Merge abotchkov@bk-internal.mysql.com:/home/bk/mysql-5.1-opt

into  mysql.com:/home/hf/work/27984/my51-27984

mysql-test/r/type_newdecimal.result

@@ -1478,6 +1478,9 @@ Error	1264	Out of range value for column 'cast(a as DECIMAL(3,2))' at row 1
 create table t1 (s varchar(100));
 insert into t1 values (0.00000000010000000000000000364321973154977415791655470655996396089904010295867919921875);
 drop table t1;
+SELECT 1.000000000000 * 99.999999999998 / 100 a,1.000000000000 * (99.999999999998 / 100) b;
+a	b
+0.9999999999999800000000000000	0.9999999999999800000000000000
 End of 5.0 tests
 select cast(143.481 as decimal(4,1));
 cast(143.481 as decimal(4,1))

mysql-test/t/type_newdecimal.test

@@ -1167,6 +1167,11 @@ create table t1 (s varchar(100));
 insert into t1 values (0.00000000010000000000000000364321973154977415791655470655996396089904010295867919921875);
 drop table t1;
 
+#
+# Bug #27984 Long Decimal Maths produces truncated results 
+#
+
+SELECT 1.000000000000 * 99.999999999998 / 100 a,1.000000000000 * (99.999999999998 / 100) b;
 --echo End of 5.0 tests
 
 #

strings/decimal.c

@@ -1523,9 +1523,10 @@ decimal_round(decimal_t *from, decimal_t *to, int scale,
     dec1 *p0= buf0+intg0+max(frac1, frac0);
     dec1 *p1= buf1+intg1+max(frac1, frac0);
 
-    to->buf[0]= 0;
     while (buf0 < p0)
       *(--p1) = *(--p0);
+    if (unlikely(intg1 > intg0))
+      to->buf[0]= 0;
 
     intg0= intg1;
     buf0=to->buf;