Bug#48157: crash in Item_field::used_tables

MySQL handles the join syntax "JOIN ... USING( field1,
... )" and natural joins by building the same parse tree as
a corresponding join with an "ON t1.field1 = t2.field1 ..."
expression would produce. This parse tree was not cleaned up
properly in the following scenario. If a thread tries to
lock some tables and finds that the tables were dropped and
re-created while waiting for the lock, it cleans up column
references in the statement by means a per-statement free
list. But if the statement was part of a stored procedure,
column references on the stored procedure's free list weren't
cleaned up and thus contained pointers to freed objects.

Fixed by adding a call to clean up the current prepared
statement's free list.

sql/sql_update.cc

@@ -23,6 +23,7 @@
 #include "sql_select.h"
 #include "sp_head.h"
 #include "sql_trigger.h"
+#include "debug_sync.h"
 
 /* Return 0 if row hasn't changed */
 
@@ -1143,8 +1144,11 @@ reopen_tables:
       items from 'fields' list, so the cleanup above is necessary to.
     */
     cleanup_items(thd->free_list);
-
+    cleanup_items(thd->stmt_arena->free_list);
     close_tables_for_reopen(thd, &table_list);
+
+    DEBUG_SYNC(thd, "multi_update_reopen_tables");
+
     goto reopen_tables;
   }