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(at)gmail(dot)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.