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, 2 insertions, 2 deletions

diff --git a/sql/table_cache.cc b/sql/table_cache.cc
index 9afe3ca61d4..d997aeff9f8 100644
--- a/sql/table_cache.cc
+++ b/sql/table_cache.cc
@@ -406,7 +406,7 @@ void tc_add_table(THD *thd, TABLE *table)
   @return TABLE object, or NULL if no unused objects.
 */
 
-static TABLE *tc_acquire_table(THD *thd, TDC_element *element)
+TABLE *tc_acquire_table(THD *thd, TDC_element *element)
 {
   uint32 n_instances=
     my_atomic_load32_explicit((int32*) &tc_active_instances,
@@ -657,7 +657,7 @@ void tdc_start_shutdown(void)
     tdc_size= 0;
     tc_size= 0;
     /* Free all cached but unused TABLEs and TABLE_SHAREs. */
-    close_cached_tables(NULL, NULL, FALSE, LONG_TIMEOUT);
+    purge_tables(true);
   }
   DBUG_VOID_RETURN;
 }