BUG #19864: pg_get_publication_tables() gets cache lookup error after concurrent DROP PUBLICATION

The following bug has been logged on the website:

Bug reference: 19864
Logged by: Noah
Email address: imchifan8(at)gmail(dot)com
PostgreSQL version: 18.6
Operating system: Linux/amd64
Description:

pg_get_publication_tables() raises an internal cache lookup error when it
blocks while opening a published table and another session drops and commits
the publication before the function resumes. A multi-call catalog SRF should
safely handle retained object identifiers after concurrent removal, either
by omitting stale rows or otherwise completing without an internal cache
lookup failure.

How to repeat:

The dblink extension must be available. Run the following as one script in a
fresh database using psql:

DROP PUBLICATION IF EXISTS race_pub;
DROP SCHEMA IF EXISTS race_schema CASCADE;
CREATE EXTENSION dblink;
CREATE SCHEMA race_schema;
CREATE TABLE race_schema.race_table (id integer);
CREATE PUBLICATION race_pub FOR TABLES IN SCHEMA race_schema;

BEGIN;
LOCK TABLE race_schema.race_table IN ACCESS EXCLUSIVE MODE;

SELECT dblink_connect(
'race_conn',
'dbname=' || current_database());
SELECT dblink_send_query(
'race_conn',
$$SELECT relid::regclass::text
FROM pg_get_publication_tables('race_pub')
ORDER BY 1$$);

SELECT pg_sleep(1);
SELECT EXISTS (
SELECT 1
FROM pg_locks
WHERE relation = 'race_schema.race_table'::regclass
AND mode = 'AccessShareLock'
AND NOT granted
) AS remote_enumerator_waiting_on_table;

DROP PUBLICATION race_pub;
COMMIT;

SELECT *
FROM dblink_get_result('race_conn')
AS result(published_table text);
SELECT dblink_disconnect('race_conn');

The lock check returned true, confirming that the remote call was waiting to
open the table. After DROP PUBLICATION committed, dblink_get_result()
reported:

ERROR: cache lookup failed for publication 17010
CONTEXT: while executing query on dblink connection named "race_conn"

The affected statement failed, but PostgreSQL remained healthy. Retrying
after the concurrent catalog DDL has finished allows the introspection
operation to proceed.

The runtime failure was reproduced on PostgreSQL 18.6 on Linux/amd64.
Separately, inspection of PostgreSQL 20devel source found the same relevant
path: pg_get_publication_tables() retains publication OIDs across SRF calls
and later passes each retained OID to GetPublication(). Its concurrent-drop
handling tolerates a missing relation through try_table_open(), but not a
publication that has disappeared.

This occurs when publication tables are enumerated while concurrent
publication DDL removes the publication.