Fix for bug #24558: Increasing decimal column length causes data loss

Altering to a decimal field we get double value then store it 
that may cause data loss. 
Fix: use store_decimal() instead.

mysql-test/r/type_newdecimal.result

@@ -1423,3 +1423,11 @@ cast(19999999999999999999 as unsigned)
 18446744073709551615
 Warnings:
 Error	1292	Truncated incorrect DECIMAL value: ''
+create table t1(a decimal(18));
+insert into t1 values(123456789012345678);
+alter table t1 modify column a decimal(19);
+select * from t1;
+a
+123456789012345678
+drop table t1;
+End of 5.0 tests

mysql-test/t/type_newdecimal.test

@@ -1120,3 +1120,14 @@ drop table t1;
 #
 select cast(19999999999999999999 as unsigned);
 
+#
+# Bug #24558: Increasing decimal column length causes data loss
+#
+
+create table t1(a decimal(18));
+insert into t1 values(123456789012345678);
+alter table t1 modify column a decimal(19);
+select * from t1;
+drop table t1;
+
+--echo End of 5.0 tests

sql/field_conv.cc

@@ -337,6 +337,13 @@ static void do_field_real(Copy_field *copy)
 }
 
 
+static void do_field_decimal(Copy_field *copy)
+{
+  my_decimal value;
+  copy->to_field->store_decimal(copy->from_field->val_decimal(&value));
+}
+
+
 /*
   string copy for single byte characters set when to string is shorter than
   from string
@@ -581,6 +588,8 @@ void (*Copy_field::get_copy_func(Field *to,Field *from))(Copy_field*)
     if (to->real_type() == FIELD_TYPE_BIT ||
         from->real_type() == FIELD_TYPE_BIT)
       return do_field_int;
+    if (to->result_type() == DECIMAL_RESULT)
+      return do_field_decimal;
     // Check if identical fields
     if (from->result_type() == STRING_RESULT)
     {