func_system.result, func_system.test:

  adding test case
item_strfunc.cc:
  Bug#12351
  CONCAT with USER()/DATEBASE() and
  a column gets strange results.
  Mark created Item_str as constant, so CONCAT
  cannot reuse it for optimization purposes.

mysql-test/r/func_system.result

@@ -68,6 +68,21 @@ drop table t1;
 select TRUE,FALSE,NULL;
 TRUE	FALSE	NULL
 1	0	NULL
+create table t1 (c1 char(5)) character set=latin1;
+insert into t1 values('row 1');
+insert into t1 values('row 2');
+insert into t1 values('row 3');
+select concat(user(), '--', c1) from t1;
+concat(user(), '--', c1)
+root@localhost--row 1
+root@localhost--row 2
+root@localhost--row 3
+select concat(database(), '--', c1) from t1;
+concat(database(), '--', c1)
+test--row 1
+test--row 2
+test--row 3
+drop table t1;
 create table t1 (a char(10)) character set latin1;
 select * from t1 where a=version();
 a

mysql-test/t/func_system.test

@@ -31,6 +31,18 @@ drop table t1;
 
 select TRUE,FALSE,NULL;
 
+#
+# Bug#12351: CONCAT with USER()/DATEBASE() and
+# a constant and a column gets strange results
+#
+create table t1 (c1 char(5)) character set=latin1;
+insert into t1 values('row 1');
+insert into t1 values('row 2');
+insert into t1 values('row 3');
+select concat(user(), '--', c1) from t1;
+select concat(database(), '--', c1) from t1;
+drop table t1;
+
 #
 # Bug#8291 Illegal collation mix with USER() function
 #

sql/item_strfunc.cc

@@ -1550,6 +1550,7 @@ Item *Item_func_sysconst::safe_charset_converter(CHARSET_INFO *tocs)
     return NULL;
   }
   conv->str_value.copy();
+  conv->str_value.shrink_to_length();
   return conv;
 }