BUG #19473: regression error in dblink: another command is already in progress

The following bug has been logged on the website:

Bug reference:      19473
Logged by:          Oleg Ivanov
Email address:      o15611@gmail.com
PostgreSQL version: 18.3
Operating system:   Linux
Description:        

Testcase:
SELECT * FROM dblink('dbname=postgres user=postgres', $$ select 7; $$ ) as
(col1 int);
SELECT * FROM dblink_connect('connection1', 'host=/var/run/postgresql
port=5432');
SELECT * FROM dblink_send_query('connection1', $$ select 8 from pg_sleep(1);
$$ );
SELECT dblink_is_busy('connection1');
SELECT * FROM dblink_get_result('connection1') as t(col1 int);
--- SELECT dblink_is_busy('connection1');
SELECT * FROM dblink_exec('connection1', $$ CHECKPOINT; $$);
SELECT * FROM dblink_disconnect('connection1');

 col1
------
    7
(1 row)

 dblink_connect
----------------
 OK
(1 row)

 dblink_send_query
-------------------
                 1
(1 row)

 dblink_is_busy
----------------
              1
(1 row)

 col1
------
    8
(1 row)

ERROR:  another command is already in progress
CONTEXT:  while executing command on dblink connection named "connection1"
 dblink_disconnect
-------------------
 OK
(1 row)

There was no error in 16 version. The error appeared in 17 version. It
seems, there is a regression bug.
The workaround is to uncomment a commented statement.