Fix for bug #28631: Bit index creation failure after alter

Problem: altering a bit field we use Field::is_equal() to check if the bit 
field is changed. Comparing the field type is not enough for bit fields.
Fix: add proper Field_bit::is_equal() that compares the field lengths as well.

mysql-test/r/type_bit.result

@@ -619,3 +619,29 @@ bit_field	int_field
 handler t1 close;
 drop table t1;
 End of 5.0 tests
+create table t1(a bit(7));
+insert into t1 values(0x40);
+alter table t1 modify column a bit(8);
+select hex(a) from t1;
+hex(a)
+40
+insert into t1 values(0x80);
+select hex(a) from t1;
+hex(a)
+40
+80
+create index a on t1(a);
+insert into t1 values(0x81);
+select hex(a) from t1;
+hex(a)
+40
+80
+81
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` bit(8) DEFAULT NULL,
+  KEY `a` (`a`)
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+drop table t1;
+End of 5.1 tests

mysql-test/t/type_bit.test

@@ -273,3 +273,20 @@ handler t1 close;
 drop table t1;
 
 --echo End of 5.0 tests
+
+#
+# Bug #28631: problem after alter
+#
+create table t1(a bit(7));
+insert into t1 values(0x40);
+alter table t1 modify column a bit(8);
+select hex(a) from t1;
+insert into t1 values(0x80);
+select hex(a) from t1;
+create index a on t1(a);
+insert into t1 values(0x81);
+select hex(a) from t1;
+show create table t1;
+drop table t1;
+
+--echo End of 5.1 tests

sql/field.cc

@@ -8239,6 +8239,13 @@ Field *Field_bit::new_key_field(MEM_ROOT *root,
 }
 
 
+uint Field_bit::is_equal(create_field *new_field) 
+{
+  return (new_field->sql_type == real_type() &&
+          new_field->length == max_display_length());
+}
+
+                       
 int Field_bit::store(const char *from, uint length, CHARSET_INFO *cs)
 {
   ASSERT_COLUMN_MARKED_FOR_WRITE;

sql/field.h

@@ -1552,6 +1552,7 @@ public:
             bit_ptr == ((Field_bit *)field)->bit_ptr &&
             bit_ofs == ((Field_bit *)field)->bit_ofs);
   }
+  uint is_equal(create_field *new_field);
   void move_field_offset(my_ptrdiff_t ptr_diff)
   {
     Field::move_field_offset(ptr_diff);