Bug#21620 ALTER TABLE affects other columns

  Problem: for character sets having mbmaxlen==2,
  any ALTER TABLE changed TEXT column type to MEDIUMTEXT,
  due to wrong "internal length to create length" formula.
  Fix: removing rounding code introduced in early 4.1 time,
  which is not correct anymore.


mysql-test/r/ctype_gbk.result:
  Adding test case
mysql-test/t/ctype_gbk.test:
  Adding test case
sql/field.cc:
  Fixing "internal length to create length" formula.

mysql-test/r/ctype_gbk.result

@@ -168,3 +168,13 @@ DROP TABLE t1;
 select hex(convert(_gbk 0xA14041 using ucs2));
 hex(convert(_gbk 0xA14041 using ucs2))
 003F0041
+create table t1 (c1 text not null, c2 text not null) character set gbk;
+alter table t1 change c1 c1 mediumtext  character set gbk not null;
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `c1` mediumtext NOT NULL,
+  `c2` text NOT NULL
+) ENGINE=MyISAM DEFAULT CHARSET=gbk
+drop table t1;
+End of 5.0 tests

mysql-test/t/ctype_gbk.test

@@ -42,3 +42,13 @@ DROP TABLE t1;
 select hex(convert(_gbk 0xA14041 using ucs2));
 
 # End of 4.1 tests
+
+#
+# Bug#21620 ALTER TABLE affects other columns
+#
+create table t1 (c1 text not null, c2 text not null) character set gbk;
+alter table t1 change c1 c1 mediumtext  character set gbk not null;
+show create table t1;
+drop table t1;
+
+--echo End of 5.0 tests

sql/field.cc

@@ -8877,7 +8877,7 @@ create_field::create_field(Field *old_field,Field *orig_field)
     case  3: sql_type= FIELD_TYPE_MEDIUM_BLOB; break;
     default: sql_type= FIELD_TYPE_LONG_BLOB; break;
     }
-    length=(length+charset->mbmaxlen-1) / charset->mbmaxlen;
+    length/= charset->mbmaxlen;
     key_length/= charset->mbmaxlen;
     break;
   case MYSQL_TYPE_STRING: