Fix for bug #10337 (cast(NULL DECIMAL) crashes the server)

mysql-test/r/cast.result:
  test result fixed
mysql-test/t/cast.test:
  test case added
sql/item_func.cc:
  checks for NULL added
strings/decimal.c:
  we need to return specified 'scale' for the rounded decimal

mysql-test/r/cast.result

@@ -344,3 +344,6 @@ select cast(s1 as decimal(7,2)) from t1;
 cast(s1 as decimal(7,2))
 111111.00
 drop table t1;
+select cast(NULL as decimal(6)) as t1;
+t1
+NULL

mysql-test/t/cast.test

@@ -168,3 +168,9 @@ create table t1(s1 time);
 insert into t1 values ('11:11:11');
 select cast(s1 as decimal(7,2)) from t1;
 drop table t1;
+
+#
+# Bug @10237 (CAST(NULL DECIMAL) crashes server)
+#
+select cast(NULL as decimal(6)) as t1;
+

sql/item_func.cc

@@ -1022,7 +1022,8 @@ longlong Item_func_unsigned::val_int()
 String *Item_decimal_typecast::val_str(String *str)
 {
   my_decimal tmp_buf, *tmp= val_decimal(&tmp_buf);
-  my_decimal_round(E_DEC_FATAL_ERROR, tmp, decimals, FALSE, &tmp_buf);
+  if (null_value)
+    return NULL;
   my_decimal2string(E_DEC_FATAL_ERROR, &tmp_buf, 0, 0, 0, str);
   return str;
 }
@@ -1032,6 +1033,8 @@ double Item_decimal_typecast::val_real()
 {
   my_decimal tmp_buf, *tmp= val_decimal(&tmp_buf);
   double res;
+  if (null_value)
+    return 0.0;
   my_decimal2double(E_DEC_FATAL_ERROR, tmp, &res);
   return res;
 }
@@ -1041,6 +1044,8 @@ longlong Item_decimal_typecast::val_int()
 {
   my_decimal tmp_buf, *tmp= val_decimal(&tmp_buf);
   longlong res;
+  if (null_value)
+    return 0;
   my_decimal2int(E_DEC_FATAL_ERROR, tmp, unsigned_flag, &res);
   return res;
 }
@@ -1049,6 +1054,8 @@ longlong Item_decimal_typecast::val_int()
 my_decimal *Item_decimal_typecast::val_decimal(my_decimal *dec)
 {
   my_decimal tmp_buf, *tmp= args[0]->val_decimal(&tmp_buf);
+  if ((null_value= args[0]->null_value))
+    return NULL;
   my_decimal_round(E_DEC_FATAL_ERROR, tmp, decimals, FALSE, dec);
   return dec;
 }

strings/decimal.c

@@ -1563,7 +1563,13 @@ decimal_round(decimal_t *from, decimal_t *to, int scale,
         break;
       if (buf1-- == to->buf)
       {
-        decimal_make_zero(to);
+        /* making 'zero' with the proper scale */
+        dec1 *p0= to->buf + frac0 + 1;
+        to->intg=1;
+        to->frac= max(scale, 0);
+        to->sign= 0;
+        for (buf1= to->buf; buf1<p0; buf1++)
+          *buf1= 0;
         return E_DEC_OK;
       }
     }