Fixed failure in rpl.rpl_change_master_demote : "IO thread should not be running..."

The issue was that the test did not take into account that the IO thread
could have been in COMMAND=Connecting state, which happens before the
COMMANMD=Slave_IO state.

The test is a bit fragile as it depends on the COMMAND state to be
syncronised with the Slave_IO_State, which is not the case.

I added a new proc state and some more information to the error
output to be able to diagnose future failures more easily.

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

@@ -295,19 +295,20 @@ eval START SLAVE UNTIL master_gtid_pos="$ssu_middle_binlog_pos";
 --let $wait_condition= SELECT count(*)=0 from information_schema.PROCESSLIST where COMMAND="Slave_SQL"
 --source include/wait_condition.inc
 --echo # Waiting for IO thread to be killed..
---let $wait_condition= SELECT count(*)=0 from information_schema.PROCESSLIST where COMMAND="Slave_IO"
+--let $wait_condition= SELECT count(*)=0 from information_schema.PROCESSLIST where COMMAND="Slave_IO" or COMMAND="Connect"
 --source include/wait_condition.inc
 
 --echo # Validating neither SQL nor IO threads are running..
 --let $io_state= query_get_value("SHOW SLAVE STATUS", Slave_IO_State, 1)
 if (`SELECT strcmp("$io_state","") != 0`)
 {
-    die "IO thread should not be running after START SLAVE UNTIL master_gtid_pos using a pre-existing GTID";
+   SELECT * from information_schema.PROCESSLIST;
+    die "IO thread should not be running after START SLAVE UNTIL master_gtid_pos using a pre-existing GTID.  Slave_IO_State=$io_state";
 }
 --let $sql_state= query_get_value("SHOW SLAVE STATUS", Slave_SQL_Running_State, 1)
 if (`SELECT strcmp("$sql_state","") != 0`)
 {
-    die "SQL thread should not be running after START SLAVE UNTIL master_gtid_pos using a pre-existing GTID";
+    die "SQL thread should not be running after START SLAVE UNTIL master_gtid_pos using a pre-existing GTID.  Slave_SQL_Running_State=$sql_state";
 }
 --echo # ..success
 

sql/mysqld.cc

@@ -9324,6 +9324,7 @@ PSI_stage_info stage_deleting_from_reference_tables= { 0, "Deleting from referen
 PSI_stage_info stage_discard_or_import_tablespace= { 0, "Discard_or_import_tablespace", 0};
 PSI_stage_info stage_enabling_keys= { 0, "Enabling keys", 0};
 PSI_stage_info stage_end= { 0, "End of update loop", 0};
+PSI_stage_info stage_ending_io_thread= { 0, "Ending IO thread", 0};
 PSI_stage_info stage_executing= { 0, "Executing", 0};
 PSI_stage_info stage_execution_of_init_command= { 0, "Execution of init_command", 0};
 PSI_stage_info stage_explaining= { 0, "Explaining", 0};

sql/mysqld.h

@@ -594,6 +594,7 @@ extern PSI_stage_info stage_deleting_from_main_table;
 extern PSI_stage_info stage_deleting_from_reference_tables;
 extern PSI_stage_info stage_discard_or_import_tablespace;
 extern PSI_stage_info stage_end;
+extern PSI_stage_info stage_ending_io_thread;
 extern PSI_stage_info stage_enabling_keys;
 extern PSI_stage_info stage_executing;
 extern PSI_stage_info stage_execution_of_init_command;

sql/slave.cc

@@ -5195,6 +5195,7 @@ log space");
 
   // error = 0;
 err:
+  THD_STAGE_INFO(thd, stage_ending_io_thread);
   // print the current replication position
   if (mi->using_gtid == Master_info::USE_GTID_NO)
     sql_print_information("Slave I/O thread exiting, read up to log '%s', "