Thread

Drop database command will raise "wrong tuple length" if pg_database tuple contains toast attribute.

Ayush Tiwari <ayushtiwari.slg01@gmail.com> — 2024-08-13T05:37:39Z

Hi hackers,

We encountered an issue lately, that if the database grants too many roles
`datacl` is toasted, following which, the drop database command will fail
with error "wrong tuple length".

To reproduce the issue, please follow below steps:

CREATE DATABASE test;

-- create helper function
CREATE OR REPLACE FUNCTION data_tuple() returns text as $body$
declare
          mycounter int;
begin
          for mycounter in select i from generate_series(1,2000) i loop
                    execute 'CREATE
ROLE aaaaaaaaaaaaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbbbbb ' || mycounter;
                    execute 'GRANT ALL ON DATABASE test to
aaaaaaaaaaaaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbbbbb ' || mycounter;
          end loop;
          return 'ok';
end;
$body$ language plpgsql volatile strict;

-- create roles and grant on the database.
SELECT data_tuple();

-- drop database command, this will result in "wrong tuple length" error.
DROP DATABASE test;

The root cause of this behaviour is that the HeapTuple in dropdb
function fetches a copy of pg_database tuple from system cache.
But the system cache flattens any toast attributes, which cause the length
check to fail in heap_inplace_update.

A patch for this issue is attached to the mail, the solution is to
change the logic to fetch the tuple by directly scanning pg_database rather
than using the catcache.

Regards,
Ayush