[BUG] Error in BRIN summarization

One of our clients caught an error "failed to find parent tuple for 
heap-only tuple at (50661,130) in table "tbl'" in PostgreSQL v12.

Steps to reproduce (REL_12_STABLE):

1) Create table with primary key, create brin index, fill table with 
some initial data:

create table tbl (id int primary key, a int) with (fillfactor=50);
create index idx on tbl using brin (a) with (autosummarize=on);
insert into tbl select i, i from generate_series(0,100000) as i;

2) Run script test_brin.sql using pgbench:

  pgbench postgres -f ../review/brin_test.sql  -n -T 120

The script is a bit messy because I was trying to reproduce a 
problematic workload. Though I didn't manage to simplify it.
The idea is that it inserts new values into the table to produce 
unindexed pages and also updates some values to trigger HOT-updates on 
these pages.

3) Open psql session and run brin_summarize_new_values

select brin_summarize_new_values('idx'::regclass::oid); \watch 2

Wait a bit. And in psql you will see the ERROR.

This error is caused by the problem with root_offsets array bounds. It 
occurs if a new HOT tuple was inserted after we've collected 
root_offsets, and thus we don't have root_offset for tuple's offnum. 
Concurrent insertions are possible, because brin_summarize_new_values() 
only holds ShareUpdateLock on table and no lock (only pin) on the page.

The draft fix is in the attachments. It saves root_offsets_size and 
checks that we only access valid fields.
Patch also adds some debug messages, just to ensure that problem was caught.

TODO:

- check if  heapam_index_validate_scan() has the same problem
- code cleanup
- test other PostgreSQL versions

[1] 
https://www.postgresql.org/message-id/flat/CA%2BTgmoYgwjmmjK24Qxb_vWAu8_Hh7gfVFcr3%2BR7ocdLvYOWJXg%40mail.gmail.com

-- 
Anastasia Lubennikova
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company