MDEV-9573 'Stop slave' hangs on replication slave

The reason for this is that stop slave takes LOCK_active_mi over the
whole operation while some slave operations will also need LOCK_active_mi
which causes deadlocks.

Fixed by introducing object counting for Master_info and not taking
LOCK_active_mi over stop slave or even stop_all_slaves()

Another benefit of this approach is that it allows:
- Multiple threads can run SHOW SLAVE STATUS at the same time
- START/STOP/RESET/SLAVE STATUS on a slave will not block other slaves
- Simpler interface for handling get_master_info()
- Added some missing unlock of 'log_lock' in error condtions
- Moved rpl_parallel_inactivate_pool(&global_rpl_thread_pool) to end
  of stop_slave() to not have to use LOCK_active_mi inside
  terminate_slave_threads()
- Changed argument for remove_master_info() to Master_info, as we always
  have this available
- Fixed core dump when doing FLUSH TABLES WITH READ LOCK and parallel
  replication. Problem was that waiting for pause_for_ftwrl was not done
  when deleting rpt->current_owner after a force_abort.

1 files changed, 2 insertions, 6 deletions

diff --git a/sql/item_func.cc b/sql/item_func.cc
index 3149ebb855d..2c43181e8bb 100644
--- a/sql/item_func.cc
+++ b/sql/item_func.cc
@@ -3958,12 +3958,7 @@ longlong Item_master_pos_wait::val_int()
   else
     connection_name= thd->variables.default_master_connection;
 
-  mysql_mutex_lock(&LOCK_active_mi);
-  if (master_info_index)  // master_info_index is set to NULL on shutdown.
-    mi= master_info_index->get_master_info(&connection_name,
-                                           Sql_condition::WARN_LEVEL_WARN);
-  mysql_mutex_unlock(&LOCK_active_mi);
-  if (!mi)
+  if (!(mi= get_master_info(&connection_name, Sql_condition::WARN_LEVEL_WARN)))
     goto err;
 
   if ((event_count = mi->rli.wait_for_pos(thd, log_name, pos, timeout)) == -2)
@@ -3971,6 +3966,7 @@ longlong Item_master_pos_wait::val_int()
     null_value = 1;
     event_count=0;
   }
+  mi->release();
 #endif
   return event_count;