Missing Column names with multi-insert

Hello all,

I'm working on a foreign data wrapper that uses INSERT, and I noticed some
odd behaviour. If I insert just one row, the
TupleDesc->attr[0]->attname.data has the column names in it. However, in a
multi-row string, all those are empty strings:

I added this debugging code to BeginForeignInsert in
https://bitbucket.org/adunstan/blackhole_fdw on postgres 10.

int i;
FormData_pg_attribute *attr;
TupleDesc tupleDesc;

tupleDesc = slot->tts_tupleDescriptor;

for (i = 0; i < tupleDesc -> natts; i++) {
attr = tupleDesc->attrs[i];
elog(WARNING, "found column '%s'", attr->attname.data);
}

Now with a single row insert, this works as you'd expect:

liz=# INSERT INTO bhtable (key, value) VALUES ('hello', 'world');
WARNING: found column 'key'
WARNING: found column 'value'
INSERT 0 1

But with a multi-row, all the column names are empty:
liz=# INSERT INTO bhtable (key, value) VALUES ('hello', 'world'),
('goodmorning', 'world');
WARNING: found column ''
WARNING: found column ''
WARNING: found column ''
WARNING: found column ''
INSERT 0 2

It doesn't seem unreasonable to me that this data wouldn't be duplicated,
but there's no mention of how I would go about retriving these column names
for my individual rows, and most foreign data wrappers I can find are
write-only.

Am I missing something obvious? Is this a bug?

Thanks,

Liz