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(at)anarazel(dot)de> wrote:
> > On 2022-03-28 15:57:37 +0300, a(dot)sokolov(at)postgrespro(dot)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