pg_upgrade --check doesn't check pg_pltemplate modifications

when upgrading postgresql 12 -> 13 the pg_upgrade --check doesn't sufficiently check for pg_pltemplate modifications (at least in version 13.5-1.pgdg110+1).

There's been a discussion on the mailing list https://postgrespro.com/list/thread-id/2521269 regarding this topic. But it doesn't seem to be implemented in the upgrade process. 

Here's a simple bash script that checks for pg_pltemplate modifications on each database:

for db in $(psql -tc "SELECT datname FROM pg_database;")
do
  if [[ "${db}" != "template0" ]]; then
    dump=$(pg_dump --schema-only --quote-all-identifiers ${db} | grep pg_pltemplate)
    if [ ! -z "$dump" ]; then
        echo "ERROR: ${db} contains pg_pltemplate modifications. pg_upgrade will fail"
        exit 1
    fi
  fi
done
echo "OK"

Any non-default GRANT/REVOKE like these:

REVOKE SELECT ON TABLE "pg_catalog"."pg_pltemplate" FROM PUBLIC;
GRANT SELECT ON TABLE "pg_catalog"."pg_pltemplate" TO "reader";

would break the upgrade process, even though the pg_upgrade --check says:

*Clusters are compatible*

then upgrade (at least with --link) fails

pg_restore: error: could not execute query: ERROR:  relation "pg_catalog.pg_pltemplate" does not exist

Including such checks in the upgrade process might make the upgrade path much easier.

Tomas Barton