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, 4 insertions, 1 deletions

diff --git a/mysys/mf_iocache.c b/mysys/mf_iocache.c
index f891a22b17d..8687c2e0c48 100644
--- a/mysys/mf_iocache.c
+++ b/mysys/mf_iocache.c
@@ -1815,6 +1815,7 @@ int my_b_flush_io_cache(IO_CACHE *info,
     It's currently safe to call this if one has called init_io_cache()
     on the 'info' object, even if init_io_cache() failed.
     This function is also safe to call twice with the same handle.
+    Note that info->file is not reset as the caller may still use ut for my_close()
 
   RETURN
    0  ok
@@ -1850,10 +1851,12 @@ int end_io_cache(IO_CACHE *info)
   if (info->type == SEQ_READ_APPEND)
   {
     /* Destroy allocated mutex */
-    info->type= TYPE_NOT_SET;
     mysql_mutex_destroy(&info->append_buffer_lock);
   }
   info->share= 0;
+  info->type= TYPE_NOT_SET;                  /* Ensure that flush_io_cache() does nothing */
+  info->write_end= 0;                        /* Ensure that my_b_write() fails */
+  info->write_function= 0;                   /* my_b_write will crash if used */
   DBUG_RETURN(error);
 } /* end_io_cache */