Very weird bug - insight would be helpful

zork42 has asked for the wisdom of the Perl Monks concerning the following question:

Hi.

Can anyone provide some insight into this weirdness please?

This was a complete pig (actually, more like a female dog) to find the cause of.

I've created a simple example script that shows the ~~bastard~~ issue.

example #1 - everything is fine here
example #2 - edit: same as #1 except filename has './' prepended to it - result: looks like it is working, but 'die' (100s of lines away in a completely different part of the program) produces a weird error message
example #3 - edit: same as #2 except the 'die' has changed - result: same as #2. Presumably 'die ""' gets optimised to 'die'?
example #4 - edit: same as #2 except the 'die' has changed - result: new weirdness, for some reason, 'die 1' no longer produces the weird error message
example #5 - edit: same as #2 except the 'die' has changed - result: as #4
example #6 - edit: same as #2 except the 'die' has changed - result: as #4

Questions:

In #2, why does prepending './' to the filename make 'die' produce the weird error message?
In #3-#6 why does a 'die' with parameters suppress the weird error message?
In #1-#6 what does '<GEN0>' in the 'die' messages mean?

Thanks!

----------------------------------------------------------------------
+----------------------------------------
                                example #1
>>>>> type weird.pl
use strict;
use warnings;
use RTF::Tokenizer;

my $tokenizer = RTF::Tokenizer->new( file => 'test.rtf' );

foreach ( 1 .. 5 )
{
    my ( $token_type, $argument, $parameter ) = $tokenizer->get_token(
+);
    print "     $token_type, $argument, $parameter\n";
}

die;

>>>>> weird.pl
     group, 1,
     control, rtf, 1
     control, ansi,
     control, ansicpg, 1252
     control, deff, 0
Died at D:\test\weird.pl line 13, <GEN0> line 1.

----------------------------------------------------------------------
+----------------------------------------
                                example #2
>>>>> type weird.pl
use strict;
use warnings;
use RTF::Tokenizer;

my $tokenizer = RTF::Tokenizer->new( file => './test.rtf' );

foreach ( 1 .. 5 )
{
    my ( $token_type, $argument, $parameter ) = $tokenizer->get_token(
+);
    print "     $token_type, $argument, $parameter\n";
}

die;

>>>>> weird.pl
     group, 1,
     control, rtf, 1
     control, ansi,
     control, ansicpg, 1252
     control, deff, 0
Can't call method "isa" without a package or object reference at C:/Pe
+rl/site/lib/RTF/Tokenizer.pm line 170.
        ...propagated at D:\test\weird.pl line 13, <GEN0> line 1.

----------------------------------------------------------------------
+----------------------------------------
                                example #3
>>>>> type weird.pl
use strict;
use warnings;
use RTF::Tokenizer;

my $tokenizer = RTF::Tokenizer->new( file => './test.rtf' );

foreach ( 1 .. 5 )
{
    my ( $token_type, $argument, $parameter ) = $tokenizer->get_token(
+);
    print "     $token_type, $argument, $parameter\n";
}

die "";

>>>>> weird.pl
     group, 1,
     control, rtf, 1
     control, ansi,
     control, ansicpg, 1252
     control, deff, 0
Can't call method "isa" without a package or object reference at C:/Pe
+rl/site/lib/RTF/Tokenizer.pm line 170.
        ...propagated at D:\test\weird.pl line 13, <GEN0> line 1.

----------------------------------------------------------------------
+----------------------------------------
                                example #4
>>>>> type weird.pl
use strict;
use warnings;
use RTF::Tokenizer;

my $tokenizer = RTF::Tokenizer->new( file => './test.rtf' );

foreach ( 1 .. 5 )
{
    my ( $token_type, $argument, $parameter ) = $tokenizer->get_token(
+);
    print "     $token_type, $argument, $parameter\n";
}

die 1;

>>>>> weird.pl
     group, 1,
     control, rtf, 1
     control, ansi,
     control, ansicpg, 1252
     control, deff, 0
1 at D:\test\weird.pl line 13, <GEN0> line 1.

----------------------------------------------------------------------
+----------------------------------------
                                example #5
>>>>> type weird.pl
use strict;
use warnings;
use RTF::Tokenizer;

my $tokenizer = RTF::Tokenizer->new( file => './test.rtf' );

foreach ( 1 .. 5 )
{
    my ( $token_type, $argument, $parameter ) = $tokenizer->get_token(
+);
    print "     $token_type, $argument, $parameter\n";
}

die "1";

>>>>> weird.pl
     group, 1,
     control, rtf, 1
     control, ansi,
     control, ansicpg, 1252
     control, deff, 0
1 at D:\test\weird.pl line 13, <GEN0> line 1.

----------------------------------------------------------------------
+----------------------------------------
                                example #6
>>>>> type weird.pl
use strict;
use warnings;
use RTF::Tokenizer;

my $tokenizer = RTF::Tokenizer->new( file => './test.rtf' );

foreach ( 1 .. 5 )
{
    my ( $token_type, $argument, $parameter ) = $tokenizer->get_token(
+);
    print "     $token_type, $argument, $parameter\n";
}

die "one";

>>>>> weird.pl
     group, 1,
     control, rtf, 1
     control, ansi,
     control, ansicpg, 1252
     control, deff, 0
one at D:\test\weird.pl line 13, <GEN0> line 1.

----------------------------------------------------------------------
+----------------------------------------
[download]

Comment on Very weird bug - insight would be helpful Download Code

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Re: Very weird bug - insight would be helpful by choroba (Cardinal) on Nov 19, 2013 at 08:37 UTC
The behaviour is described in the first several paragraphs of perldoc -f die. Search for "if the output is empty". GEN0 refers to an input file handle. لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ	[reply]
Re: Very weird bug - insight would be helpful by zork42 (Monk) on Nov 19, 2013 at 16:10 UTC
That makes sense, thanks!! So the eval at C:/Perl/site/lib/RTF/Tokenizer.pm line 170 just left this error lying around in $@: Can't call method "isa" without a package or object reference at C:/Perl/site/lib/RTF/Tokenizer.pm line 170. ? So good practice would be to always go `$@ = ''` after a failed eval? or use `local $@`? Should I report this as a bug to the Tokenizer.pm maintainer?	[reply] [d/l] [select]
Re^2: Very weird bug - insight would be helpful by choroba (Cardinal) on Nov 19, 2013 at 16:38 UTC
Good practice is to use Try::Tiny, or at least `eval { # Code that might die. 1; } or { # Handle the exception. }` [download] BTW, check whom you are replying to. لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ	[reply] [d/l]
Re^3: Very weird bug - insight would be helpful by zork42 (Monk) on Nov 20, 2013 at 17:21 UTC
Thanks! Try::Tiny sounds good, I'll use that in my code. Please can you answer these questions if possible (to help my Perl indoctrination ;) ) 1. Would going `$@ = ''` after a failed eval work? 2. Would `use local $@` also work? 3. Should I report this as a bug to the Tokenizer.pm maintainer? BTW, check whom you are replying to. I normally (almost always) use the "Comment on" link just under the OP. I'm not used to forums where you can reply to individual posts, most of the forums I use just allow you to add a new post to the end of the topic. Also I find I sometimes miss new comments that are added as replies to earlier comments. Is using the "Comment on" link OK? Or is using the reply-to-comment link preferred?	[reply] [d/l] [select]
Re^4: Very weird bug - insight would be helpful by choroba (Cardinal) on Nov 21, 2013 at 07:24 UTC