Re[2]: On login trigger: take three

 
Hi, 
>Tue, March 29, 2022, 0:31 +03:00 from Andres Freund <andres@anarazel.de>:
> 
>Hi,
>
>On 2022-03-28 23:27:56 +0200, Daniel Gustafsson wrote:
>> > On 28 Mar 2022, at 19:10, Andres Freund < andres@anarazel.de > wrote:
>> > On 2022-03-28 15:57:37 +0300,  a.sokolov@postgrespro.ru wrote:
>>
>> >> + data initialization. It is vital that any event trigger using the
>> >> + <literal>login</literal> event checks whether or not the database is in
>> >> + recovery.
>> »
 
>> >> Does any trigger really have to contain a pg_is_in_recovery() call?
>> >
>> > Not *any* trigger, just any trigger that writes.
>>
>> Thats correct, the docs should be updated with something like the below I
>> reckon.
>>
>> It is vital that event trigger using the <literal>login</literal> event
>> which has side-effects checks whether or not the database is in recovery to
>> ensure they are not performing modifications to hot standby nodes.
>
>Maybe side-effects is a bit too general? Emitting a log message, rejecting a
>login, setting some GUCs, etc are all side-effects too.
Something like this:
 
<important>
    <para>
      The <literal>login</literal> triggers fire also on standby servers.
      To keep them from becoming inaccessible, such triggers should
      avoid writing anything to the database when running on a standby.
      This can be achieved by checking <function>pg_is_in_recovery</function>(), see an example below.
    </para>
</important>
> 
Also, please fix a typo in doc/src/sgml/ref/create_event_trigger.sgml :
 
- single-user mode and you'll be able to do that. Even triggers can also be
+ single-user mode and you'll be able to do that. Event triggers can also be
 
Regarding the trigger function example:
It does not do anything if run on a standby. To show that it can do something on a standby to, I propose to move throwing the night exception to the beginning.
So it will be:
 
CREATE OR REPLACE FUNCTION init_session() 
RETURNS event_trigger SECURITY DEFINER LANGUAGE plpgsql AS 
$$ 
DECLARE 
  hour integer = EXTRACT('hour' FROM current_time); 
  rec boolean;
BEGIN

-- 1) Forbid logging in late:
IF hour BETWEEN 2 AND 4 THEN
  RAISE EXCEPTION 'Login forbidden';  -- do not allow to login these hours
END IF;

-- The remaining stuff cannot be done on standbys,
-- so ensure the database is not in recovery
SELECT pg_is_in_recovery() INTO rec;
IF rec THEN
  RETURN;
END IF

-- 2) Assign some roles

IF hour BETWEEN 8 AND 20 THEN         -- at daytime grant the day_worker role
  EXECUTE 'REVOKE night_worker FROM ' || quote_ident(session_user);
  EXECUTE 'GRANT    day_worker TO '   || quote_ident(session_user);
ELSE                                  -- at other time grant the night_worker role
  EXECUTE 'REVOKE day_worker FROM ' || quote_ident(session_user);
  EXECUTE 'GRANT  night_worker TO ' || quote_ident(session_user);
END IF;

-- 3) Initialize some user session data

CREATE TEMP TABLE session_storage (x float, y integer);

-- 4) Log the connection time

INSERT INTO user_login_log VALUES (session_user, current_timestamp);

END;
$$;
Finally, let me propose to append to the regression test the following:
 
 
\c 
SELECT dathasloginevt FROM pg_database WHERE datname= :'DBNAME';
 
which should output:
 dathasloginevt 
----------------
 f
(1 row)
 
So we can check that removal of the event trigger resets this flag in pg_database. Note that reconnect (\c) is necessary here.
 
Regards,
Ivan
 
>
>> >> In this message
>> >> ( https://www.postgresql.org/message-id/20220312024652.lvgehszwke4hhove%40alap3.anarazel.de )
>> >> it was only about triggers on hot standby, which run not read-only queries
>> >
>> > The problem precisely is that the login triggers run on hot standby nodes, and
>> > that if they do writes, you can't login anymore.
>>
>> Do you think this potential foot-gun is scary enough to reject this patch?
>> There are lots of creative ways to cause Nagios alerts from ones database, but
>> this has the potential to do so with a small bug in userland code. Still, I
>> kind of like the feature so I'm indecisive.
>
>It does seem like a huge footgun. But also kinda useful. So I'm really +-0.
>
>Greetings,
>
>Andres Freund