Fix brin_form_tuple to properly detoast data

Hi,

As pointed out in [1], BRIN is not properly handling toasted data, which
may easily lead to index tuples referencing TOAST-ed values. Which is
clearly wrong - it's trivial to trigger failues after a DELETE.

Attached is a patch that aims to fix this - AFAIK the brin_form_tuple
was simply missing the TOAST_INDEX_HACK stuff from index_form_tuple,
which ensures the data is detoasted and (possibly) re-compressed. The
code is mostly the same, with some BRIN-specific tweaks (looking at
oi_typecache instead of the index descriptor, etc.).

I also attach a simple SQL script that I used to trigger the issue. This
needs to be turned into a regression test, I'll work on that tomorrow.

A separate question is what to do about existing indexes - ISTM the only
thing we can do is to tell the users to reindex all BRIN indexes on
varlena values. Something like this:

select * from pg_class
where relam = (select oid from pg_am where amname = 'brin')
and oid in (select attrelid from pg_attribute where attlen = -1
and attstorage in ('e', 'x'));

regards

[1] https://www.postgresql.org/message-id/20201001184133.oq5uq75sb45pu3aw%40development

--
Tomas Vondra http://www.2ndQuadrant.com
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