Modules
-------

The new debian installer is highly modular. Modules are loaded at several
points in the install process to add capabilities to the installer. Modules
are essentially sets of files that get unpacked onto the installation
system, plus some metadata.

Because there is already so much infrastructure to build and manage debian
packages, and because it allows metadata to accompany the files in a module,
we have decided to use debian packages as the modules for the installer. To 
differentiate them from normal Debian packages, modules have filenames
ending in ".udeb". However, modules must not have the same package
name as any regular Debian package. The 'u' in udeb stands for Greek
mu, meaning micro (udebs take less space than regular debian packages).

Unlike regular debian packages, modules for the installer will not be
policy compliant. They will not contain documentation in /usr/share/doc.
They need not comply with the FHS. They may be statically linked. They are
built with options such as -Os and -fomit-frame-pointer to reduce binary
size. They may conflict with essential "real" debian packages, and thus be
non-installable on a real debian system, although it is recommended that if
possible, this be avoided.

Here is a short rundown on differences between deb packages and udebs:

All udeb packages should be able to build debug versions if
DEB_BUILD_OPTIONS="debug".

Misc.Differences
================

* Udebs must not include .md5sum files.

New Control Scripts
===================

menutest
--------

Menutest scripts should return a true value (0) if they think it would be a
good idea if their menu item was default, and a false value if it doesn't
matter. For example in case of a udeb providing an ethernet driver, the
menutest script would run a hardware detection routine and return 0 if the
particular ethernet card was installed.

Menutest scripts are optional. If a module does not have one, a simpler
default test is used.

isinstallable
-------------

If this script exists, and when run returns a non-zero value, then
its menu item is not displayed.

New Headers
===========

Installer-Menu-Item
-------------------

A priority (a number). The priority number influences the ordering of
items in the menu; higher numbered items are closer to the end of the
menu.  See the file menu-item-numbers.txt for the currently used
numbers.  If this field does not appear, or has a value of 0, a module
will not appear on the main menu.

Subarchitecture
---------------

Whitespace separated list of supported subarchitectures.

Kernel-Version
--------------

The uname -r of the version of the kernel that this udeb contains files
(eg, kernel modules) for. If specified, must exactly match the running
kernel or anna will skip installing the udeb.


Modules will not be installed by a full-fledged dpkg implementation, so the
following features of regular debs will not be supported:

Disallowed Control Files
========================

preinst, postrm, prerm, md5sums, conffiles

Disallowed Headers
==================

* Pre-Depends
* Conflicts
* Essential
* Suggests

Disallowed Header Options
=========================

* `|' in dependencies.

Supported Control Files
=======================

postinst

Supported Headers
=================

* Depends, Recommends
  only simple dependencies, no alternatives (versioned dependencies may be
  used, but the version part will be ignored by most of d-i)
* Enhances, Provides


A 2 minute primer on building udebs.
====================================

1. Add a Build-Dependency against debhelper (>= 4.2).
2. Add "XC-Package-Type: udeb" to the package's stanza in the control file.
3. Build the package!