Fixed bug#15962: CONCAT() in UNION may lead to a data trucation.

To calculate its max_length the CONCAT() function is simply sums max_lengths
of its arguments but when the collation of an argument differs from the 
collation of the CONCAT() max_length will be wrong. This may lead to a data
truncation when a tmp table is used, in UNIONS for example.

The Item_func_concat::fix_length_and_dec() function now recalculates the 
max_length of an argument when the mbmaxlen of the argument differs from the
mbmaxlen of the CONCAT().

mysql-test/r/func_concat.result

@@ -68,3 +68,10 @@ select 'a' union select concat('a', -0.0000);
 a
 a
 a0.0000
+create table t1(f1 varchar(6)) charset=utf8;
+insert into t1 values ("123456");
+select concat(f1, 2) a from t1 union select 'x' a from t1;
+a
+1234562
+x
+drop table t1;

mysql-test/t/func_concat.test

@@ -53,3 +53,11 @@ select 'a' union select concat('a', -0.0);
 select 'a' union select concat('a', -0.0000);
 
 # End of 4.1 tests
+
+#
+# Bug#15962: CONCAT() in UNION may lead to a data trucation.
+#
+create table t1(f1 varchar(6)) charset=utf8;
+insert into t1 values ("123456");
+select concat(f1, 2) a from t1 union select 'x' a from t1;
+drop table t1;

sql/item_strfunc.cc

@@ -389,7 +389,14 @@ void Item_func_concat::fix_length_and_dec()
     return;
 
   for (uint i=0 ; i < arg_count ; i++)
+  {
+    if (args[i]->collation.collation->mbmaxlen != collation.collation->mbmaxlen)
+      max_result_length+= (args[i]->max_length /
+                           args[i]->collation.collation->mbmaxlen) *
+                           collation.collation->mbmaxlen;
+    else
       max_result_length+= args[i]->max_length;
+  }
 
   if (max_result_length >= MAX_BLOB_WIDTH)
   {