I have a wild idea of renaming nextOffset in SpGistLeafTupleData. Or at least documenting in comments that this field is more than just an offset.
Seems reasonable as previous changes localized mentions of nextOffset only to leaf tuple definition and access macros. So I've done this renaming.
This looks like assert rather than real runtime check in spgLeafTupleSize()
+ if (state->includeTupdesc->natts + 1 >= INDEX_MAX_KEYS) + ereport(ERROR, + (errcode(ERRCODE_TOO_MANY_COLUMNS), + errmsg("number of index columns (%d) exceeds limit (%d)", + state->includeTupdesc->natts, INDEX_MAX_KEYS))); Even if you go with check, number of columns is state->includeTupdesc->natts + 1.
I placed this check only once on SpGist state creation and replaced the other checks to asserts.
Also I'd refactor this + /* Form descriptor for INCLUDE columns if any */
