Bug #27984 Long Decimal Maths produces truncated results.

decimal_round failed to perform a correct rounding 
of a decimal number if its first nine digits were '9'.
It just sets those digits to 0.

mysql-test/r/type_newdecimal.result

@@ -1465,4 +1465,7 @@ Error	1264	Out of range value adjusted for column 'cast(a as DECIMAL(3,2))' at r
 Error	1264	Out of range value adjusted for column 'cast(a as DECIMAL(3,2))' at row 1
 Error	1264	Out of range value adjusted for column 'cast(a as DECIMAL(3,2))' at row 1
 Error	1264	Out of range value adjusted for column 'cast(a as DECIMAL(3,2))' at row 1
+SELECT 1.000000000000 * 99.999999999998 / 100 a,1.000000000000 * (99.999999999998 / 100) b;
+a	b
+0.9999999999999800000000000000	0.9999999999999800000000000000
 End of 5.0 tests

mysql-test/t/type_newdecimal.test

@@ -1149,4 +1149,9 @@ select cast(a as DECIMAL(3,2)), count(*)
   UNION select 12.1234
  ) t group by 1;
 
+#
+# 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

@@ -1517,9 +1517,10 @@ 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;