Log4perl swallowing $@

[saberworks]

I have been beating my head against this for a bit and I feel like I'm fundamentally misunderstanding how eval {} and $@ work. Given the following script with the following following log4perl configuration, I would expect that $@ wouldn't be unset in the middle of the if($@) block. What I'm seeing is that if I call any loglevel method on the $logger object it clears $@. Is this expected behavior?

#!perl

use warnings;
use strict;

use Log::Log4perl;

Log::Log4perl->init_and_watch('yourlogger.conf', 10);

my $logger = Log::Log4perl::get_logger('test_die');

$logger->warn("before eval");

eval {
    die 'foo';
};

if($@) {
    print $@; # I expect this to print "foo", it does
    $logger->debug('test');
    print $@; # I expect this to print "foo", it doesn't!
    print 'after printing $@';
}

print "\n";

yourlogger.conf:

log4perl.rootLogger = DEBUG, stdout, file

log4perl.appender.stdout = Log::Log4perl::Appender::Screen
log4perl.appender.stdout.layout = Log::Log4perl::Layout::SimpleLayout
log4perl.appender.stdout.stderr = 1
log4perl.appender.stdout.Threshold = DEBUG

log4perl.appender.file = Log::Dispatch::FileRotate
log4perl.appender.file.mode = append
log4perl.appender.file.size = 10_000_000
log4perl.appender.file.max = 10
log4perl.appender.file.filename = foo.txt
log4perl.appender.file.layout=PatternLayout
log4perl.appender.file.layout.ConversionPattern=%H:%P (%c) %p - %d: {%
+X{work}} %m%n

What am I doing wrong?

[ikegami]

You're surprised that a commonly used global variable gets used? Yeah, the logger should localise it's use — file a bug report — but that's rarely done, and it's not biggy that it doesn't.

if (my $e = $@) {
    print $e;
    $logger->debug('test');
    print $e;
    print 'after printing $e';
}

[saberworks]

You're right, it should be obvious that any external bit of code could use $@, thanks for that. I'm so used to seeing if($@) { # handle error } that I didn't really think about it properly.

[Urthas]

Oh I don't know about that. Actually, I think you *are* thinking about it properly. Although it makes sense that external code will refer to certain variables, it is natural to expect that said code won't walk all over those variables without restoring them or at least telling you about it in some way. File that report, in my opinion.

[ELISHEVA]

-- I'm so used to seeing if($@) { # handle error } --

Unless this is throwaway code, in the meantime until that bug is fixed, it wouldn't hurt to add a comment to your code to explain why you needed to do $e=$@, just so that someone further down the pike doesn't think that your own assignment code is redundant or unnecessarily confusing and try to "clean" it up.

If you are so used to seeing if ($@)..., the same is likely true for others.

[saintmike]

Sorry, I'm late to the party, but just for the record: Log4perl isn't resetting $@ here, it's a 3rd party appender from CPAN called Log::Dispatch::FileRotate. (You can easily verify this by replacing it by Log4perl's native file appender Log::Log4perl::Appender::File).

Might be a good idea to file a bug against it in order to have it localize its use of $@, although, as others have pointed out, it's a common mistake.

[Anonymous Monk]

$@ can't be relied on?