PostgreSQL logical decoder output plugin - unchanged toast data

Hi. I have a question about PostgreSQL logical decoder output plugin.

I am not specialist in Postgres at all, so maybe I miss some very
basic point. In the plugin, I want to always get all the values (even
those that are unchanged toast data)

When I try to get the datum that is internal on disk (here is code)

```
struct varlena *s = (struct varlena *)DatumGetPointer(origval);
struct varlena * ret = heap_tuple_untoast_attr(s);
Datum result = PointerGetDatum(PG_DETOAST_DATUM(ret));
```
it fails with no known snapshots error (from heap_tuple_untoast_attr).
My question is why is it that.

So, even theoretically it is not possible to the the varlena on disk
from logical replication plugin?

Mit freundlichen Grüßen,
Georgy Buranov