Re: [PATCH bpf v3 2/4] selftests/bpf: Test refcount_acquire return nullability

> > +       tmp = bpf_obj_new(typeof(*tmp));
> > +       if (!tmp)
> > +               return 3;
> > +       bpf_obj_drop(tmp);

This dummy code creates a temporary node_refcount_only object and
immediately releases it, the actual bug test starts afterward. Its
purpose is compiler scaffolding for BTF: The dummy bpf_obj_new()
forces Clang to emit the complete BTF definition of that type,
including its bpf_refcount field. bpf_obj_drop() then releases the
temporary object so it won't affect the actual bug checking code.

If we remove the dummy block, Clang might not include the complete
node_refcount_only description in BTF. The program could then fail
during loading (e.g. while creating the map, since it can't validate
the kptr's complete type, or during verification, since the verifier
is not sure the object contains bpf_refcount field) before reaching
the actual test. (whether it still works depends on the clang version)