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Re: BUG #5066: plperl issues with perl_destruct() and ENDblocks

From: David Fetter <david(at)fetter(dot)org>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Alvaro Herrera <alvherre(at)commandprompt(dot)com>,Tim Bunce <Tim(dot)Bunce(at)pobox(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>,pgsql-bugs(at)postgresql(dot)org
Subject: Re: BUG #5066: plperl issues with perl_destruct() and ENDblocks
Date: 2009-09-21 18:45:19
Message-ID: 20090921184519.GM31599@fetter.org (view raw or flat)
Thread:
Lists: pgsql-bugs
On Mon, Sep 21, 2009 at 02:28:11PM -0400, Robert Haas wrote:
> On Mon, Sep 21, 2009 at 2:17 PM, David Fetter <david(at)fetter(dot)org> wrote:
> > On Mon, Sep 21, 2009 at 01:06:17PM -0400, Alvaro Herrera wrote:
> >> David Fetter escribió:
> >> > On Mon, Sep 21, 2009 at 12:06:30PM -0400, Alvaro Herrera wrote:
> >> > > David Fetter escribió:
> >> > >
> >> > > > Taken literally, that would mean, "the last action before the
> >> > > > backend exits," but at least to me, that sounds troubling for
> >> > > > the same reasons that "end of transaction" triggers do.  What
> >> > > > happens when there are two different END blocks in a session?
> >> > >
> >> > > The manual is clear that both are executed.
> >> >
> >> > So it is, but does order matter, and if so, how would PostgreSQL
> >> > know?
> >>
> >> The fine manual saith
> >>
> >>       You may have multiple "END" blocks within a file--they will
> >>       execute in reverse order of definition; that is: last in, first
> >>       out (LIFO).
> >>
> >> But then, why would we care?  We just call the destructor and Perl
> >> ensures that the blocks are called in the right order.
> >
> > This is not quite what I meant.  Let's say we have two or more different
> > PL/Perl functions executed over the course of a backend.  Which one's
> > END block gets executed last?  Do we need to warn people about this?
> > Generate a WARNING, even?
> 
> This is a feature of the Perl language.  I don't think it's our job to
> second-guess the language design, however good or bad it may be.  As a
> long-time Perl programmer, I would certainly say that if you are
> counting on the execution ordering of your END blocks, you are
> probably playing with fire and likely ought to rethink your
> application design, because there are all kinds of ways this could
> fail spectacularly as a result of apparently innocuous application
> changes (like, say, alphabetizing the list of "use" declarations in
> some package).  But that's true not only with PL/perl but with just
> plain old perl, and I don't see that it's substantially more dangerous
> here than anywhere else.

OK, we've considered it and decided it's people's own foot-gun :)

Cheers,
David.
-- 
David Fetter <david(at)fetter(dot)org> http://fetter.org/
Phone: +1 415 235 3778  AIM: dfetter666  Yahoo!: dfetter
Skype: davidfetter      XMPP: david(dot)fetter(at)gmail(dot)com

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