Bug #34678 @@debug variable's incremental mode

Description:
@@debug variable handles dbug incremental mode incorrectly

How to repeat:
mysql> select @@debug;
+---------+
| @@debug |
+---------+
|         | 
+---------+
mysql> set debug='T';
Query OK, 0 rows affected (0.00 sec)
mysql> select @@debug;
+---------+
| @@debug |
+---------+
| T       | 
+---------+
mysql> set debug='+P';
Query OK, 0 rows affected (0.00 sec)
mysql> select @@debug;
+---------+
| @@debug |
+---------+
| P       | 
+---------+

must be 'T:P'

Suggested fix:
use DBUG_SET instead of a pair DBUG_POP/DBUG_PUSH in sys_var_thd_dbug::update()

Queued in 5.1-runtime

http://lists.mysql.com/commits/42996

ChangeSet@1.2580, 2008-02-26 12:03:59-03:00, davi@mysql.com +7 -0
  Bug#34424 query_cache_debug.test leads to valgrind warnings
  Bug#34678 @@debug variable's incremental mode

Pushed into 5.1.24-rc

Pushed into 6.0.5-alpha

Pushed into 5.1.23-ndb-6.3.11.

Noted in 5.1.24, 6.0.5 changelogs.

Assigning an incremental value to the debug system variable did not
add the new value to the current value. For example, if the current
debug value was 'T', the statement SET debug = '+P' resulted in a
value of 'P' rather than the correct value of 'P:T'.