On Sat, Jun 24, 2017 at 11:41:39AM -0700, Junio C Hamano wrote: > > The other nice thing about whence_only is that it flips the logic. So > > any existing callers which depend on filling the union automatically > > will not be affected (though I would be surprised if there are any such > > callers; most of that information isn't actually that interesting). > > Hmph, but the solution does not scale. When a caller wants whence > and something else that cannot be asked for or ignored by being a > "pointer to a result" field, such a request cannot be expressed. We > either need to make all fields in oi request to "pointer to a > result, if the result is needed, or NULL when the result is not of > interest", or give a bit for each non-pointer field to allow the > caller to express "I am not interested in the value of this field". True. > In the usecase Jonathan has, the caller's wish is a very narrow "I > am interested in nothing; just checking if the object is there", and > passing NULL for oi works fine. So I'm inclined to suggest that we > take that approach now and worry about a more generic and scalable > "how would one tell the machinery that the value for a field is > uninteresting when the field is not a pointer to result?" mechanism > until a real need arises. If we are open to writing anything, then I think it should follow the same pointer-to-data pattern that the rest of the struct does. I.e., declare the extra pack-data struct as a pointer, and let callers fill it in or not. It's excessive in the sense that it's not a variable-sized answer, but it at least makes the interface consistent. And no callers who read it would be silently broken; the actual data type in "struct object_info" would change, so they'd get a noisy compile error. -Peff
