Re: FATAL: simple query "BEGIN" arrived before ending an extended query message

Hi,

There's been some recent development on the side of pgjdbc as well:
https://github.com/pgjdbc/pgjdbc/pull/3728

We are currently testing that fix. With that patch, pgjdbc will always send
a sync first when in extended mode before sending a simple query. Do you
think it's still a good idea to support mixing simple and extended mode in
pgpool? It's difficult for me to assess the risk of your proposed patch.
Maybe it's better to just not allow it if that makes it easier to implement?

Ps. Sorry for the bad quoting. I'm writing this on my phone, as I'm still
on vacation.

Op do 24 jul 2025, 03:36 schreef Tatsuo Ishii <ishii(at)postgresql(dot)org>:

> >> Hi Emond,
> >>
> >>>> > The problem however, is that previously the error was only
> triggered when
> >>>> > using autosave, which is not a common setup, but now the error is
> >>>> triggered
> >>>> > even when using default configuration. This makes it impossible to
> >>>> combine
> >>>> > PgJDBC 42.7.6 and up with pgpool-II. I do not know what the plans
> are on
> >>>> > this from the side of PostgreSQL and PgJDBC, but I just wanted to
> raise
> >>>> > some awareness on this issue here. IMHO PostgreSQL should either
> >>>> explicitly
> >>>> > allow this (in which case pgpool-II needs to be fixed) or explicitly
> >>>> > disallow it (in which case PgJDBC needs to be fixed).
> >>>>
> >>>> As far as I know about PostgreSQL's side, Tom Lane said about this:
> >>>>
> https://www.postgresql.org/message-id/2069511.1706571615@sss.pgh.pa.us
> >>>>
> >>>> > I think it's poor practice, at best. You should end the
> >>>> > extended-protocol query cycle before invoking simple query.
> >>>> >
> >>>> > I'm disinclined to document, or make any promises about,
> >>>> > what happens if you mix the protocols.
> >>>>
> >>>> In my understanding he does not say PostgreSQL explicitely allows this
> >>>> (mixing extended and simple protocol message).
> >>>>
> >>>
> >>> It's more or less allowed without any errors, but the expected
> behavior is
> >>> not clear nor is it documented. It seems libpq even throws an error on
> the
> >>> client side when you try to do this:
> >>>
> >>> Jelte Fennema-Nio in
> >>>
> https://www.postgresql.org/message-id/CAGECzQQ1hs2DU9pmQq18Y%3DqK4nZqhXDVg-sGEa5K01Lj4XMmxw%40mail.gmail.com
> >>>> I totally agree that it makes sense to throw an error in this case.
> >>>> Libpq actually throws an error client side when a caller attempts to
> >>>> do this, but this is something that should be checked server side,
> >>>> given that the protocol docs specify this:
> >>>> docs> At completion of each series of extended-query messages, the
> >>>> frontend should issue a Sync message.
> >>>
> >>>
> >>>> > The current situation
> >>>> > is no good as we now simply cannot upgrade PgJDBC anymore (and the
> same
> >>>> > will be true for all other users of pgpool-II).
> >>>>
> >>>> Yeah.
> >>>>
> >>>> What I don't understand is, why PgJDBC decided to make it default
> >>>> (sending simple protocol query after extended query protocl without
> >>>> sync) even without autosave being set when they update PgJDBC to
> >>>> 42.7.7.
> >>>>
> >>>
> >>> It wasn't a real decision to make this happen more often. They fixed a
> bug
> >>> that caused many queries to be sent using the extended protocol even
> when
> >>> explicitly asked for a simple query:
> >>> https://github.com/pgjdbc/pgjdbc/issues/3724#issuecomment-3051773696
> >>>
> >>> In our case we got sporadic errors. These errors are very hard to
> >>> reproduce. Some users were not able to login into our application at
> all,
> >>> others had no trouble whatsoever and some users got an error every
> once in
> >>> a while. Even when tests seem fine, you might get these errors later
> on.
> >>> None of our automated tests failed. On our testing environment we've
> only
> >>> seen the error twice in a period of 3 weeks. When we moved to
> production,
> >>> we triggered the error over 100 times in just 1 day! I would certainly
> not
> >>> recommend using PgJDBC 42.7.6 and up when using pgpool.
> >>
> >> I created a patch for pgpool to allow a simple query without ending a
> >> sequence of extended query protocol message by a sync message.
> >>
> >> The idea is, within SimpleQuery() (a function responsible for
> >> processing simple protocol query), check whether extended query
> >> protocol ends. If not, call ProcessBackendReposnse() to process any
> >> replies from backend such as parse complete, bind complete or command
> >> complete and so. After this, SimpleQuery() can process simple query
> >> as usual. I hope this solves the issue with PgJDBC 42.7.6.
> >
> > After thinking more, I concluded that it would be better to do the
> > task in ProcessFrontendReposnse(), rather than in SimpleQuery()
> > because we usually do not call ProcessBackendResponse() inside
> > functions such as SimpleQuery() that are responsible for processing
> > particular frontend message.
> >
> > Attached is the v2 patch.
>
> This is the v3 patch. Fix regression test 082.guard_against_bad_protocol.
> Slightly modify commit message.
>
> Best regards,
> --
> Tatsuo Ishii
> SRA OSS K.K.
> English: http://www.sraoss.co.jp/index_en/
> Japanese:http://www.sraoss.co.jp
>