Fix for bug #50908 "Assertion `handler_tables_hash.records == 0'

failed in enter_locked_tables_mode".

Server was aborted due to assertion failure when one tried to 
execute statement requiring prelocking (i.e. firing triggers
or using stored functions) while having open HANDLERs.

The problem was that THD::enter_locked_tables_mode() method
which was called at the beginning of execution of prelocked 
statement assumed there are no open HANDLERs. It had to do 
so because corresponding THD::leave_locked_tables_mode()
method was unable to properly restore MDL sentinel when
leaving LOCK TABLES/prelocked mode in the presence of open 
HANDLERs.

This patch solves this problem by changing the latter method
to properly restore MDL sentinel and thus removing need for 
this assumption. As a side-effect, it lifts unjustified
limitation by allowing to keep HANDLERs open when entering 
LOCK TABLES mode.

1 files changed, 24 insertions, 0 deletions

diff --git a/sql/sql_handler.cc b/sql/sql_handler.cc
index 9f365d0cf2f..4a69b46ddb7 100644
--- a/sql/sql_handler.cc
+++ b/sql/sql_handler.cc
@@ -837,3 +837,27 @@ void mysql_ha_cleanup(THD *thd)
   DBUG_VOID_RETURN;
 }
 
+
+/**
+  Move tickets for metadata locks corresponding to open HANDLERs
+  after transaction sentinel in order to protect them from being
+  released at the end of transaction.
+
+  @param thd Thread identifier.
+*/
+
+void mysql_ha_move_tickets_after_trans_sentinel(THD *thd)
+{
+  TABLE_LIST *hash_tables;
+  DBUG_ENTER("mysql_ha_move_tickets_after_trans_sentinel");
+
+  for (uint i= 0; i < thd->handler_tables_hash.records; i++)
+  {
+    hash_tables= (TABLE_LIST*) my_hash_element(&thd->handler_tables_hash, i);
+    if (hash_tables->table && hash_tables->table->mdl_ticket)
+      thd->mdl_context.
+             move_ticket_after_trans_sentinel(hash_tables->table->mdl_ticket);
+  }
+  DBUG_VOID_RETURN;
+}
+