MDEV-11668 rpl.rpl_heartbeat_basic fails sporadically in buildbot

On a slow builder, a delay between binlog events on master could
occur, which would cause a heartbeat which is not expected by the
test. The solution is to monitor the timing of binlog events
on the master and only perform the heartbeat check if no critical
delays have happened.

Additionally, an unused variable was removed (this change is
unrelated to the bugfix).

mysql-test/suite/rpl/r/rpl_heartbeat_basic.result

@@ -7,7 +7,6 @@ RESET SLAVE;
 SET @restore_slave_net_timeout=@@global.slave_net_timeout;
 RESET MASTER;
 SET @restore_slave_net_timeout=@@global.slave_net_timeout;
-SET @restore_event_scheduler=@@global.event_scheduler;
 
 *** Default value ***
 CHANGE MASTER TO MASTER_HOST='127.0.0.1', MASTER_PORT=MASTER_PORT, MASTER_USER='root';
@@ -223,7 +222,7 @@ RESET SLAVE;
 CHANGE MASTER TO MASTER_HOST='127.0.0.1', MASTER_PORT=MASTER_PORT, MASTER_USER='root', MASTER_CONNECT_RETRY=20, MASTER_HEARTBEAT_PERIOD=5;
 include/start_slave.inc
 SET @@global.event_scheduler=1;
-Number of received heartbeat events: 0
+Received heartbeats meet expectations: TRUE
 DELETE FROM t1;
 DROP EVENT e1;
 

mysql-test/suite/rpl/t/rpl_heartbeat_basic.test

@@ -34,7 +34,6 @@ eval SET @restore_slave_heartbeat_timeout=$slave_heartbeat_timeout;
 --connection master
 RESET MASTER;
 SET @restore_slave_net_timeout=@@global.slave_net_timeout;
-SET @restore_event_scheduler=@@global.event_scheduler;
 --echo
 
 #
@@ -352,21 +351,54 @@ eval CHANGE MASTER TO MASTER_HOST='127.0.0.1', MASTER_PORT=$MASTER_MYPORT, MASTE
 --connection master
 # Enable scheduler
 SET @@global.event_scheduler=1;
+
 --sync_slave_with_master
 let $rcvd_heartbeats_before= query_get_value(SHOW STATUS LIKE 'slave_received_heartbeats', Value, 1);
-# Wait some updates for table t1 from master
-let $wait_condition= SELECT COUNT(*)=1 FROM t1 WHERE a > 5;
---source include/wait_condition.inc
+
+--connection master
+
+# Whether or not to send a heartbeat is decided on the master, based on
+# whether the binlog was updated during the period or not.
+# Even with the 1-second event, we cannot make the master to write binary
+# logs (or execute SQL) in a timely manner. We can only check that they
+# were executed in a timely manner, and if they were not, neutralize the
+# heartbeat check on the slave.
+# We will wait for 5 events, and keep checking 'Binlog_commits' on master.
+# Time interval between consequent events will be measured.
+# We can only expect that no heartbeats have been sent if the interval
+# between events never exceeded MASTER_HEARTBEAT_PERIOD.
+# If it has exceeded the value at least once, the slave can legitimately
+# receive a heartbeat (but we cannot require it, because the delay
+# could have occurred somewhere else, e.g. upon checking the status).
+# So, if the delay is detected, we will signal slave to ignore possible
+# heartbeats.
+
+let $possible_heartbeats= 0;
+let $commits_to_wait= 5;
+while ($commits_to_wait)
+{
+  let $tm= `SELECT UNIX_TIMESTAMP(NOW(3))`;
+  let $binlog_commits= query_get_value(SHOW STATUS LIKE 'Binlog_commits', Value, 1);
+  let $wait_condition= SELECT VARIABLE_VALUE > $binlog_commits FROM INFORMATION_SCHEMA.GLOBAL_STATUS WHERE VARIABLE_NAME= 'BINLOG_COMMITS';
+  --source include/wait_condition.inc
+  dec $commits_to_wait;
+  if (`SELECT UNIX_TIMESTAMP(NOW(3)) > $tm + 5`)
+  {
+    let $possible_heartbeats= 1;
+    let $commits_to_wait= 0;
+  }
+}
+
+--connection slave
 let $rcvd_heartbeats_after= query_get_value(SHOW STATUS LIKE 'slave_received_heartbeats', Value, 1);
-let $result= query_get_value(SELECT ($rcvd_heartbeats_after - $rcvd_heartbeats_before) > 0 AS Result, Result, 1);
---echo Number of received heartbeat events: $result
+let $result= `SELECT CASE WHEN $possible_heartbeats THEN 'TRUE' WHEN $rcvd_heartbeats_after - $rcvd_heartbeats_before > 0 THEN 'FALSE' ELSE 'TRUE' END`;
+--echo Received heartbeats meet expectations: $result
 --connection master
 DELETE FROM t1;
 DROP EVENT e1;
 --sync_slave_with_master
 --echo
 
-
 # Check received heartbeat events while logs flushed on slave
 --echo *** Flush logs on slave ***
 STOP SLAVE;