MDEV-10149: sys_vars.rpl_init_slave_func fails sporadically in buildbot

problem:
========
mysqltest: In included file "./include/assert.inc":
included from mysql-test/suite/sys_vars/t/rpl_init_slave_func.test at line 69:
Assertion text: '@@global.max_connections = @start_max_connections'
Assertion result: '0'


mysqltest: In included file "./include/assert.inc":
included from mysql-test/suite/sys_vars/t/rpl_init_slave_func.test at line 86:
Assertion text: '@@global.max_connections = @start_max_connections + 1'
Assertion result: '0'

Analysis:
=========
A slave SQL thread sets its Running state to Yes very early in its
initialisation, before the majority of initialisation actions, including
executing the init_slave command, are done. Thus the testcase has a race
condition where the initial replication setup might finish executing later
than the testcase SET GLOBAL init_slave, making the testcase see its effect
where it checks for its absence.

Fix:
===
Include 'sync_slave_sql_with_master.inc' at the beginning of the test to
ensure that slave applier has completed the execution of 'init_slave' command
and proceeded to event application. Replace the apparently needless RESET
MASTER / RESET SLAVE etc.

Patch is based on:
https://github.com/percona/percona-server/pull/1464/commits/b91e2e6f90611aa299c302929fb8b068e8ac0dee
Author: laurynas-biveinis

mysql-test/suite/sys_vars/r/rpl_init_slave_func.result

 include/master-slave.inc
 [connection master]
-connection slave
+include/sync_slave_sql_with_master.inc
 SET @start_max_connections= @@global.max_connections;
 SET @start_init_slave= @@global.init_slave;
 SET NAMES utf8;
@@ -19,18 +19,11 @@ SELECT @@global.init_slave = 'SET @@global.max_connections = @@global.max_connec
 1
 Expect 1
 include/assert.inc [@@global.max_connections = @start_max_connections]
-STOP SLAVE;
-RESET MASTER;
-RESET SLAVE;
-START SLAVE;
-include/wait_for_slave_to_start.inc
+include/restart_slave.inc
+include/sync_slave_sql_with_master.inc
 include/assert.inc [@@global.max_connections = @start_max_connections + 1]
 SET @@global.init_slave = "SET @a=5";
-STOP SLAVE;
-RESET MASTER;
-RESET SLAVE;
-START SLAVE;
-include/wait_for_slave_to_start.inc
+include/restart_slave.inc
 SHOW VARIABLES LIKE 'init_slave';
 Variable_name	Value
 init_slave	SET @a=5

mysql-test/suite/sys_vars/t/rpl_init_slave_func.test

@@ -29,8 +29,13 @@
 ###############################################################################
 
 source include/master-slave.inc;
---echo connection slave
-connection slave;
+
+# Since a part of slave SQL thread initialisation happens after Slave_SQL_Running
+# has been set to Yes, there is a race condition between initialisation above and
+# init_slave setting given below. Synchronise slave applier with master to ensure
+# init_slave is complete and applier had processed few events like FD.
+--source include/sync_slave_sql_with_master.inc
+
 --disable_query_log
 call mtr.add_suppression("Unsafe statement written to the binary log using statement format since BINLOG_FORMAT = STATEMENT");
 --enable_query_log
@@ -67,14 +72,15 @@ let $wait_condition= SELECT @@global.max_connections = @start_max_connections;
 --let $assert_text= @@global.max_connections = @start_max_connections
 --let $assert_cond= @@global.max_connections = @start_max_connections
 --source include/assert.inc
-#
-# reset of the server
-STOP SLAVE;
---wait_for_slave_to_stop
-RESET MASTER;
-RESET SLAVE;
-START SLAVE;
-source include/wait_for_slave_to_start.inc;
+
+--source include/restart_slave_sql.inc
+
+# Upon slave start, sync the applier with master, to ensure slave has
+# completed init_slave command execution and processed FD event from the
+# master.
+--connection master
+--source include/sync_slave_sql_with_master.inc
+
 #
 # wait for the slave threads have set the global variable.
 let $wait_timeout= 90;
@@ -88,12 +94,7 @@ let $wait_condition= SELECT @@global.max_connections = @start_max_connections +
 # Setting a variable(which is local to a session) and must not be visible
 SET @@global.init_slave = "SET @a=5";
 #
-STOP SLAVE;
---wait_for_slave_to_stop
-RESET MASTER;
-RESET SLAVE;
-START SLAVE;
-source include/wait_for_slave_to_start.inc;
+--source include/restart_slave_sql.inc
 #
 SHOW VARIABLES LIKE 'init_slave';
 # expect NULL