Meaning and intention of glue field in zonetree::types::ZoneCut is unclear. #579
Description
See also: NLnetLabs/cascade#222
It is unclear what data should or does end up in the glue field of an in-memory zone. The actual determination of glue is done in zonefile and ZoneBuilder.
The use of glue, ignoring zone walking for a moment.
IIRC it was meant to be as a way to cheaply reference actual glue records in the zone from the points in the tree at which the glue records will be needed during query response construction, e.g. "these records might also be useful to you" to be included in the ADDITIONAL section of a response.
I suspect that #222 correctly skips walking of glue records as the original glue record is probably stored as a Normal record as well as being referenced from glue at ZoneCuts, and only the original record should be emitted while walking the zone.
We should document how the glue field is supposed to be used and what it contains, especially whether or not it "duplicates" records stored elsewhere in the ApexZone data structures.