I suppose what you're doing is returning a pointer to a GraphBLAS object as a Datum (or part of a pass-by-ref Datum)? If so, that's not going to work terribly well, because it ignores the problem that datatype- independent code is going to assume it can copy Datum values using datumCopy() or equivalent logic. More often than not, such copying is done to move the value into a different memory context in preparation for freeing the original context. If you delete the GraphBLAS object when the original context is deleted, you now have a dangling pointer in the copy.
We did invent some infrastructure awhile ago that could potentially handle this sort of situation: it's the "expanded datum" stuff. The idea here would be that your representation involving a GraphBLAS pointer would be an efficient-to-operate-on expanded object. You would need to be able to serialize and deserialize that representation into plain self-contained Datums (probably varlena blobs), but hopefully GraphBLAS is capable of going along with that. You'd still need a memory context reset callback attached to each expanded object, to free the associated GraphBLAS object --- but expanded objects are explicitly aware of which context they're in, so at least in principle that should work. (I'm not sure anyone's actually tried to build an expanded-object representation that has external resources, so we might find there are some bugs to fix there.)
Ah I see, the water is much deeper here. Thanks for the detailed explanation, expandeddatum.h was very helpful and I see now how array_expanded works. If I run into any problems registering my callback in the expanded context I'll repost back.
