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The Little Deferral Boy

Email::Pipemailer::DieHandler - 2009-12-03

Avoid Unwanted Returns

I write a whole lot of code that takes a piece of mail and does something with it. Some of these are SMTP servers, some process mailboxes on disk, but at least half of them are pipemailers. A pipemailer reads a message on STDIN and exits with a code that lets the calling program know what happened. To grossly simplify, the possible exit values are:

  • 0  - everything is okay; the message has been handled

  • 75 - we couldn't handle the message right now, try again later

  • anything else - something is wrong; report failure and never try again

Why does this matter? Well, say you're using Email::Filter or Mail::Audit or some other Perl library for delivering your mail. I mean, you're not using procmail are you? One day you decide you're going to start checking your mail with SpamAssassin, so you copy this blurb from the documentation:


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$mail−>exit(0);
$mail−>simple(Email::Simple−>new($mail−>pipe("spamassassin")));
$mail−>exit(1);
 

It's in the documentation, so it should be fine... but you've copied and pasted from perldoc and you're getting bitten by its stupid use of <U+2122> instead of a hyphen in the arrow operator. (This gets me about once a week.) Unfortunately, you don't notice, and now your program dies because perl has no idea what to do with that fake hyphen. Your program exits 9 (or something else seemingly random) and your mail server starts bouncing absolutely all your mail.

A test suite would help, and so would lots of other things, but Email::Pipemailer::DieHandler is dead simple, and a good first line of defense. When you install its "SIGDIE" handler, any fatal exception outside of an eval will be logged and will cause the program to exit 75. Your mail might not get delivered, but at least it won't get bounced. Hopefully you'll notice before it all times out!

Email::Pipemailer::DieHandler is a pretty tiny module. It's so short that I can reproduce all of its effective code right here:


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package Email::Pipemailer::DieHandler;
sub import {
  my ($self, $install, $arg) = @_;
  return unless $install and $install eq '-install';

  $arg ||= {};
  my $logger = $arg->{logger} || sub {};

  $SIG{__DIE__} = sub {
    return if $^S; # don't interfere with evals
    my ($e) = @_;
    defined $^S and eval { $logger->($e); };
    $! = 75;
    die $e;
  };
}
 

It might be the shortest module I've published on the CPAN, but don't let that fool you: this module has saved me quite a lot of embarassment by converting stupid bounces into errors that only I needed to see.

See Also