Fix for bug #12652385 - "61493: REORDERING COLUMNS

TO POSITION FIRST CAN CAUSE DATA TO BE CORRUPTED".
 
ALTER TABLE MODIFY/CHANGE ... FIRST did nothing except renaming
columns if new version of the table had exactly the same 
structure as the old one (i.e. as result of such statement, names 
of columns changed their order as specified but data in columns 
didn't). The same thing happened for ALTER TABLE DROP COLUMN/ADD 
COLUMN statements which were supposed to produce new version of
table with exactly the same structure as the old version of table.
I.e. in the latter case the result was the same as if old column 
was renamed instead of being dropped and new column with default
as value being created.
 
Both these problems were caused by the fact that ALTER TABLE
implementation incorrectly interpreted both these situations as 
simple renaming of columns and assumed that in-place ALTER TABLE
algorithm could have been used for them.
 
This patch fixes this problem by ensuring that in cases when some
column is moved to the first position or some column is dropped
the default ALTER TABLE algorithm involving table copying is 
always used. This is achieved by detecting such situations in
mysql_prepare_alter_table() and setting Alter_info::change_level
to ALTER_TABLE_DATA_CHANGED for them.

mysql-test/r/alter_table.result

@@ -1345,4 +1345,33 @@ DROP TABLE t1;
 CREATE TABLE t1 (a TEXT, id INT, b INT);
 ALTER TABLE t1 DROP COLUMN a, ADD COLUMN c TEXT FIRST;
 DROP TABLE t1;
+#
+# Test for bug #12652385 - "61493: REORDERING COLUMNS TO POSITION
+#                           FIRST CAN CAUSE DATA TO BE CORRUPTED".
+#
+drop table if exists t1;
+# Use MyISAM engine as the fact that InnoDB doesn't support
+# in-place ALTER TABLE in cases when columns are being renamed
+# hides some bugs.
+create table t1 (i int, j int) engine=myisam;
+insert into t1 value (1, 2);
+# First, test for original problem described in the bug report.
+select * from t1;
+i	j
+1	2
+# Change of column order by the below ALTER TABLE statement should
+# affect both column names and column contents.
+alter table t1 modify column j int first;
+select * from t1;
+j	i
+2	1
+# Now test for similar problem with the same root.
+# The below ALTER TABLE should change not only the name but
+# also the value for the last column of the table.
+alter table t1 drop column i, add column k int default 0;
+select * from t1;
+j	k
+2	0
+# Clean-up.
+drop table t1;
 End of 5.1 tests

mysql-test/t/alter_table.test

@@ -1073,4 +1073,31 @@ ALTER TABLE t1 DROP COLUMN a, ADD COLUMN c TEXT FIRST;
 DROP TABLE t1;
 
 
+--echo #
+--echo # Test for bug #12652385 - "61493: REORDERING COLUMNS TO POSITION
+--echo #                           FIRST CAN CAUSE DATA TO BE CORRUPTED".
+--echo #
+--disable_warnings
+drop table if exists t1;
+--enable_warnings
+--echo # Use MyISAM engine as the fact that InnoDB doesn't support
+--echo # in-place ALTER TABLE in cases when columns are being renamed
+--echo # hides some bugs.
+create table t1 (i int, j int) engine=myisam;
+insert into t1 value (1, 2);
+--echo # First, test for original problem described in the bug report.
+select * from t1;
+--echo # Change of column order by the below ALTER TABLE statement should
+--echo # affect both column names and column contents.
+alter table t1 modify column j int first;
+select * from t1;
+--echo # Now test for similar problem with the same root.
+--echo # The below ALTER TABLE should change not only the name but
+--echo # also the value for the last column of the table.
+alter table t1 drop column i, add column k int default 0;
+select * from t1;
+--echo # Clean-up.
+drop table t1;
+
+
 --echo End of 5.1 tests

sql/sql_table.cc

@@ -6170,6 +6170,12 @@ mysql_prepare_alter_table(THD *thd, TABLE *table,
     if (drop)
     {
       drop_it.remove();
+      /*
+        ALTER TABLE DROP COLUMN always changes table data even in cases
+        when new version of the table has the same structure as the old
+        one.
+      */
+      alter_info->change_level= ALTER_TABLE_DATA_CHANGED;
       continue;
     }
     /* Check if field is changed */
@@ -6247,7 +6253,14 @@ mysql_prepare_alter_table(THD *thd, TABLE *table,
     if (!def->after)
       new_create_list.push_back(def);
     else if (def->after == first_keyword)
+    {
       new_create_list.push_front(def);
+      /*
+        Re-ordering columns in table can't be done using in-place algorithm
+        as it always changes table data.
+      */
+      alter_info->change_level= ALTER_TABLE_DATA_CHANGED;
+    }
     else
     {
       Create_field *find;
@@ -6263,6 +6276,10 @@ mysql_prepare_alter_table(THD *thd, TABLE *table,
         goto err;
       }
       find_it.after(def);			// Put element after this
+      /*
+        Re-ordering columns in table can't be done using in-place algorithm
+        as it always changes table data.
+      */
       alter_info->change_level= ALTER_TABLE_DATA_CHANGED;
     }
   }