MDEV-6321: close_temporary_tables() in format description event not serialised correctly

When a master server starts up, it logs a special format_description event at
the start of a new binlog to mark that is has restarted. This is used by a
slave to drop all temporary tables - this is needed in case the master crashed
and did not have a chance to send explicit DROP TEMPORARY TABLE statements to
the slave.

In parallel replication, we need to be careful when dropping the temporary
tables - we need to be sure that no prior events are still executing that
might be using the temporary tables to be dropped, _and_ that no following
events have started executing that might have created new temporary tables
that should not be dropped.

This was not handled correctly, which could cause errors about access to not
existing temporary tables or even crashes. This patch implements that such
format_description events cause serialisation of event execution; all prior
events are executed to completion first, then the format_description event is
executed, dropping temporary tables, then following events are queued for
execution.

Master restarts should be sufficiently infrequent that the resulting loss of
parallelism should be of minimal impact.

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

@@ -0,0 +1,143 @@
+--source include/have_binlog_format_statement.inc
+--let $rpl_topology=1->2
+--source include/rpl_init.inc
+
+--echo *** MDEV-6321: close_temporary_tables() in format description event not serialised correctly ***
+
+--connection server_2
+SET @old_parallel_threads=@@GLOBAL.slave_parallel_threads;
+--source include/stop_slave.inc
+SET GLOBAL slave_parallel_threads=5;
+CHANGE MASTER TO master_use_gtid= current_pos;
+--source include/start_slave.inc
+
+--connection server_1
+CREATE TABLE t1 (a INT PRIMARY KEY, b VARCHAR(100) CHARACTER SET utf8);
+--save_master_pos
+
+--connection server_2
+--sync_with_master
+--source include/stop_slave.inc
+
+
+--connection server_1
+SET gtid_domain_id= 1;
+INSERT INTO t1 VALUES (1, 0);
+
+CREATE TEMPORARY TABLE t2 (a int);
+
+--connection default
+SET gtid_domain_id= 2;
+CREATE TEMPORARY TABLE t3 (a INT PRIMARY KEY);
+CREATE TEMPORARY TABLE t4 (a int);
+INSERT INTO t3 VALUES (100);
+INSERT INTO t4 SELECT a+1 FROM t3;
+
+--connection server_1
+INSERT INTO t2 VALUES (2), (4), (6), (8), (10), (12), (14), (16), (18), (20);
+INSERT INTO t2 VALUES (3), (6), (9), (12), (15), (18);
+INSERT INTO t2 VALUES (4), (8), (12), (16), (20);
+
+--connection default
+INSERT INTO t3 SELECT a+2 FROM t4;
+INSERT INTO t4 SELECT a+4 FROM t3;
+
+--connection server_1
+INSERT INTO t2 VALUES (5), (10), (15), (20);
+INSERT INTO t2 VALUES (6), (12), (18);
+INSERT INTO t2 VALUES (7), (14);
+INSERT INTO t2 VALUES (8), (16);
+INSERT INTO t2 VALUES (9), (18);
+INSERT INTO t2 VALUES (10), (20);
+
+--connection default
+INSERT INTO t3 SELECT a+8 FROM t4;
+INSERT INTO t4 SELECT a+16 FROM t3;
+
+--connection server_1
+INSERT INTO t2 VALUES (11);
+INSERT INTO t2 VALUES (12);
+INSERT INTO t2 VALUES (13);
+
+--connection default
+INSERT INTO t3 SELECT a+32 FROM t4;
+
+--connection server_1
+INSERT INTO t2 VALUES (14);
+INSERT INTO t2 VALUES (15);
+INSERT INTO t2 VALUES (16);
+
+--connection default
+INSERT INTO t4 SELECT a+64 FROM t3;
+
+--connection server_1
+INSERT INTO t2 VALUES (17);
+INSERT INTO t2 VALUES (18);
+INSERT INTO t2 VALUES (19);
+
+--connection default
+INSERT INTO t3 SELECT a+128 FROM t4;
+
+--connection server_1
+INSERT INTO t2 VALUES (20);
+
+--connection default
+INSERT INTO t1 SELECT a, a MOD 7 FROM t3;
+INSERT INTO t1 SELECT a, a MOD 7 FROM t4;
+
+--connection server_1
+INSERT INTO t1 SELECT a, COUNT(*) FROM t2 GROUP BY a;
+
+# Crash the master server, so that temporary tables are implicitly dropped.
+--write_file $MYSQLTEST_VARDIR/tmp/mysqld.1.expect
+wait
+EOF
+
+FLUSH TABLES;
+SET SESSION debug_dbug="+d,crash_dispatch_command_before";
+--error 2006,2013
+SELECT 1;
+
+--source include/wait_until_disconnected.inc
+--connection default
+--source include/wait_until_disconnected.inc
+
+--append_file $MYSQLTEST_VARDIR/tmp/mysqld.1.expect
+restart
+EOF
+
+--connection default
+--enable_reconnect
+--source include/wait_until_connected_again.inc
+
+--connection server_1
+--enable_reconnect
+--source include/wait_until_connected_again.inc
+
+INSERT INTO t1 VALUES (0, 1);
+--save_master_pos
+
+--connection server_2
+# Start the slave replicating the events.
+# The bug was that the format description event written after the crash could
+# be fetched ahead of the execution of the temporary table events and executed
+# out-of-band. This would cause drop of all temporary tables and thus failure
+# for execution of remaining events.
+
+--source include/start_slave.inc
+--sync_with_master
+
+SELECT * FROM t1 WHERE a <= 20 ORDER BY a;
+SELECT COUNT(*) FROM t1 WHERE a BETWEEN 100+0 AND 100+256;
+SHOW STATUS LIKE 'Slave_open_temp_tables';
+
+
+--connection server_2
+--source include/stop_slave.inc
+SET GLOBAL slave_parallel_threads=@old_parallel_threads;
+--source include/start_slave.inc
+
+--connection server_1
+DROP TABLE t1;
+
+--source include/rpl_end.inc