Optimize flush tables with read lock (FTWRL) to not wait for select's

Part of MDEV-5336 Implement LOCK FOR BACKUP

The idea is that instead of waiting in close_cached_tables() for all
tables to be closed, we instead call flush_tables() that does:
- Flush not used objects in table cache to free memory
- Collect all tables that are open
- Call HA_EXTRA_FLUSH on the objects, to get them into "closed state"
- Added HA_EXTRA_FLUSH support to archive and CSV
- Added multi-user protection to HA_EXTRA_FLUSH in MyISAM and Aria

The benefit compared to old code is:
- FTWRL doesn't have to wait for long running read operations or
  open HANDLER's

1 files changed, 5 insertions, 0 deletions

diff --git a/storage/myisam/mi_extra.c b/storage/myisam/mi_extra.c
index c10bf61a477..194ad5bf07a 100644
--- a/storage/myisam/mi_extra.c
+++ b/storage/myisam/mi_extra.c
@@ -332,7 +332,11 @@ int mi_extra(MI_INFO *info, enum ha_extra_function function, void *extra_arg)
     if (!share->temporary)
       flush_key_blocks(share->key_cache, share->kfile, &share->dirty_part_map,
                        FLUSH_KEEP);
+    mysql_mutex_lock(&share->intern_lock);
+    /* Tell mi_lock_database() that we locked the intern_lock mutex */
+    info->intern_lock_locked= 1;
     _mi_decrement_open_count(info);
+    info->intern_lock_locked= 0;
     if (share->not_flushed)
     {
       share->not_flushed=0;
@@ -349,6 +353,7 @@ int mi_extra(MI_INFO *info, enum ha_extra_function function, void *extra_arg)
     }
     if (share->base.blobs)
       mi_alloc_rec_buff(info, -1, &info->rec_buff);
+    mysql_mutex_unlock(&share->intern_lock);
     break;
   case HA_EXTRA_NORMAL:				/* Theese isn't in use */
     info->quick_mode=0;