Bug #15948123: SERVER WORKS INCORRECT WITH LONG TABLE ALIASES

Code in MDL subsystem assumes that identifiers of objects can't
be longer than NAME_LEN characters. This assumption was broken
when one tried to construct MDL_key based on table alias, which
can have arbitrary length. Since MDL_key's (and MDL locks) are
not really used for table aliases this patch changes code to
not initialize MDL_key object for table list element representing
aliases.

sql/mdl.h

@@ -246,6 +246,8 @@ public:
   }
   void mdl_key_init(const MDL_key *rhs)
   {
+    DBUG_ASSERT(rhs->m_length <= NAME_LEN);
+    DBUG_ASSERT(rhs->m_db_name_length <= NAME_LEN);
     memcpy(m_ptr, rhs->m_ptr, rhs->m_length);
     m_length= rhs->m_length;
     m_db_name_length= rhs->m_db_name_length;

sql/sql_parse.cc

@@ -6000,8 +6000,13 @@ TABLE_LIST *st_select_lex::add_table_to_list(THD *thd,
   ptr->next_name_resolution_table= NULL;
   /* Link table in global list (all used tables) */
   lex->add_to_query_tables(ptr);
-  ptr->mdl_request.init(MDL_key::TABLE, ptr->db, ptr->table_name, mdl_type,
-                        MDL_TRANSACTION);
+
+  // Pure table aliases do not need to be locked:
+  if (!test(table_options & TL_OPTION_ALIAS))
+  {
+    ptr->mdl_request.init(MDL_key::TABLE, ptr->db, ptr->table_name, mdl_type,
+                          MDL_TRANSACTION);
+  }
   DBUG_RETURN(ptr);
 }