Why does pg_rewind deny permission for pg_read_binary_file() other than 'dbname=postgres'?

[Zhaoxun Yan <yan.zhaoxun@xxxxxxxxx>]

Hi there!

I am using repmgr and I have to use the command repmgr node rejoin --force-rewind under 'dbname=repmgr'. It always fail on using pg_rewind, the error is like this:
pg_rewind: error: could not fetch remote file "global/pg_control": ERROR:  permission denied for function pg_read_binary_file

I look into pg_rewind, and found that for a rewind user defined like
https://www.postgresql.org/docs/16/app-pgrewind.html

It always encounters such a problem if database != postgres but functions when 'dbname=postgres'

$ pg_rewind -D /pgdata --source-server='host=172.17.1.2 port=5432 user=rep dbname=repmgr connect_timeout=5'
pg_rewind: error: could not fetch remote file "global/pg_control": ERROR:  permission denied for function pg_read_binary_file
$ pg_rewind -D /pgdata --source-server='host=172.17.1.2 port=5432 user=rep dbname=postgres connect_timeout=5'
pg_rewind: source and target cluster are on the same timeline
pg_rewind: no rewind required

What is the problem with it?

BTW, below is what I have done to USER rewinder:

CREATE USER rewinder;
GRANT EXECUTE ON function pg_catalog.pg_ls_dir(text, boolean, boolean) TO rewinder;
GRANT EXECUTE ON function pg_catalog.pg_stat_file(text, boolean) TO rewinder;
GRANT EXECUTE ON function pg_catalog.pg_read_binary_file(text) TO rewinder;
GRANT EXECUTE ON function pg_catalog.pg_read_binary_file(text, bigint, bigint, boolean) TO rewinder;

# below is irrelevant to postgresql's pg_rewind

GRANT ALL PRIVILEGES ON DATABASE repmgr TO rewinder;
GRANT ALL PRIVILEGES ON SCHEMA repmgr TO rewinder;
GRANT pg_read_all_stats TO rewinder;
GRANT ALL ON SCHEMA repmgr TO rewinder;
GRANT SELECT  ON DATABASE repmgr TO rewinder;
GRANT SELECT ON ALL TABLES IN SCHEMA repmgr TO rewinder;
GRANT SELECT ON SCHEMA repmgr TO rewinder;