPtr virClassForObjectRWLockable(void);
>  #  define VIR_PARENT_REQUIRED ATTRIBUTE_NONNULL(1)
>  # endif
> 
> +# define VIR_CLASS_HAS_PARENT(name) \
> +        !sizeof(char[0-offsetof(name, parent)])

I don't quite understand why this is so obfuscated. Anyway, since
VIR_CLASS_NEW() is going to be a stand alone macro (like VIR_ENUM_DECL
for instance) we can do plain:

#define VIR_CLASS_NEW(prt, name) \
  verify(offsetof(name, parent) == 0); \
  if (!(name##Class = virClassNew(prt, #name, sizeof(name), name##Dispose))) \
    return -1;

(written from the top of my head, not tested, not compiled, don't take
it too much literally)

We couldn't do that if VIR_CLASS_NEW() is still a function-like macro
( if (!(nameClass = VIR_CLASS_NEW(...))) return -1; ).

Michal