Re: On login trigger: take three

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.


> >> 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