--disable_warnings
drop table if exists t1;
--enable_warnings

set @@autocommit=0;

create table t1 (a int not null, b int not null, index (a,b)) engine=innodb;
insert into t1 values (1,2), (1,3), (2,3), (2,4);

drop trigger if exists tr1;
delimiter |;
create trigger tr1 after update on t1 for each row begin
  set @a = 1;
  rollback to savepoint s1;
  set @b = 1;
end
|
delimiter ;|

start transaction;
savepoint s1;
select @a, @b;
--error 1305
update t1 set a= 21 where b = 2;
select @a, @b;