mod_perl/FreeBSD Bug

[ti...@sophos.com]

Hello,

We are using the latest version of Apache (1.3.33) and mod_perl (1.29)
running on FreeBSD 4.9. When mod_perl is compiled as a DSO the server 
grows
by approximately 12MB every time it does a graceful restart. When it gets
greater than 512MB FreeBSD stops giving it more memory (this can be 
changed
in the kernel) the server will die. I have heard of other people with this
problem and it seems to be widely known. In our environment this only
happens on the FreeBSD servers, Linux is unaffected

The advice appears to be either do a full restart or compile in mod_perl
with Apache. A full restart would inconvenience many users and is not
practical. Does anyone know of a fix?

We compiled in mod_perl with Apache which fixed the memory leak but
introduced a very strange bug. A hash was being corrupted, only some data
was being set when a package is preloaded. This does not happen when
compiled as a DSO. Again, Linux is unaffected by this bug even when
mod_perl is compiled in with Apache.

I produced a short test case to demonstrate this bug. It appears that on
FreeBSD when packages are preloaded they are compiled twice. This does not
happen on Linux. We found this out by tie()ing a hash and writing every
access to a file. The test case sets two keys on a hash. When this is
preloaded on Linux the following happens to the hash on Apache start up:

  Hash is created
  Key 'a' is set
  Key 'b' is set
 
and the hash is destroyed when Apache exits. I would expect this 
behaviour.
On FreeBSD when compiled as a DSO the following happens:

  Hash 1 is created
  Key 'a' is set on hash 1
  Key 'b' is set hash 1
  Hash 1 is destroyed
  Hash 2 is created
  Key 'a' is set on hash 2
  Key 'b' is set hash 2

Hash 1 and 2 are the same declared hash but different instances. As you 
can
see it appears the code has been compiled twice but it runs as normal. 
When
the same code is preloaded on a build where mod_perl is compiled into
Apache the following happens which demonstrates the bug:

  Hash 1 is created
  Key 'a' is set on hash 1
  Key 'b' is set hash 1
  Key 'a' is set on hash 1
  Hash 2 is created
  Hash 1 is destroyed
  Key 'b' is set hash 2

Therefore the hash is only populated with 'b' as 'a' has been set twice on
the first instance of the hash. This does not happen if the
PerlFreshRestart is not defined in the Apache configuration, but this 
means
we cannot update code with a graceful restart. Does anyone know of a fix?

The code for the test case is defined below. The bug only appears when the
Test::A and Test::B packages are defined in separate files. If they are
defined in-line, even in a BEGIN block the bug does not appear. The bug
also disappears when the hash is defined as a package variable 'our' 
rather
than a lexically scoped variable 'my'. Note the 'use Test::A' and 'use
Test::B' must come after the 'set' subroutine as they call it.

We are therefore in a situation where on FreeBSD if we use mod_perl as a
DSO the server crashes and if we compile it in with Apache data gets
corrupted. We have tested this on vanilla and FreeBSD ports builds. None 
of
these bugs are present on Linux.

So can anyone give us advice on fixing the memory leak and/or fixing the
data corruption?

Any suggestions are gratefully received.


Kind regards,
Tim





Test code (Test.pm, Test/A.pm, Test/B.pm)

Test.pm
------------------
package Test;

use strict;
use warnings;

use Data::Dumper;

my %hash;

sub set {
    $hash{$_[1]} = '';
};

sub handler {
    print Dumper \%hash; 
} 

use Test::A;
use Test::B;

1;
------------------


Test/A.pm
------------------
package Test::A;

use strict;
use warnings;

Test->set('a');

1;
------------------


Test/B.pm
------------------
package Test::B;

use strict;
use warnings;

Test->set('b');

1;
------------------


-- 
Timothy Coggins
Perl Programmer, Sophos

Tel: 01235 559933
Web: www.sophos.com
Sophos - protecting businesses against viruses and spam

Re: mod_perl/FreeBSD Bug

[David Dorward <do...@gmail.com>]

tim.coggins@sophos.com wrote:

> We are using the latest version of Apache (1.3.33) and mod_perl (1.29)
> running on FreeBSD 4.9. 

Scott McWhirter asked me to mention that this issue now has a bug report 
filed for it.

<http://rt.cpan.org/NoAuth/Bug.html?id=11254>

-- 
David Dorward