[BUG] false positive in bt_index_check in case of short 4B varlena datum

Hi,

Following example produces error raised from bt_index_check.

drop table if exists t;
create table t (v text);
alter table t alter column v set storage plain;
insert into t values ('x');
copy t to '/tmp/1.lst';
copy t from '/tmp/1.lst';
create index t_idx on t(v);
create extension if not exists amcheck;
select bt_index_check('t_idx', true);

postgres=# select bt_index_check('t_idx', true);
ERROR:  heap tuple (0,2) from table "t" lacks matching index tuple 
within index "t_idx"
HINT:  Retrying verification using the function bt_index_parent_check() 
might provide a more specific error.

As result table contains 2 logically identical tuples:
  - one contains varlena 'x' with 1B (1-byte) header (added by INSERT 
statement)
  - one contains varlena 'x' with 4B (4-bytes) header (added by COPY 
statement)
CREATE INDEX statement builds index with posting list referencing both 
heap tuples.
The function bt_index_check calculates fingerprints of 1B and 4B header 
datums,
they are different and function returns error.

The attached patch allows to avoid such kind of false positives by 
converting short
4B datums to 1B before fingerprinting. Also it contains test for 
provided case.

Thank you,
  Michael

-- 
Michael Zhilin
Postgres Professional