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Postings from February 2000
[ID 20000209.003] Fwd: Re: FIX: Apache::Session & perldie/DESTROY bug
From:
Michael Blakeley
Date:
February 9, 2000 21:50
Subject:
[ID 20000209.003] Fwd: Re: FIX: Apache::Session & perldie/DESTROY bug
Message ID:
v04220807b4c8001ca79a@blakeley.com
I really hope that I'm not wasting your time. This appears to be a
scoping bug in perl up to and including 5.0005_03. I have not checked
developer releases.
For background, from
http://www.bitmechanic.com/mail-archives/modperl/May1999/0943.html
>FIX: Apache::Session & perl die/DESTROY bug
>
>dean@brettle.com
>Sat, 29 May 1999 12:08:59 -0700
>
>For the second time in as many weeks I've been bitten by a bug in perl
>5.004's error handling. The first time the bug was triggered by
>DBIx::Recordset-0.19beta. This time, the same bug is triggered by
>Apache::Session-0.99.8. I've attached a patch (use patch -p1).
>
>Here is a test case to illustrate the problem:
>
>use Apache::Session;
>use Apache::Session::File;
>
>eval {
>my %session;
>tie (%session, 'Apache::Session::File', undef);
>$session{'foo'} = 'bar';
>die "This error should be caught but isn't";
>};
>
>if ($@) {
>print "Success: Error caught.\n";
>} else {
>print "Failure: Error missed due to perl bug.\n";
>}
>
>Here is what is going on:
>
>0. We eval some code, expecting to catch errors.
>
>1. We create an object in some scope within the eval. In our example
>the object is a hash tied to an Apache::Session::File and the scope is
>just the scope of the eval, but it could really be any object and any
>scope contained within the eval.
>
>2. Later in the scope, we die. This sets $@ and goes to the next line
>past our eval, exiting our scope in the process.
>
>3. When Perl exits the scope, it invokes our object's DESTROY() to clean
>it up.
>
>4. DESTROY() happens to contain an eval which doesn't die(). This
>*clears* $@. Oooops! In the case of Apache::Session, this eval is
>actually
>appears to be inside Storable's nstore_fd. Storable is used by
>Apache::Session::File.
>
>5. When we check $@ after our eval, it's empty. :-(
>
>This is a *really* evil bug because it means that anytime you create an
>object using someone else's package, you run the risk of completely
>hosing any exception handling you are doing.
>
>This appears to be an acknowledged bug in perl as of 5.004_05. I don't
>know about 5.005. Can someone who has 5.005 installed tell me if the
>above test script works correctly?
>
>For more info on the bug, see:
>
>http://x43.deja.com/getdoc.xp?AN=389699069&CONTEXT=926789340.1861025869
>
>Unfortunately, due to $@'s unintuitive scoping, a complete solution
>isn't quite as simple as adding 'local $@' at the beginning of each
>DESTROY. Instead, I needed to write a psuedo-control construct
>(Apache::Session::Destroy::carefully) which I wrap around the code
>executed in each DESTROY. For more info on $@'s unintuitive scoping,
>see:
>
>http://x28.deja.com/getdoc.xp?AN=339328324&CONTEXT=926784629.777191445
I've removed the workaround patch that Dean gives for
Apache::Session. Suffice to say that this issue/bug/whatever still
exists in 5.003_05:
>Date: Wed, 09 Feb 2000 20:44:34 -0800
>From: Dean Brettle <dean@brettle.com>
>To: Michael Blakeley <mike@blakeley.com>
>Subject: Re: FIX: Apache::Session & perl die/DESTROY bug
>
>Michael Blakeley wrote:
> >
> > I saw your mod_perl article on the $@ scoping problem in perl 5.003.
> > So just FYI...
> >
> > $ perl -v
> >
> > This is perl, version 5.005_03 built for sun4-solaris
> >
> > $ perl
> > use Apache::Session;
> > use Apache::Session::File;
> >
> > eval {
> > my %session;
> > tie (%session, 'Apache::Session::File', undef);
> > $session{'foo'} = 'bar';
> > die "This error should be caught but isn't";
> > };
> >
> > if ($@) {
> > print "Success: Error caught.\n";
> > } else {
> > print "Failure: Error missed due to perl bug.\n";
> > }
> > Failure: Error missed due to perl bug.
> > $
>
>Sigh...
>
> > Did you ever get any p5p feedback on this?
> >
>
>Nope. I posted it but never got any response.
And so I decided to post to this list, myself. Again, I hope that I'm
not simply wasting your time. Having run into this problem myself
(and been suprised, since the example code for Apache::Session
expects $@ to scope properly), I'd love to hear that this has been
fixed. Yes, I should pull the latest source and do it myself - but
I'm a much better gadfly than I am a C programmer.
-- Mike