Comments - Wait for changes on a table

Like is mentioned in MDEV-16590, this can be emulated in SQL. Here's a quick little prototype of a pair of stored procedures and a table that can be used to send and wait for signals. Adding signal IDs and versions for notify_one/notify_all should be possible.

delimiter $$ ;
CREATE OR REPLACE PROCEDURE wait_for_signal(IN timeout INT)
BEGIN
    INSERT INTO signal_waiters VALUES (CONNECTION_ID());
    EXECUTE IMMEDIATE CONCAT('SELECT ''wait_for_signal'', SLEEP (', timeout, ')');
    DELETE FROM signal_waiters WHERE id = CONNECTION_ID();
END $$

CREATE OR REPLACE PROCEDURE send_signal()
BEGIN
    START TRANSACTION;
    FOR rec IN (SELECT id FROM signal_waiters FOR UPDATE)
    DO
      EXECUTE IMMEDIATE CONCAT('KILL QUERY ', rec.id);
      DELETE FROM signal_waiters WHERE id = rec.id;
    END FOR;
    COMMIT;
END $$
delimiter ; $$

CREATE OR REPLACE TABLE signal_waiters(id BIGINT PRIMARY KEY);

-- To wait for a signal
CALL wait_for_signal(10);

-- To send a signal
CALL send_signal();