Fixed bug #29251.

Sometimes special 0 ENUM values was ALTERed to normal
empty string ENUM values.

Special 0 ENUM value has the same string representation
as normal ENUM value defined as '' (empty string).
The do_field_string function was used to convert
ENUM data at an ALTER TABLE request, but this
function doesn't care about numerical "indices" of
ENUM values, i.e. do_field_string doesn't distinguish
a special 0 value from an empty string value.

A new copy function called do_field_enum has been added to
copy special 0 ENUM values without conversion to an empty
string.


sql/field_conv.cc:
  Fixed bug #29251.
  The Copy_field::get_copy_func method has been modified to
  return a pointer to the do_field_enum function if a conversion
  between two columns of incompatible enum types is required.
  The do_field_enum function has been added for the correct
  conversion of special 0 enum values.
mysql-test/t/type_enum.test:
  Updated test case for bug #29251.
mysql-test/r/type_enum.result:
  Updated test case for bug #29251.

mysql-test/r/type_enum.result

@@ -1778,4 +1778,27 @@ drop table t1;
 create table t1(exhausting_charset enum('ABCDEFGHIJKLMNOPQRSTUVWXYZ','
	
 
 !"','#$%&\'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{|}~','xx\','yy\','zz'));
 ERROR 42000: Field separator argument is not what is expected; check the manual
+CREATE TABLE t1 (
+id INT AUTO_INCREMENT PRIMARY KEY,
+c1 ENUM('a', '', 'b')
+);
+INSERT INTO t1 (c1) VALUES (0), ('a'), (''), ('b');
+Warnings:
+Warning	1265	Data truncated for column 'c1' at row 1
+SELECT id, c1 + 0, c1 FROM t1;
+id	c1 + 0	c1
+1	0	
+2	1	a
+3	2	
+4	3	b
+ALTER TABLE t1 CHANGE c1 c1 ENUM('a', '') NOT NULL;
+Warnings:
+Warning	1265	Data truncated for column 'c1' at row 4
+SELECT id, c1 + 0, c1 FROM t1;
+id	c1 + 0	c1
+1	0	
+2	1	a
+3	2	
+4	0	
+DROP TABLE t1;
 End of 4.1 tests

mysql-test/t/type_enum.test

@@ -156,4 +156,21 @@ drop table t1;
 create table t1(exhausting_charset enum('ABCDEFGHIJKLMNOPQRSTUVWXYZ','
	
 
 !"','#$%&\'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{|}~','xx\','yy\','zz'));
 
+#
+# Bug #29251: MySQL coerces special 0 enum values to normal '' value
+# when ALTERing the column
+#
+
+CREATE TABLE t1 (
+  id INT AUTO_INCREMENT PRIMARY KEY,
+  c1 ENUM('a', '', 'b')
+);
+INSERT INTO t1 (c1) VALUES (0), ('a'), (''), ('b');
+SELECT id, c1 + 0, c1 FROM t1;
+
+ALTER TABLE t1 CHANGE c1 c1 ENUM('a', '') NOT NULL;
+SELECT id, c1 + 0, c1 FROM t1;
+
+DROP TABLE t1;
+
 --echo End of 4.1 tests

sql/field_conv.cc

@@ -311,6 +311,15 @@ static void do_field_string(Copy_field *copy)
 }
 
 
+static void do_field_enum(Copy_field *copy)
+{
+  if (copy->from_field->val_int() == 0)
+    ((Field_enum *) copy->to_field)->store_type((ulonglong) 0);
+  else
+    do_field_string(copy);
+}
+
+
 static void do_field_int(Copy_field *copy)
 {
   longlong value=copy->from_field->val_int();
@@ -538,8 +547,14 @@ void (*Copy_field::get_copy_func(Field *to,Field *from))(Copy_field*)
 	  to->real_type() == FIELD_TYPE_SET)
       {
 	if (!to->eq_def(from))
+        {
+          if (from->real_type() == MYSQL_TYPE_ENUM &&
+              to->real_type() == MYSQL_TYPE_ENUM)
+            return do_field_enum;
+          else
             return do_field_string;
         }
+      }
       else if (to->charset() != from->charset())
 	return do_field_string;
       else if (to->real_type() == FIELD_TYPE_VAR_STRING && to_length !=