MDEV-28967 Assertion `marked_for_write_or_computed()' failed in...

MDEV-28967 Assertion `marked_for_write_or_computed()' failed in Field_new_decimal::store_value / online_alter_read_from_binlog`

in the catch-up phase of the online alter we apply row events,
they're unpacked into `from->record[0]` and then converted
to `to->record[0]`.

This needs all fields of `from` to be in the `write_set`.

Although practically `Field::unpack()` does not assert the `write_set`,
and `Field::reset()` - used when a field value is not present in the
after-image - also doesn't assert the `write_set` for many types,
`Field_new_decimal::reset()` does.

mysql-test/main/alter_table_online_debug.result

@@ -608,3 +608,22 @@ NULL
 connection default;
 drop table t1;
 set debug_sync= reset;
+#
+# MDEV-28967 Assertion `marked_for_write_or_computed()' failed in Field_new_decimal::store_value / online_alter_read_from_binlog`
+#
+create  table t1 (a decimal(8,2), b varchar(8));
+insert into t1 (b) values ('x');
+set debug_sync= 'now wait_for downgraded';
+connection con2;
+set debug_sync= 'alter_table_online_downgraded signal downgraded wait_for goforit';
+alter table t1 force, algorithm=copy, lock=none;
+connection default;
+insert t1 (b) values ('k');
+insert t1 (b) values ('m');
+set debug_sync= 'now signal goforit';
+connection con2;
+drop table t1;
+set debug_sync= reset;
+#
+# End of 11.2 tests
+#

mysql-test/main/alter_table_online_debug.test

@@ -742,3 +742,31 @@ select * from t1;
 --connection default
 drop table t1;
 set debug_sync= reset;
+
+--echo #
+--echo # MDEV-28967 Assertion `marked_for_write_or_computed()' failed in Field_new_decimal::store_value / online_alter_read_from_binlog`
+--echo #
+create  table t1 (a decimal(8,2), b varchar(8));
+insert into t1 (b) values ('x');
+
+--send set debug_sync= 'now wait_for downgraded'
+
+--connection con2
+set debug_sync= 'alter_table_online_downgraded signal downgraded wait_for goforit';
+--send alter table t1 force, algorithm=copy, lock=none
+
+--connection default
+--reap
+insert t1 (b) values ('k');
+insert t1 (b) values ('m');
+set debug_sync= 'now signal goforit';
+
+--connection con2
+--reap
+
+drop table t1;
+set debug_sync= reset;
+
+--echo #
+--echo # End of 11.2 tests
+--echo #

sql/sql_table.cc

@@ -11993,6 +11993,8 @@ copy_data_between_tables(THD *thd, TABLE *from, TABLE *to,
     rli.relay_log.description_event_for_exec=
                                             new Format_description_log_event(4);
 
+    // We'll be filling from->record[0] from row events
+    bitmap_set_all(from->write_set);
     // We restore bitmaps, because update event is going to mess up with them.
     to->default_column_bitmaps();