Improve the API/implementation for HPyCapsule_Destructor

[antocuni]

this is best explained by this comment:
#308 (comment)

[fangerer]

@antocuni I started to work on that and now remember some details: Even if we introduce a macro like you suggested, the problem is that there are the functions PyCapsule_GetDestructor and HPyCapsule_GetDestructor (note that capsule objects will be exchanged via Python modules and may be used from C API and HPy extensions). Those functions return the function pointer to the destructor. Now there are two cases:

1. If the capsule object was created in an HPy extension:
   PyCapsule_GetDestructor returns the ptr to the trampoline.
   HPyCapsule_GetDestructor returns the ptr to the actual function impl.
2. If the capsule object was created in a C API extension:
   PyCapsule_GetDestructor returns the ptr to the actual function impl..
   HPyCapsule_GetDestructor returns NULL (since we cannot provide a useful trampoline).

I can think of following solutions:

1. Just don't provide HPyCapsule_GetDestructor.
2. Always set a name in case the capsule is created from HPy. Then we can over-allocate the name and store the HPy destructor function pointer there.
3. Create a HPyCapsule (see below) and don't use CPython's PyCapsule_New but implement a custom allocator. Then we just need to have a way to identify a capsule object is an HPy capsule such that we can do the cast and access the additional field. But that is possible.

typedef struct { 
    PyCapsule base; 
    HPyCapsule_Destructor hpy_dtor;
} HPyCapsule;

To be honest, I think we should just kill HPyCapsule_GetDestructor. What is that function good for? Why would you want to get the destructor of a foreign capsule object? If it is your own capsule object, you have the destructor anyway.

[fangerer]

I just did a quick grep in the top4000 packages for PyCapsule_GetDestructor and didn't find a single usage. So, let's kill it!

[antocuni]

I just did a quick grep in the top4000 packages for PyCapsule_GetDestructor and didn't find a single usage. So, let's kill it!

yes, kill it!