MDEV-37968 GTT: is allowed to us as mysql.help_topic, failing assertions

HELP caused opening a child GTT, whish has table category TEMPORARY,
while only SYSTEM/STATISTICS tables are allowed.

In another assertion, ref_count didn't reach 0 in innodb on shutdown.

We really don't want allowing GTT to work as a system table.

The problem is table->s->table_category is determined at *open* stage,
merely by a name:
share->table_category= get_table_category(share->db, share->table_name)

We could forbid GTT creation when its name maches non-user category.
However, RENAME never opens a table, which means, we anyway can rename
GTT to a system table.

All we can do is make sure we don't open it.

Parent issue:
MDEV-35915

Author: FooBarrior

mysql-test/main/global_temporary_table.result

@@ -1342,4 +1342,12 @@ create or replace global temporary table t2 select 0;
 alter table t2 force;
 flush table t2;
 drop table t2;
+# MDEV-37968 GTT: is allowed to us as mysql.help_topic, causing various assertion failures
+rename table mysql.help_topic to mysql.help_topic1;
+create global temporary table test.t1 (help_topic_id int unsigned,name char);
+rename table test.t1 to mysql.help_topic;
+help '';
+ERROR HY000: 'mysql.help_topic' is not of type 'BASE TABLE'
+drop table mysql.help_topic;
+rename table mysql.help_topic1 to mysql.help_topic;
 disconnect con1;

mysql-test/main/global_temporary_table.test

@@ -1223,4 +1223,14 @@ alter table t2 force;
 flush table t2;
 drop table t2;
 
+--echo # MDEV-37968 GTT: is allowed to us as mysql.help_topic, causing various assertion failures
+rename table mysql.help_topic to mysql.help_topic1;
+create global temporary table test.t1 (help_topic_id int unsigned,name char);
+rename table test.t1 to mysql.help_topic;
+--error ER_WRONG_OBJECT
+help '';
+
+drop table mysql.help_topic;
+rename table mysql.help_topic1 to mysql.help_topic;
+
 --disconnect con1

mysql-test/suite/binlog/r/global_temporary_table.result

@@ -1345,6 +1345,14 @@ create or replace global temporary table t2 select 0;
 alter table t2 force;
 flush table t2;
 drop table t2;
+# MDEV-37968 GTT: is allowed to us as mysql.help_topic, causing various assertion failures
+rename table mysql.help_topic to mysql.help_topic1;
+create global temporary table test.t1 (help_topic_id int unsigned,name char);
+rename table test.t1 to mysql.help_topic;
+help '';
+ERROR HY000: 'mysql.help_topic' is not of type 'BASE TABLE'
+drop table mysql.help_topic;
+rename table mysql.help_topic1 to mysql.help_topic;
 disconnect con1;
 include/show_binlog_events.inc
 Log_name	Pos	Event_type	Server_id	End_log_pos	Info
@@ -2113,5 +2121,15 @@ master-bin.000001	#	Gtid	#	#	GTID #-#-#
 master-bin.000001	#	Query	#	#	use `test`; flush table t2
 master-bin.000001	#	Gtid	#	#	GTID #-#-#
 master-bin.000001	#	Query	#	#	use `test`; DROP TABLE `t2` /* generated by server */
+master-bin.000001	#	Gtid	#	#	GTID #-#-#
+master-bin.000001	#	Query	#	#	use `test`; rename table mysql.help_topic to mysql.help_topic1
+master-bin.000001	#	Gtid	#	#	GTID #-#-#
+master-bin.000001	#	Query	#	#	use `test`; create global temporary table test.t1 (help_topic_id int unsigned,name char)
+master-bin.000001	#	Gtid	#	#	GTID #-#-#
+master-bin.000001	#	Query	#	#	use `test`; rename table test.t1 to mysql.help_topic
+master-bin.000001	#	Gtid	#	#	GTID #-#-#
+master-bin.000001	#	Query	#	#	use `test`; DROP TABLE `mysql`.`help_topic` /* generated by server */
+master-bin.000001	#	Gtid	#	#	GTID #-#-#
+master-bin.000001	#	Query	#	#	use `test`; rename table mysql.help_topic1 to mysql.help_topic
 set global binlog_annotate_row_events= @old_binlog_annotate_row_events;
 reset master;

mysql-test/suite/rpl/r/global_temporary_table.result

@@ -1345,6 +1345,14 @@ create or replace global temporary table t2 select 0;
 alter table t2 force;
 flush table t2;
 drop table t2;
+# MDEV-37968 GTT: is allowed to us as mysql.help_topic, causing various assertion failures
+rename table mysql.help_topic to mysql.help_topic1;
+create global temporary table test.t1 (help_topic_id int unsigned,name char);
+rename table test.t1 to mysql.help_topic;
+help '';
+ERROR HY000: 'mysql.help_topic' is not of type 'BASE TABLE'
+drop table mysql.help_topic;
+rename table mysql.help_topic1 to mysql.help_topic;
 disconnect con1;
 connection slave;
 connection master;

sql/sql_table.cc

@@ -6239,6 +6239,13 @@ my_bool open_global_temporary_table(THD *thd, TABLE_SHARE *source,
     return TRUE;
   }
 
+  if (source->table_category != TABLE_CATEGORY_USER)
+  {
+    my_error(ER_WRONG_OBJECT, MYF(0), source->db.str, source->table_name.str,
+             "BASE TABLE");
+    return TRUE;
+  }
+
   /*
     First, lookup in tmp tables list for cases like "t join t"
     This also could happen if tl->open_type is OT_BASE_ONLY