Fix for bug #9796 "Query Cache caches queries with CURRENT_USER()

function".

We should not cache queries using CURRENT_USER() function as we do it
for some other functions, e.g. USER() function.

mysql-test/r/query_cache.result

@@ -299,6 +299,8 @@ select UNIX_TIMESTAMP() from t1;
 UNIX_TIMESTAMP()
 select USER() from t1;
 USER()
+select CURRENT_USER() from t1;
+CURRENT_USER()
 select benchmark(1,1) from t1;
 benchmark(1,1)
 show status like "Qcache_queries_in_cache";

mysql-test/t/query_cache.test

@@ -182,6 +182,7 @@ select LAST_INSERT_ID() from t1;
 select RAND() from t1;
 select UNIX_TIMESTAMP() from t1;
 select USER() from t1;
+select CURRENT_USER() from t1;
 select benchmark(1,1) from t1;
 show status like "Qcache_queries_in_cache";
 #

sql/item_create.cc

@@ -297,6 +297,7 @@ Item *create_func_current_user()
   char buff[HOSTNAME_LENGTH+USERNAME_LENGTH+2];
   uint length;
 
+  thd->safe_to_cache_query= 0;
   length= (uint) (strxmov(buff, thd->priv_user, "@", thd->priv_host, NullS) -
 		  buff);
   return new Item_string(NullS, thd->memdup(buff, length), length);