Re: exactly what is COPY BOTH mode supposed to do in case of an error?

On 27 April 2013 20:12, Robert Haas <robertmhaas@gmail.com> wrote:

> My feeling is that it would be better not to back-patch this, but just
> fix it in master.  Given the present uses of COPY-BOTH mode, the
> problems seem to be limited to bad error messages, so it's arguably
> not a critical bug fix.  Also, I think that no matter which way we fix
> it, people who upgrade the master to a new point release, but not
> pg_receivexlog, would in some unlikely cases actually experience a
> regression in the quality of error messages.  I would say we have to
> live with that if the consequences were any worse than bad error
> messages in the first place, but as far as I can tell they're not.  If
> someone can contrive a scenario where this causes outright breakage,
> that would tip the balance for me, but I don't at present see such a
> hazard.
>
> On a practical level, the main thing I didn't like about trying to fix
> the server was the same issue that Tom mentioned: we'd need code in
> the server to track whether COPY-BOTH mode is active and skip client
> messages until we hit a CopyDone or CopyFail message.  And I suspect
> that code would be somewhat fragile, because having sent an
> ErrorResponse already, we'd have no straightforward way to report a
> further error - we'd need to report follow-on errors via NOTICE or
> FATAL.  Now this is not a disaster, but it's not great, either,
> because there's a lot of code (including, notably, palloc) which
> assumes that it can throw an ERROR whenever it likes.  And in this
> case, it couldn't.
>
> The second thing I didn't like about that approach was that it would
> make COPY-BOTH quite asymmetrical with both COPY-OUT and COPY-IN.
> That didn't seem like a great idea, either.
>
> A further point is that the problems in the back branches are less
> serious anyway, because the timeline-switching code is the only thing
> that ever tries to exit COPY-BOTH mode without closing the connection,
> and that's new in 9.3.
>
> So for all those reasons, my vote is for a client-side, master-only fix.

+1, very sound

--Simon Riggs                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services