Fix for MDEV-4117 @@global.relay_log_purge not per-master, conflicts between...

Fix for MDEV-4117 @@global.relay_log_purge not per-master, conflicts between different masters in multisource replication

The fix is to not change @relay_log_purge as part of the CHANGE MASTER.
(There is no logical reason why this is done in the current source)

mysql-test/suite/rpl/r/rpl_slave_status.result:
  Ensure that CHANGE MASTER doesn't change relay_log_purge
mysql-test/suite/rpl/t/rpl_slave_status.test:
  Ensure that CHANGE MASTER doesn't change relay_log_purge
sql/sql_repl.cc:
  Don't change relay_log_purge in CHANGE MASTER

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

@@ -5,6 +5,18 @@ include/master-slave.inc
 GRANT REPLICATION SLAVE ON *.* TO rpl@127.0.0.1 IDENTIFIED BY 'rpl';
 [on slave]
 include/stop_slave.inc
+set @save_relay_log_purge=@@global.relay_log_purge;
+set @@global.relay_log_purge=0;
+CHANGE MASTER TO master_user='rpl', master_password='rpl';
+select @@global.relay_log_purge;
+@@global.relay_log_purge
+0
+set @@global.relay_log_purge=1;
+CHANGE MASTER TO master_user='rpl', master_password='rpl';
+select @@global.relay_log_purge;
+@@global.relay_log_purge
+1
+set @@global.relay_log_purge=@save_relay_log_purge;
 CHANGE MASTER TO master_user='rpl', master_password='rpl';
 include/start_slave.inc
 ==== Do replication as new user ====

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

@@ -2,7 +2,7 @@
 #
 # Verify that a slave without replication privileges has
 # Slave_IO_Running = No
-#
+# Check that relay_log_status doesn't change by CHANGE MASTER
 # ==== Method ====
 #
 # We do the following steps:
@@ -28,6 +28,17 @@ GRANT REPLICATION SLAVE ON *.* TO rpl@127.0.0.1 IDENTIFIED BY 'rpl';
 --echo [on slave]
 sync_slave_with_master;
 source include/stop_slave.inc;
+
+# Test that relay_log_purge doesn't change because of CHANGE MASTER
+set @save_relay_log_purge=@@global.relay_log_purge;
+set @@global.relay_log_purge=0;
+CHANGE MASTER TO master_user='rpl', master_password='rpl';
+select @@global.relay_log_purge;
+set @@global.relay_log_purge=1;
+CHANGE MASTER TO master_user='rpl', master_password='rpl';
+select @@global.relay_log_purge;
+set @@global.relay_log_purge=@save_relay_log_purge;
+
 CHANGE MASTER TO master_user='rpl', master_password='rpl';
 source include/start_slave.inc;
 

sql/sql_repl.cc

@@ -3444,7 +3444,6 @@ bool change_master(THD* thd, Master_info* mi, bool *master_info_added)
   }
   if (need_relay_log_purge)
   {
-    relay_log_purge= 1;
     THD_STAGE_INFO(thd, stage_purging_old_relay_logs);
     if (purge_relay_logs(&mi->rli, thd,
 			 0 /* not only reset, but also reinit */,
@@ -3458,7 +3457,6 @@ bool change_master(THD* thd, Master_info* mi, bool *master_info_added)
   else
   {
     const char* msg;
-    relay_log_purge= 0;
     /* Relay log is already initialized */
     if (init_relay_log_pos(&mi->rli,
 			   mi->rli.group_relay_log_name,