[Mimedefang] Next generation mimedefang-filter

[Jan Pieter Cornet]

On Thu, May 12, 2005 at 11:36:22AM +0100, Nik Clayton wrote:
> Ditch mimedefang-filter.  Have mimedefang.pl use something like 
> Module::Pluggable to find everything in the MimeDefang::Filter::Plugin 
> namespace.
> 
> Then have every filter_* subroutine in mimedefang-filter be something like:

Hm. So all the fun happens on this list while I'm temporarily away :)

This is precisely the thing I ran into when we started building our
MIMEDefang filters, so I just went ahead and created a way to modularize
the mimedefang-filter.

As an example, I've rewritten suggested-minimum-filter-for-windows-clients
into separate modules, and made that available here:

http://www.xs4all.nl/~johnpc/mimedefang-modular/

(note that the example was done rather quickly, and without too much
testing. For example this still contains a (C) Roaring Penguin...
which is not too strange since a large part is a literal copy of
the example filter. The example primairily serves as a coding example).

The heart of this is a mimedefang-filter-select file, which acts as
a replacement to mimedefang-filter. It does exactly what you describe:
loop over all modules for all filter_* calls... except that modules
that "take action" obviously abort the loop. Since this installs as
a replacement "mimedefang-filter", this requires no changes to
mimedefang itself, so you can still install mimedefang upgrades
when necessary.

This also uses a module "Mimedefang.pm" which makes Mimedefang appear
like a regular perl module/namespace that allows you to import symbols
from. Somewhat hackish, but all the complexity is compile-time complexity
and therefore hardly worth looking at.

All the filters are stored in the MailFilter/ subdirectory, all filters
reside in the MailFilter::* namespace. MailFilter::Settings is hardcoded
in mimedefang-filter-select, and contains the list of modules to use.

This works, rather well even, because it allows us to have multiple
developers working on different parts of the filter without conflicts,
and it allows for easier testing. I haven't yet made a real independent
test-setup yet, but at least you can install one module on, say, your
desktop machine and telnet to localhost:25 to test it.

Anyone is free to use this, it can be distributed with the same conditions
as MIMEDefang itself. We're using it ourselves, on production systems
handling millions of emails per day (at least the mimedefang-filter-
select and Mimedefang.pm, not the rest of the example).

However, this isn't fully complete, for example filter_map is missing.
Also, I might change the naming scheme at any point since I'm not too
happy about the MailFilter:: namespace (and CPAN maintainers would likely
kill me for using that). However, since I'm using this myself, any renaming
will likely only require a few well chosen global search/replace actions
on your custom filters, to get updated.

Also, please notice: this adds a lot of "scary perl" to your mimedefang
filter. If you're intimidated by the perl in suggested-minimum-filter-for-
windows-clients, then this is probably not for you.

Feedback is welcome. I can't promise to implement all suggestions, but
I'll definately consider good ones (and I get to decide which suggestions
are good :)

-- 
#!perl -wpl # mmfppfmpmmpp mmpffm <pmmppfmfpppppfmmmf at fpffmm4mmmpmfpmf.ppppmf>
$p=3-2*/[^\W\dmpf_]/i;s.[a-z]{$p}.vec($f=join('',$p-1?chr(sub{$_[0]*9+$_[1]*3+
$_[2]}->(map{/p|f/i+/f/i}split//,$&)+97):qw(m p f)[map{((ord$&)%32-1)/$_%3}(9,
3,1)]),5,1)='`'lt$&;$f.eig;                                # Jan-Pieter Cornet