Bug #54105 assert in MDL_context::release_locks_stored_before

The problem was that SHOW CREATE EVENT released all metadata locks
held by the current transaction. This made any exisiting savepoints
invalid, triggering the assert when ROLLBACK TO SAVEPOINT later
was executed.

This patch fixes the problem by making sure SHOW CREATE EVENT only
releases metadata locks acquired by the statement itself.

Test case added to event_trans.test.

1 files changed, 14 insertions, 5 deletions

diff --git a/sql/event_db_repository.cc b/sql/event_db_repository.cc
index d47f1641bb0..db508e4ea41 100644
--- a/sql/event_db_repository.cc
+++ b/sql/event_db_repository.cc
@@ -996,24 +996,33 @@ Event_db_repository::load_named_event(THD *thd, LEX_STRING dbname,
                                       LEX_STRING name, Event_basic *etn)
 {
   bool ret;
-  TABLE *table= NULL;
   ulong saved_mode= thd->variables.sql_mode;
+  Open_tables_backup open_tables_backup;
+  TABLE_LIST event_table;
 
   DBUG_ENTER("Event_db_repository::load_named_event");
   DBUG_PRINT("enter",("thd: 0x%lx  name: %*s", (long) thd,
                       (int) name.length, name.str));
 
+  event_table.init_one_table("mysql", 5, "event", 5, "event", TL_READ);
+
   /* Reset sql_mode during data dictionary operations. */
   thd->variables.sql_mode= 0;
 
-  if (!(ret= open_event_table(thd, TL_READ, &table)))
+  /*
+    We don't use open_event_table() here to make sure that SHOW
+    CREATE EVENT works properly in transactional context, and
+    does not release transactional metadata locks when the
+    event table is closed.
+  */
+  if (!(ret= open_system_tables_for_read(thd, &event_table, &open_tables_backup)))
   {
-    if ((ret= find_named_event(dbname, name, table)))
+    if ((ret= find_named_event(dbname, name, event_table.table)))
       my_error(ER_EVENT_DOES_NOT_EXIST, MYF(0), name.str);
-    else if ((ret= etn->load_from_row(thd, table)))
+    else if ((ret= etn->load_from_row(thd, event_table.table)))
       my_error(ER_CANNOT_LOAD_FROM_TABLE, MYF(0), "event");
 
-    close_mysql_tables(thd);
+    close_system_tables(thd, &open_tables_backup);
   }
 
   thd->variables.sql_mode= saved_mode;