Fixed a bug that allowed:

SET @foo=bar;

This would cause a deadlock in mysqld, it now generates the error:
ER_SET_CONSTANTS_ONLY as follows:

mysql> set @foo=bar;
ERROR 1204: You may only use constant expressions with SET

BitKeeper/etc/logging_ok

 mwagner@evoq.mwagner.org
 heikki@donna.mysql.fi
+jcole@tetra.spaceapes.com

include/mysqld_error.h

@@ -204,4 +204,5 @@
 #define ER_MASTER_INFO 1201
 #define ER_SLAVE_THREAD 1202
 #define ER_TOO_MANY_USER_CONNECTIONS 1203
-#define ER_ERROR_MESSAGES 204
+#define ER_SET_CONSTANTS_ONLY 1204
+#define ER_ERROR_MESSAGES 205

sql/share/czech/errmsg.txt

@@ -214,3 +214,4 @@
 "Could not initialize master info structure, check permisions on master.info",
 "Could not create slave thread, check system resources",
 "User %-.64s has already more than 'max_user_connections' active connections",
+"You may only use constant expressions with SET",

sql/share/danish/errmsg.txt

@@ -208,3 +208,4 @@
 "Could not initialize master info structure, check permisions on master.info",
 "Could not create slave thread, check system resources",
 "User %-.64s has already more than 'max_user_connections' active connections",
+"You may only use constant expressions with SET",

sql/share/dutch/errmsg.txt

@@ -205,3 +205,4 @@
 "Could not initialize master info structure, check permisions on master.info",
 "Could not create slave thread, check system resources",
 "User %-.64s has already more than 'max_user_connections' active connections",
+"You may only use constant expressions with SET",

sql/share/english/errmsg.txt

@@ -205,3 +205,4 @@
 "Could not initialize master info structure, check permisions on master.info",
 "Could not create slave thread, check system resources",
 "User %-.64s has already more than 'max_user_connections' active connections",
+"You may only use constant expressions with SET",

sql/share/estonian/errmsg.txt

@@ -209,3 +209,4 @@
 "Could not initialize master info structure, check permisions on master.info",
 "Could not create slave thread, check system resources",
 "User %-.64s has already more than 'max_user_connections' active connections",
+"You may only use constant expressions with SET",

sql/share/french/errmsg.txt

@@ -205,3 +205,4 @@
 "Could not initialize master info structure, check permisions on master.info",
 "Could not create slave thread, check system resources",
 "User %-.64s has already more than 'max_user_connections' active connections",
+"You may only use constant expressions with SET",

sql/share/german/errmsg.txt

@@ -208,3 +208,4 @@
 "Could not initialize master info structure, check permisions on master.info",
 "Could not create slave thread, check system resources",
 "User %-.64s has already more than 'max_user_connections' active connections",
+"You may only use constant expressions with SET",

sql/share/greek/errmsg.txt

@@ -205,3 +205,4 @@
 "Could not initialize master info structure, check permisions on master.info",
 "Could not create slave thread, check system resources",
 "User %-.64s has already more than 'max_user_connections' active connections",
+"You may only use constant expressions with SET",

sql/share/hungarian/errmsg.txt

@@ -207,3 +207,4 @@
 "Could not initialize master info structure, check permisions on master.info",
 "Could not create slave thread, check system resources",
 "User %-.64s has already more than 'max_user_connections' active connections",
+"You may only use constant expressions with SET",

sql/share/italian/errmsg.txt

@@ -205,3 +205,4 @@
 "Could not initialize master info structure, check permisions on master.info",
 "Could not create slave thread, check system resources",
 "User %-.64s has already more than 'max_user_connections' active connections",
+"You may only use constant expressions with SET",

sql/share/japanese/errmsg.txt

@@ -207,3 +207,4 @@
 "Could not initialize master info structure, check permisions on master.info",
 "Could not create slave thread, check system resources",
 "User %-.64s has already more than 'max_user_connections' active connections",
+"You may only use constant expressions with SET",

sql/share/korean/errmsg.txt

@@ -205,3 +205,4 @@
 "Could not initialize master info structure, check permisions on master.info",
 "Could not create slave thread, check system resources",
 "User %-.64s has already more than 'max_user_connections' active connections",
+"You may only use constant expressions with SET",

sql/share/norwegian-ny/errmsg.txt

@@ -207,3 +207,4 @@
 "Could not initialize master info structure, check permisions on master.info",
 "Could not create slave thread, check system resources",
 "User %-.64s has already more than 'max_user_connections' active connections",
+"You may only use constant expressions with SET",

sql/share/norwegian/errmsg.txt

@@ -207,3 +207,4 @@
 "Could not initialize master info structure, check permisions on master.info",
 "Could not create slave thread, check system resources",
 "User %-.64s has already more than 'max_user_connections' active connections",
+"You may only use constant expressions with SET",

sql/share/polish/errmsg.txt

@@ -209,3 +209,4 @@
 "Could not initialize master info structure, check permisions on master.info",
 "Could not create slave thread, check system resources",
 "User %-.64s has already more than 'max_user_connections' active connections",
+"You may only use constant expressions with SET",

sql/share/portuguese/errmsg.txt

@@ -205,3 +205,4 @@
 "Could not initialize master info structure, check permisions on master.info",
 "Could not create slave thread, check system resources",
 "User %-.64s has already more than 'max_user_connections' active connections",
+"You may only use constant expressions with SET",

sql/share/romanian/errmsg.txt

@@ -209,3 +209,4 @@
 "Could not initialize master info structure, check permisions on master.info",
 "Could not create slave thread, check system resources",
 "User %-.64s has already more than 'max_user_connections' active connections",
+"You may only use constant expressions with SET",

sql/share/russian/errmsg.txt

@@ -208,3 +208,4 @@
 "    master info,  persmissions   master.info",
 "    SLAVE,   ",
 "User %-.64s has already more than 'max_user_connections' active connections",
+"You may only use constant expressions with SET",

sql/share/slovak/errmsg.txt

@@ -213,3 +213,4 @@
 "Could not initialize master info structure, check permisions on master.info",
 "Could not create slave thread, check system resources",
 "User %-.64s has already more than 'max_user_connections' active connections",
+"You may only use constant expressions with SET",

sql/share/spanish/errmsg.txt

@@ -206,3 +206,4 @@
 "Could not initialize master info structure, check permisions on master.info",
 "Could not create slave thread, check system resources",
 "User %-.64s has already more than 'max_user_connections' active connections",
+"You may only use constant expressions with SET",

sql/share/swedish/errmsg.txt

@@ -205,3 +205,4 @@
 "Kunde inte initializera replications-strukturerna. Kontrollera privilegerna för 'master.info'",
 "Kunde inte starta en tråd för replikering",
 "Användare '%-.64s' har redan 'max_user_connections' aktiva inloggningar",
+"You may only use constant expressions with SET",

sql/sql_yacc.yy

@@ -2666,7 +2666,7 @@ option_value:
 	  {
 	     Item_func_set_user_var *item = new Item_func_set_user_var($2,$4);
 	     if (item->fix_fields(current_thd,0) || item->update())
-		YYABORT;
+		send_error(&current_thd->net, ER_SET_CONSTANTS_ONLY);
 	  }
          | SQL_SLAVE_SKIP_COUNTER equal ULONG_NUM
           {