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[ID 20000209.003] Fwd: Re: FIX: Apache::Session & perldie/DESTROY bug

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Michael Blakeley
February 9, 2000 21:50
[ID 20000209.003] Fwd: Re: FIX: Apache::Session & perldie/DESTROY bug
Message ID:
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

>FIX: Apache::Session & perl die/DESTROY bug
>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
>appears to be inside Storable's nstore_fd. Storable is used by
>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:
>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,

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 <>
>To: Michael Blakeley <>
>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.
>  > $
>  > 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

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