MDEV-21695 Server crashes in TABLE::evaluate_update_default_function upon UPDATE on temporary table

copy_data_between_tables() sets to->s->default_fields to 0, as a part
of the code disabling ON UPDATE actions for all old fields
(so ON UPDATE is enable only for new fields during copying).
After the actual copying, copy_data_between_tables() did not restore
to->s->default_fields to the original value. As a result, the
TABLE_SHARE to->s was left in a wrong state after copy_data_between_tables()
and further open_table_from_share() using this TABLE_SHARE did not
populate TABLE::default_field, which further made
TABLE::evaluate_update_default_function() crash on access to NULL
pointer.

Fix:
Changing copy_data_between_tables() to restore to->s->default_fields
to its original value after the copying loop.

mysql-test/r/temp_table.result

@@ -584,3 +584,21 @@ ALTER TABLE t1 CHANGE no_such_col1 col1 BIGINT NULL;
 ERROR 42S22: Unknown column 'no_such_col1' in 't1'
 TRUNCATE TABLE t1;
 DROP TEMPORARY TABLE t1;
+#
+# MDEV-21695 Server crashes in TABLE::evaluate_update_default_function upon UPDATE on temporary table
+#
+SET timestamp=UNIX_TIMESTAMP('2001-01-01 10:20:30');
+CREATE TEMPORARY TABLE t1 (a DATETIME ON UPDATE CURRENT_TIMESTAMP);
+ALTER TABLE t1 ADD b INT;
+INSERT INTO t1 (b) VALUES (1),(2);
+ALTER TABLE t1 CHANGE COLUMN x xx INT;
+ERROR 42S22: Unknown column 'x' in 't1'
+UPDATE t1 SET b = 3;
+SELECT * FROM t1;
+a	b
+2001-01-01 10:20:30	3
+2001-01-01 10:20:30	3
+DROP TEMPORARY TABLE t1;
+#
+# End of 10.2 tests
+#

mysql-test/t/temp_table.test

@@ -639,3 +639,22 @@ ALTER TABLE t1 CHANGE no_such_col1 col1 BIGINT NULL;
 # was not dropped during the first TRUNCATE due to extra table handles.
 TRUNCATE TABLE t1;
 DROP TEMPORARY TABLE t1;
+
+--echo #
+--echo # MDEV-21695 Server crashes in TABLE::evaluate_update_default_function upon UPDATE on temporary table
+--echo #
+
+SET timestamp=UNIX_TIMESTAMP('2001-01-01 10:20:30');
+CREATE TEMPORARY TABLE t1 (a DATETIME ON UPDATE CURRENT_TIMESTAMP);
+ALTER TABLE t1 ADD b INT;
+INSERT INTO t1 (b) VALUES (1),(2);
+--error ER_BAD_FIELD_ERROR
+ALTER TABLE t1 CHANGE COLUMN x xx INT;
+UPDATE t1 SET b = 3;
+SELECT * FROM t1;
+DROP TEMPORARY TABLE t1;
+
+
+--echo #
+--echo # End of 10.2 tests
+--echo #

sql/sql_table.cc

@@ -10057,6 +10057,7 @@ copy_data_between_tables(THD *thd, TABLE *from, TABLE *to,
   sql_mode_t save_sql_mode= thd->variables.sql_mode;
   ulonglong prev_insert_id, time_to_report_progress;
   Field **dfield_ptr= to->default_field;
+  uint save_to_s_default_fields= to->s->default_fields;
   DBUG_ENTER("copy_data_between_tables");
 
   /* Two or 3 stages; Sorting, copying data and update indexes */
@@ -10338,6 +10339,7 @@ copy_data_between_tables(THD *thd, TABLE *from, TABLE *to,
   *copied= found_count;
   *deleted=delete_count;
   to->file->ha_release_auto_increment();
+  to->s->default_fields= save_to_s_default_fields;
 
   if (!cleanup_done)
   {