MDEV-28545 MyISAM reorganize partition corrupt older table format

The ALTER related code cannot do at the same time both:
- modify partitions
- change column data types

Explicit changing of a column data type together with a partition change is
prohibited by the parter, so this is not allowed and returns a syntax error:

  ALTER TABLE t MODIFY ts BIGINT, DROP PARTITION p1;

This fix additionally disables implicit data type upgrade
(e.g. from "MariaDB 5.3 TIME" to "MySQL 5.6 TIME", or the other way
around according to the current mysql56_temporal_format) in case of
an ALTER modifying partitions, e.g.:

  ALTER TABLE t DROP PARTITION p1;

In such commands now only the partition change happens, while
the data types stay unchanged.

One can additionally run:

  ALTER TABLE t FORCE;

either before or after the ALTER modifying partitions to
upgrade data types according to mysql56_temporal_format.

mysql-test/main/partition_alter.result

@@ -212,3 +212,26 @@ test.t	check	status	OK
 delete from t order by b limit 1;
 drop table t;
 # End of 10.3 tests
+#
+# Start of 10.4 tests
+#
+#
+# MDEV-28545 MyISAM reorganize partition corrupt older table format
+#
+SET GLOBAL mysql56_temporal_format=OFF;
+CREATE TABLE t (ts timestamp, KEY (ts)) ENGINE=MyISAM
+PARTITION BY RANGE (unix_timestamp(ts)) (
+PARTITION p1 VALUES LESS THAN (1645398000),
+PARTITION pn VALUES LESS THAN MAXVALUE
+);
+SET GLOBAL mysql56_temporal_format=ON;
+FLUSH TABLES;
+ALTER TABLE t DROP PARTITION p1;
+CHECK TABLE t;
+Table	Op	Msg_type	Msg_text
+test.t	check	status	OK
+DROP TABLE t;
+SET GLOBAL mysql56_temporal_format=DEFAULT;
+#
+# End of 10.4 tests
+#

mysql-test/main/partition_alter.test

@@ -197,3 +197,29 @@ delete from t order by b limit 1;
 drop table t;
 
 --echo # End of 10.3 tests
+
+--echo #
+--echo # Start of 10.4 tests
+--echo #
+
+--echo #
+--echo # MDEV-28545 MyISAM reorganize partition corrupt older table format
+--echo #
+
+SET GLOBAL mysql56_temporal_format=OFF;
+CREATE TABLE t (ts timestamp, KEY (ts)) ENGINE=MyISAM
+PARTITION BY RANGE (unix_timestamp(ts)) (
+  PARTITION p1 VALUES LESS THAN (1645398000),
+  PARTITION pn VALUES LESS THAN MAXVALUE
+);
+
+SET GLOBAL mysql56_temporal_format=ON;
+FLUSH TABLES;
+ALTER TABLE t DROP PARTITION p1;
+CHECK TABLE t;
+DROP TABLE t;
+SET GLOBAL mysql56_temporal_format=DEFAULT;
+
+--echo #
+--echo # End of 10.4 tests
+--echo #

sql/field.cc

@@ -11020,8 +11020,6 @@ Column_definition::Column_definition(THD *thd, Field *old_field,
 
   type_handler()->Column_definition_reuse_fix_attributes(thd, this, old_field);
 
-  type_handler()->Column_definition_implicit_upgrade(this);
-
   /*
     Copy the default (constant/function) from the column object orig_field, if
     supplied. We do this if all these conditions are met:

sql/field.h

@@ -5203,6 +5203,13 @@ public:
 
   bool vers_check_timestamp(const Lex_table_name &table_name) const;
   bool vers_check_bigint(const Lex_table_name &table_name) const;
+
+  static void upgrade_data_types(List<Create_field> &list)
+  {
+    List_iterator<Create_field> it(list);
+    while (Create_field *f= it++)
+      f->type_handler()->Column_definition_implicit_upgrade(f);
+  }
 };
 
 

sql/sql_insert.cc

@@ -4346,6 +4346,16 @@ TABLE *select_create::create_table_from_items(THD *thd, List<Item> *items,
     alter_info->create_list.push_back(cr_field, thd->mem_root);
   }
 
+  /*
+    Item*::type_handler() always returns pointers to
+    type_handler_{time2|datetime2|timestamp2} no matter what
+    the current mysql56_temporal_format says.
+    Let's convert them according to mysql56_temporal_format.
+    QQ: This perhaps should eventually be fixed to have Item*::type_handler()
+    respect mysql56_temporal_format, and remove the upgrade from here.
+  */
+  Create_field::upgrade_data_types(alter_info->create_list);
+
   if (create_info->check_fields(thd, alter_info,
                                 create_table->table_name,
                                 create_table->db,

sql/sql_table.cc

@@ -10068,6 +10068,18 @@ do_continue:;
 
   set_table_default_charset(thd, create_info, alter_ctx.db);
 
+  /*
+    The ALTER related code cannot alter partitions and change column data types
+    at the same time. So in case of partition change statements like:
+      ALTER TABLE t1 DROP PARTITION p1;
+    we skip implicit data type upgrade (such as "MariaDB 5.3 TIME" to
+    "MySQL 5.6 TIME" or vice versa according to mysql56_temporal_format).
+    Note, one can run a separate "ALTER TABLE t1 FORCE;" statement
+    before or after the partition change ALTER statement to upgrade data types.
+  */
+  if (IF_PARTITIONING(!fast_alter_partition, 1))
+    Create_field::upgrade_data_types(alter_info->create_list);
+
   if (create_info->check_fields(thd, alter_info,
                                 table_list->table_name, table_list->db) ||
       create_info->fix_period_fields(thd, alter_info))