tima has asked for the wisdom of the Perl Monks concerning the following question:

I'm trying to use Filter::Handle in a script I'm writing (it does exact what I want I need) and see that its tests have been segfaulting on Perl 5.8+. Perl 5.6 seems uneffected. The problem lies in this test for the Filter and UnFilter methods:

my $out;
Filter \*STDOUT, sub {
    $out = sprintf "%d: %s\n", 1, "@_";
    ()
};
print "Foo";
UnFilter \*STDOUT;
print $out eq "1: Foo\n" ? "ok 3\n" : "not ok 3\n";

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Here is the code to those two methods:

sub Filter {
    my $fh = $_[0];
    tie *{ $fh }, __PACKAGE__, @_;
}

sub UnFilter {
    my $fh = shift;
    { local $^W = 0; untie *{ $fh } }
}

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Perl's internals are not my forte so I don't see or understand what is going wrong. I was hoping someone here could clue me in. The maintainer of the module has his hands full with a few other things right now.

Thanks.

Replies are listed 'Best First'.
Re: Filter::Handle segfault in Perl 5.8+
by tima (Acolyte) on Oct 26, 2007 at 22:42 UTC

    I uncovered some more information on this problem. While the test code I originally posted didn't report anythingon the segfault, this code...

    #!/usr/bin/perl -w
use strict;
use Filter::Handle qw/subs/;

Filter \*STDOUT,
    sub { local $_ = "@_"; s/blue/red/g; $_ };

print "My house is blue.\n";
print "So is my cat, whose nose is blue.\n";

UnFilter \*STDOUT;

print "And the plane is also blue.\n";

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    ... gave me a "Deep recursion on subroutine "Filter::Handle::PRINT" at /Library/Perl/5.8.6/Filter/Handle.pm line 42." That gave me some clues, but what I thought was the solution isn't working.

    Here is the subroutine with line 42 in it.

    sub PRINT {
    my $self = shift;
    my $fh = *{ $self->{fh} };
    print $fh, $self->{output}->(@_);
}

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    The last line in #42. Immediately I thought perl is getting confused to which print to call and going into an infinite loop until a segfault happens. Thing is prepending CORE:: still ends in the same results.

    sub PRINT {
    my $self = shift;
    my $fh = *{ $self->{fh} };
    CORE::print $fh $self->{output}->(@_);
}

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    I also added a "use subs" for print and printf to no avail.

    use subs qw(print printf);

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    My experiments with Tie::Handle have return similar results so far.

    What is happening here to confuse Perl that it goes into an infinite loop? Why is the use of CORE:: getting ignored or changed by this code?

    Any ideas would be appreciated.

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[select]
      OK I finally got my head out of my backside and figured it out -- at least for Perl 5.8. Here is the patch I'll be submitting to Ben: 
      41,42c41,42
<     my $fh = *{ $self->{fh} };
<     print $fh $self->{output}->(@_);
---
>     open FH, ">&", \*{ $self->{fh} }; # dup to avoid potential STDOU
+T recursion
>     print FH $self->{output}->(@_);

      [download]
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