MySQL Bugs: #38962: Assignment of a variable to "max_allowed

Bug #38962	Assignment of a variable to "max_allowed_packet" fails.
Submitted:	22 Aug 2008 14:00	Modified:	26 Aug 2008 1:37
Reporter:	Horst Hunger	Email Updates:
Status:	Not a Bug	Impact on me:	None
Category:	MySQL Server	Severity:	S3 (Non-critical)
Version:	5.1.28-debug	OS:	Linux (2.6.22.18-0.2-default #1 SMP)
Assigned to:		CPU Architecture:	Any

Description:
The following statement:

SET @@global.max_allowed_packet= @start_max_allowed_packet;

failed with the following error message:

mysqltest: At line 94: query 'SET @@global.max_allowed_packet= @start_max_allowed_packet' failed: 1232: Incorrect argument type to variable 'max_allowed_packet'

How to repeat:
execute the main test "max_allowed_packet_func.test" after enabling or inserting (if not yet existing) of the "SET" statement at the beginning and end of the program. Comment the "let ..." at both places out, if not yet done. The test exists in main, but not in the newest version. 
You can also execute the newest version which is attached.

Suggested fix:
Allow the requested assignment as it is allowed for other system variables or describe that restriction in the manual.

On 5.1.26 I've got:

C:\Program Files\MySQL\MySQL Server 5.1\bin>mysql -uroot -proot -P3310 test
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 7
Server version: 5.1.26-rc-community-log MySQL Community Server (GPL)

Type 'help;' or '\h' for help. Type '\c' to clear the buffer.

mysql> SET @start_max_allowed_paket= @@global.max_allowed_packet;
Query OK, 0 rows affected (0.00 sec)

mysql> SET  @@global.max_allowed_packet=@start_max_allowed_paket;
Query OK, 0 rows affected (0.03 sec)

So, it worked in 5.1.26.

I was able to repeat on FC 8 64-bit bzr tree:

Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 3
Server version: 5.1.28-debug Source distribution

Type 'help;' or '\h' for help. Type '\c' to clear the buffer.

mysql> SET @@global.max_allowed_packet= @start_max_allowed_packet;
ERROR 1232 (42000): Incorrect argument type to variable 'max_allowed_packet'
mysql>

Thank you for the bug report.

This behavior doesn't show up if the variable is properly initialized.  Setting to Not a Bug, since it's OK if assigning from an uninitialized variable doesn't take effect.