Re: Question about MemoryContextRegisterResetCallback

On Sun, Jan 13, 2019 at 9:30 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> 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
>
>
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.

Thanks Tom!

-Michel

> regards, tom lane
>