MDEV-14528 Track master timestamp in case rolling back to serial replication

When replicated events are from Master unaware of MariaDB GTID their handling by
the Parallel slave misses Seconds_Behind_Master updating.

In the bug condition the Show-Slave-Status' field remains unchanged.

Because in such case event execution is sequential the bug
is fixed with deploying the same logics as in the explicit single-threaded mode
with is to set

  Relay_log_event::last_master_timestamp

member early at the end of event reading from the relay log.

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

@@ -9,7 +9,10 @@ connection slave;
 SET @old_parallel= @@GLOBAL.slave_parallel_threads;
 SET GLOBAL slave_parallel_threads=10;
 CHANGE MASTER TO master_host='127.0.0.1', master_port=SERVER_MYPORT_1, master_user='root', master_log_file='master-bin.000001', master_log_pos=4;
+FLUSH TABLES WITH READ LOCK;
 include/start_slave.inc
+include/wait_for_slave_param.inc [Seconds_Behind_Master]
+UNLOCK TABLES;
 connection master;
 CREATE TABLE t2 (a INT PRIMARY KEY) ENGINE=InnoDB;
 INSERT INTO t2 VALUES (1);

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

@@ -27,7 +27,15 @@ SET @old_parallel= @@GLOBAL.slave_parallel_threads;
 SET GLOBAL slave_parallel_threads=10;
 --replace_result $SERVER_MYPORT_1 SERVER_MYPORT_1
 eval CHANGE MASTER TO master_host='127.0.0.1', master_port=$SERVER_MYPORT_1, master_user='root', master_log_file='master-bin.000001', master_log_pos=4;
+
+# Block execution yet when the blocked query timestamp has been already accounted
+FLUSH TABLES WITH READ LOCK;
 --source include/start_slave.inc
+--let $slave_param           = Seconds_Behind_Master
+--let $slave_param_value     = 1
+--let $slave_param_comparison= >=
+--source include/wait_for_slave_param.inc
+UNLOCK TABLES;
 
 --connection master
 CREATE TABLE t2 (a INT PRIMARY KEY) ENGINE=InnoDB;

sql/slave.cc

@@ -3960,6 +3960,11 @@ static int exec_relay_log_event(THD* thd, Relay_log_info* rli,
         This is the case for pre-10.0 events without GTID, and for handling
         slave_skip_counter.
       */
+      if (!(ev->is_artificial_event() || ev->is_relay_log_event() || (ev->when == 0)))
+      {
+        rli->last_master_timestamp= ev->when + (time_t) ev->exec_time;
+        DBUG_ASSERT(rli->last_master_timestamp >= 0);
+      }
     }
 
     if (typ == GTID_EVENT)