MDEV-29750 triggers can modify history

should be the same behavior as for virtual columns:
* a warning on every inserted row
* silently ignored in a trigger

mysql-test/suite/versioning/r/misc.result

+set time_zone='+00:00';
+#
+# MDEV-29750 triggers can modify history
+#
+set sql_mode='', timestamp=unix_timestamp('2010-10-10 10:10:10');
+create table t (a int, b int as (a+1), s timestamp(6) as row start, e timestamp(6) as row end, period for system_time(s,e)) with system versioning;
+insert into t values (1,1, '2022-01-01','2023-01-01'),(2,2, '2022-02-02','2023-02-02');
+Warnings:
+Warning	1906	The value specified for generated column 'b' in table 't' has been ignored
+Warning	1906	The value specified for generated column 's' in table 't' has been ignored
+Warning	1906	The value specified for generated column 'e' in table 't' has been ignored
+Warning	1906	The value specified for generated column 'b' in table 't' has been ignored
+Warning	1906	The value specified for generated column 's' in table 't' has been ignored
+Warning	1906	The value specified for generated column 'e' in table 't' has been ignored
+create trigger tr before insert on t for each row set new.b=1, new.s = '2022-03-03', new.e = '2023-03-03';
+insert into t (a) values (3),(4);
+select * from t for system_time all;
+a	b	s	e
+1	2	2010-10-10 10:10:10.000000	2038-01-19 03:14:07.999999
+2	3	2010-10-10 10:10:10.000000	2038-01-19 03:14:07.999999
+3	4	2010-10-10 10:10:10.000000	2038-01-19 03:14:07.999999
+4	5	2010-10-10 10:10:10.000000	2038-01-19 03:14:07.999999
+drop table t;
+set sql_mode=default, timestamp=default;
+#
+# End of 10.3 tests
+#

mysql-test/suite/versioning/t/misc.test

+#
+# simple tests that don't need to be run in multiple various combinations
+#
+set time_zone='+00:00';
+
+--echo #
+--echo # MDEV-29750 triggers can modify history
+--echo #
+set sql_mode='', timestamp=unix_timestamp('2010-10-10 10:10:10');
+create table t (a int, b int as (a+1), s timestamp(6) as row start, e timestamp(6) as row end, period for system_time(s,e)) with system versioning;
+insert into t values (1,1, '2022-01-01','2023-01-01'),(2,2, '2022-02-02','2023-02-02');
+create trigger tr before insert on t for each row set new.b=1, new.s = '2022-03-03', new.e = '2023-03-03';
+insert into t (a) values (3),(4);
+select * from t for system_time all;
+drop table t;
+set sql_mode=default, timestamp=default;
+
+--echo #
+--echo # End of 10.3 tests
+--echo #

sql/item.cc

@@ -9804,6 +9804,8 @@ bool Item_trigger_field::set_value(THD *thd, sp_rcontext * /*ctx*/, Item **it)
 
   if (!item || fix_fields_if_needed(thd, NULL))
     return true;
+  if (field->vers_sys_field())
+    return false;
 
   // NOTE: field->table->copy_blobs should be false here, but let's
   // remember the value at runtime to avoid subtle bugs.