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, 9 insertions, 3 deletions

diff --git a/sql/sql_handler.cc b/sql/sql_handler.cc
index 72df8367dc7..08114b99757 100644
--- a/sql/sql_handler.cc
+++ b/sql/sql_handler.cc
@@ -1196,10 +1196,10 @@ void mysql_ha_flush(THD *thd)
   @note Broadcasts refresh if it closed a table with old version.
 */
 
-void mysql_ha_cleanup(THD *thd)
+void mysql_ha_cleanup_no_free(THD *thd)
 {
   SQL_HANDLER *hash_tables;
-  DBUG_ENTER("mysql_ha_cleanup");
+  DBUG_ENTER("mysql_ha_cleanup_no_free");
 
   for (uint i= 0; i < thd->handler_tables_hash.records; i++)
   {
@@ -1207,9 +1207,15 @@ void mysql_ha_cleanup(THD *thd)
     if (hash_tables->table)
       mysql_ha_close_table(hash_tables);
   }
+  DBUG_VOID_RETURN;
+}
 
-  my_hash_free(&thd->handler_tables_hash);
 
+void mysql_ha_cleanup(THD *thd)
+{
+  DBUG_ENTER("mysql_ha_cleanup");
+  mysql_ha_cleanup_no_free(thd);
+  my_hash_free(&thd->handler_tables_hash);
   DBUG_VOID_RETURN;
 }