Re: Question about MemoryContextRegisterResetCallback

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.)

Take a look at

src/include/utils/expandeddatum.h
src/backend/utils/adt/expandeddatum.c
src/backend/utils/adt/array_expanded.c
src/backend/utils/adt/expandedrecord.c

                        regards, tom lane