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Packaging Guide

6. Patches to Packages

Sometimes, Ubuntu package maintainers have to change the upstream source code in order to make it work properly on Ubuntu. Examples include, patches to upstream that haven’t yet made it into a released version, or changes to the upstream’s build system needed only for building it on Ubuntu. We could change the upstream source code directly, but doing this makes it more difficult to remove the patches later when upstream has incorporated them, or extract the change to submit to the upstream project. Instead, we keep these changes as separate patches, in the form of diff files.

There are a number of different ways of handling patches in Debian packages, fortunately we are standardizing on one system, Quilt, which is now used by most packages.

Let’s look at an example package, kamoso in Trusty:

$ bzr branch ubuntu:trusty/kamoso

The patches are kept in debian/patches. This package has one patch kubuntu_01_fix_qmax_on_armel.diff to fix a compile failure on ARM. The patch has been given a name to describe what it does, a number to keep the patches in order (two patches can overlap if they change the same file) and in this case the Kubuntu team adds their own prefix to show the patch comes from them rather than from Debian.

The order of patches to apply is kept in debian/patches/series.

6.1. Patches with Quilt

Before working with Quilt you need to tell it where to find the patches. Add this to your ~/.bashrc:

export QUILT_PATCHES=debian/patches

And source the file to apply the new export:

$ . ~/.bashrc

By default all patches are applied already to UDD checkouts or downloaded packages. You can check this with:

$ quilt applied

If you wanted to remove the patch you would run pop:

$ quilt pop
Removing patch kubuntu_01_fix_qmax_on_armel.diff
Restoring src/kamoso.cpp

No patches applied

And to apply a patch you use push:

$ quilt push
Applying patch kubuntu_01_fix_qmax_on_armel.diff
patching file src/kamoso.cpp

Now at patch kubuntu_01_fix_qmax_on_armel.diff

6.2. Adding a New Patch

To add a new patch you need to tell Quilt to create a new patch, tell it which files that patch should change, edit the files then refresh the patch:

$ quilt new kubuntu_02_program_description.diff
Patch kubuntu_02_program_description.diff is now on top
$ quilt add src/main.cpp
File src/main.cpp added to patch kubuntu_02_program_description.diff
$ sed -i "s,Webcam picture retriever,Webcam snapshot program,"
$ quilt refresh
Refreshed patch kubuntu_02_program_description.diff

The quilt add step is important, if you forget it the files will not end up in the patch.

The change will now be in debian/patches/kubuntu_02_program_description.diff and the series file will have had the new patch added to it. You should add the new file to the packaging:

$ bzr add debian/patches/kubuntu_02_program_description.diff
$ bzr add .pc/*
$ dch -i "Add patch kubuntu_02_program_description.diff to improve the program description"
$ bzr commit

Quilt keeps its metadata in the .pc/ directory, so currently you need to add that to the packaging too. This should be improved in future.

As a general rule you should be careful adding patches to programs unless they come from upstream, there is often a good reason why that change has not already been made. The above example changes a user interface string for example, so it would break all translations. If in doubt, do ask the upstream author before adding a patch.

6.3. Patch Headers

We recommend that you tag every patch with DEP-3 headers by putting them at the top of patch file. Here are some headers that you can use:

Description:Description of what the patch does. It is formatted like Description field in debian/control: first line is short description, starting with lowercase letter, the next lines are long description, indented with a space.
Author:Who wrote the patch (i.e. “Jane Doe <packager@example.com>”).
Origin:Where this patch comes from (i.e. “upstream”), when Author is not present.
Bug-Ubuntu:A link to Launchpad bug, a short form is preferred (like https://bugs.launchpad.net/bugs/XXXXXXX). If there are also bugs in upstream or Debian bugtrackers, add Bug or Bug-Debian headers.
Forwarded:Whether the patch was forwarded upstream. Either “yes”, “no” or “not-needed”.
Last-Update:Date of the last revision (in form “YYYY-MM-DD”).

6.4. Upgrading to New Upstream Versions

To upgrade to the new version, you can use bzr merge-upstream command:

$ bzr merge-upstream --version 2.0.2 https://launchpad.net/ubuntu/+archive/primary/+files/kamoso_2.0.2.orig.tar.bz2

When you run this command, all patches will be unapplied, because they can become out of date. They might need to be refreshed to match the new upstream source or they might need to be removed altogether. To check for problems, apply the patches one at a time:

$ quilt push
Applying patch kubuntu_01_fix_qmax_on_armel.diff
patching file src/kamoso.cpp
Hunk #1 FAILED at 398.
1 out of 1 hunk FAILED -- rejects in file src/kamoso.cpp
Patch kubuntu_01_fix_qmax_on_armel.diff can be reverse-applied

If it can be reverse-applied this means the patch has been applied already by upstream, so we can delete the patch:

$ quilt delete kubuntu_01_fix_qmax_on_armel
Removed patch kubuntu_01_fix_qmax_on_armel.diff

Then carry on:

$ quilt push
Applied kubuntu_02_program_description.diff

It is a good idea to run refresh, this will update the patch relative to the changed upstream source:

$ quilt refresh
Refreshed patch kubuntu_02_program_description.diff

Then commit as usual:

$ bzr commit -m "new upstream version"

6.5. Making A Package Use Quilt

Modern packages use Quilt by default, it is built into the packaging format. Check in debian/source/format to ensure it says 3.0 (quilt).

Older packages using source format 1.0 will need to explicitly use Quilt, usually by including a makefile into debian/rules.

6.6. Configuring Quilt

You can use ~/.quiltrc file to configure quilt. Here are some options that can be useful for using quilt with debian/packages:

# Set the patches directory
# Remove all useless formatting from the patches
QUILT_REFRESH_ARGS="-p ab --no-timestamps --no-index"
# The same for quilt diff command, and use colored output
QUILT_DIFF_ARGS="-p ab --no-timestamps --no-index --color=auto"

6.7. Other Patch Systems

Other patch systems used by packages include dpatch and cdbs simple-patchsys, these work similarly to Quilt by keeping patches in debian/patches but have different commands to apply, un-apply or create patches. You can find out which patch system is used by a package by using the what-patch command (from the ubuntu-dev-tools package). You can use edit-patch, shown in previous chapters, as a reliable way to work with all systems.

In even older packages changes will be included directly to sources and kept in the diff.gz source file. This makes it hard to upgrade to new upstream versions or differentiate between patches and is best avoided.

Do not change a package’s patch system without discussing it with the Debian maintainer or relevant Ubuntu team. If there is no existing patch system then feel free to add Quilt.