file corruption goes undetected

*ISSUE*

External modified datafile not detected, no error given but result produced;

*REPRODUCTION*

script.sh
---

psql   <<DO_CLEAN_1
drop database corruption;
DO_CLEAN_1

rm -rf /tmp/oscmd.sh
rm rf /tmp/output_1.txt
rm rf /tmp/output_2.txt

psql   <<END_SCRIPT

\o '/tmp/output_1.txt'
select version();
create database corruption;
\c corruption
show data_checksums;
show ignore_checksum_failure;
create table demo ( a serial, t timestamp with time zone);
insert into demo (t)  values (now());
select max(t)  from demo;
select pg_relation_filepath('demo');
copy (select command || setting ||'/'||file
      from (select *  from (select 1 as my_order, 'rm -rf ' as command,
pg_relation_filepath('demo') as file
          union all
            select 2 as my_order , 'touch ' as command ,
pg_relation_filepath('demo') as file )
          cross join (select setting from  pg_settings where
name='data_directory'))  order by my_order)  to '/tmp/oscmd.sh';
END_SCRIPT

cat /tmp/output_1.txt
chmod 700 /tmp/oscmd.sh
/tmp/oscmd.sh

psql -d corruption  <<END_SCRIPT_2

\o '/tmp/output_2.txt'
select max(t)  from demo;
END_SCRIPT_2

cat /tmp/output_2.txt
---

*ETC*
Result in tmp/output_1.txt  en tmp/output_2.txt.    After file has been
removed and recreated with touch  a NULL result is returned. The same
happens when the has been replaced with dd -if /dev/zero of=<file> 
bs=1024 count=64000

When replacing file with bogus data (/dev/random)  an error is being
returned.   Result when the datafile is being nullified

side info: pg_backrest does recognise the file as not valid an replaces
is during restore

regards

Bram

--
vrijdags afwezig
LOGO <https://www.axisintoict.nl>
Bram van der Vos
bram(dot)van(dot)der(dot)vos(at)axisinto(dot)nl
06 127 27 547
Albert Schweitzerlaan 10b
3451 EC Vleuten
Twitter <https://twitter.com/AxisintoICT>Linkedin
<https://nl.linkedin.com/in/bramvandervos>