[Bug default/27715] Fails to detect GObjectClass ABI changes

["maennich at android dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=27715

--- Comment #9 from Matthias Maennich <maennich at android dot com> ---
Anything passed to the API via opaque pointers is essentially hidden from ABI
analysis as no type information (other than void*) is attached to it. From an
ABI point of view the ABI of the symbol that passes void* is unchanged. Though
further down this of course can become incompatible.

There are cases that you intentionally want to pass void* to be ABI compatible
even though the type changes. E.g. if your library has such an interface:

void* get_handle(){
  // allocates the resources and returns just a handle to it.
  ...
  return (void*) some_object;
}

void do_something(void* handle) {
  // uses the handle as a reference
}

The ABI is stable even though the type could have changed. The client code
might not even know the concrete type and is also not supposed to cast to it.

So, as much as I would also like to sometimes inspect underlying types of
void*, flagging changes to those types is probably wrong. If you want this type
to be stable, you need to let it participate as a type in the interface.

One way of achieving this is to define a symbol just for the sake of ABI
analysis:

void (struct _GTypeModuleClass obj) { }

This will get picked up by libabigail and the type will participate in the
analysis then.

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