Reference – Page 2

5 reasons why a Debian package is more than a simple file archive

November 8, 2010 by Raphaël Hertzog

Folder with gears You’re probably manipulating Debian packages everyday, but do you know what those files are? This article will show you their bowels… Surely they are more than file archives otherwise we would just use TAR archives (you know those files ending with .tar.gz). Let’s have a look!

1. It’s two TAR file archives in an AR file archive!

A .deb file is actually an archive using the AR format, you can manipulate it with the ar command. This archive contains 3 files, you can check it yourself, download any .deb file and run “ar t” on it:

$ ar t gwibber_2.31.91-1_all.deb
debian-binary
control.tar.gz
data.tar.gz

debian-binary is a text file indicating the version of the format of the .deb file, the current version is “2.0”.

$ ar p gwibber_2.31.91-1_all.deb debian-binary
2.0

data.tar.gz contains the real files of the package, the content of that archive gets installed in your root directory when you run “dpkg --unpack“.

But the most interesting part—which truly makes .deb files more than a file archive—is the last file. control.tar.gz contains meta-information used by the package manager. What are they?

$ ar p gwibber_2.31.91-1_all.deb control.tar.gz | tar tzf -
./
./postinst
./prerm
./preinst
./postrm
./conffiles
./md5sums
./control

2. It contains meta-information defining the package and its relationships

The control file within the control.tar.gz archive is the most fundamental file. It contains basic information about the package like its name, its version, its description, the architecture it runs on, who is maintaining it and so on. It also contains dependency fields so that the package manager can ensure that everything needed by the package is installed before-hand. If you want to learn more about those fields, you can check Binary control files in the Debian Policy.

Those information end up in /var/lib/dpkg/status once the package is installed.

3. It contains maintainer scripts so that everything can just work out of the box

At various steps of the installation/upgrade/removal process, dpkg is executing the maintainer scripts provided by the package:

postinst: after installation
preinst: before installation
postrm: after removal
prerm: before removal

Note that this description is largely simplified. In fact the scripts are executed on many other occasions with different parameters. There’s an entire chapter of the Debian Policy dedicated to this topic. But you might find this wiki page easier to grasp: http://wiki.debian.org/MaintainerScripts.

While this looks scary, it’s a very important feature. It’s required to cope with non-backwards compatible upgrades, to provide automatic configuration, to create system users on the fly, etc.

4. Configuration files are special files

Unpacking a file archive overwrites the previous version of the files. This is the desired behavior when you upgrade a package, except for configuration files. You prefer not to loose your customizations, don’t you?

That’s why packages can list configuration files in the conffiles file provided by control.tar.gz. That way dpkg will deal with them in a special way.

5. You can always add new meta-information

And in fact many tools already exploit the possibility to provide supplementary files in control.tar.gz:

debsums use the md5sums file to ensure no files were accidentally modified
dpkg-shlibdeps uses shlibs and symbols files to generate dependencies on libraries
debconf uses config scripts to collect configuration information from the user

Once installed, those files are kept by dpkg in /var/lib/dpkg/info/package.* along with maintainer scripts.

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Understanding Debian’s release process

October 18, 2010 by Raphaël Hertzog

Currently, the main product of the Debian project is its stable release^[1]. Those release come out approximately every 18-24 months. This article gives a short overview of the process leading to the next stable release.

Creating a new distribution

Immediately after a stable release, a new distribution is created in the Debian archive. Its initial content is a copy of the (just released) stable distribution. Its codename is decided by the release managers and there’s a tradition of picking a character’s name from the Toy Story movie.

As an example, the “wheezy” distribution will be created once “squeeze” (aka Debian 6.0) is out.

For simplicity there’s a generic name to refer to the distribution used to prepare the next stable release: it’s testing. In the Debian archive, testing is just a symbolic link pointing to the right directory (squeeze currently).

Updating packages, working on release goals

During most of the cycle, developers work on packaging new upstream versions and implementing release goals. They upload their packages in the unstable distribution.

From there packages trickle to the testing distribution once they satisfy some quality checks: they must not have new release-critical bugs, they must have been built on all architectures that were previously supported, they must not break any dependency in testing, and they must have spent at least 10 days in unstable.

This minimal period ensures the package has been tested and gives enough time to users to file bugs if the package is suffering from problems. If the problems are deemed release-critical, they will block the migration of the package to testing.

During this part of the release cycle, the main work of the release team is ensuring that updated packages flow from unstable to testing. It can be a tricky task: package dependencies frequently tie packages together so that they can only migrate to testing together. If only one of the tied packages is not ready (for example if a new revision has been uploaded and has not spent 10 days in unstable yet), then none of them can migrate.

Stabilizing, polishing, fixing release-critical issues

The constant churn of new packages makes it very difficult to build a very polished release. That’s why, at some point, release managers freeze the testing distribution: automatic updates are stopped and they vet every single update made to testing. They have strong requirements, the goal is to only allow updates fixing release critical bugs, or those which are low-risk and bring significant value to the user experience (like new translations, updated documentation, etc.).

During freeze, some packages are also removed because the current upstream version can’t be supported for the lifetime of the stable release.

The freeze tends to slow down the pace of changes in unstable. Many maintainers opt to push new upstream versions in experimental instead so that if they need to update their packages in testing, they can still do it through unstable. This procedure is recommended by the release managers because it means that updates that they unblock have been tested as usual. It’s not the case for updates uploaded directly to testing (through testing-proposed-updates).

This behavior is rather annoying for the bleeding-edge users that use testing or unstable like a rolling release.

Release time

Once release managers are satisfied of the quality of the new distribution, some last minute work is needed, like generating the CD images. In the Debian archive, the release is made official by pointing the “stable” symbolic link to the new distribution (and the “oldstable” one to the previous distribution).

Now it’s party time, the cycle is over, and a new one can start. 🙂

^[1] The Constantly Usable Testing project aims to make testing a first-class product like stable—but with a very different update policy.

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Everything you need to know about conffiles: configuration files managed by dpkg

September 21, 2010 by Raphaël Hertzog

The Debian policy dictates that package upgrades must take care of preserving user changes to configuration files. This article will explain you how most packages ensure this. This is important knowledge for anyone who has to manage upgrades: knowing how it works lets you easily automate most of it and deal correctly with the fallout.

How dpkg manages configuration files

Most packages rely on dpkg to properly install configuration files. Dpkg keeps a checksum of the last installed version of configuration file. When it must install a new version, it calculates the checksum of the currently installed file and if it doesn’t match anymore, it knows that the user has edited the file. In that case, instead of overwriting the configuration file, it asks the user what to do. You probably already have seen those questions, they look like this:

Configuration file `/etc/bash.bashrc'
 ==> Modified (by you or by a script) since installation.
 ==> Package distributor has shipped an updated version.
   What would you like to do about it ?  Your options are:
    Y or I  : install the package maintainer's version
    N or O  : keep your currently-installed version
      D     : show the differences between the versions
      Z     : start a shell to examine the situation
 The default action is to keep your current version.
*** bash.bashrc (Y/I/N/O/D/Z) [default=N] ?

In this specific example, if you answer “Y” or “I” (for “yes” or “install”), dpkg will install the new version of /etc/bash.bashrc but it will also backup the current version in /etc/bash.bashrc.dpkg-old. If you answer “N” or “O” (for “no” or “old”), dpkg will install the new version in /etc/bash.bashrc.dpkg-dist and /etc/bash.bashrc is left untouched. The two other answers allow you to examine the differences before taking a decision. Note that if you choose to start a shell, the new version is currently available as /etc/bash.bashrc.dpkg-new (and since Squeeze there are convenient environment variables $DPKG_CONFFILE_OLD and $DPKG_CONFFILE_NEW in case you want to create a custom review script).

All configurations files managed by dpkg are called “conffiles” because that’s the name of the field where they are recorded in the dpkg database. You can display the list of conffiles for any package:

$ dpkg --status bash
[...]
Conffiles:
 /etc/skel/.profile ecb6d3479ac3823f1da7f314d871989b
 /etc/skel/.bashrc 2afdd6c53990f2387a7ef9989af0bc07
 /etc/skel/.bash_logout 22bfb8c1dd94b5f3813a2b25da67463f
 /etc/bash.bashrc 5b3c3bc73d236e4e1b6f9b6c1ed5964e
[...]

The command “dpkg-query --showformat='${Conffiles}\n' --show bash” can give you the same information if you need to retrieve only that field. The 32 characters after the filename are the MD5 checksum of the original configuration file provided by the package.

Avoiding the conffile prompt

Every time that dpkg must install a new conffile that you have modified (and a removed file is only a particular case of a modified file in dpkg’s eyes), it will stop the upgrade and wait your answer. This can be particularly annoying for major upgrades. That’s why you can give predefined answers to dpkg with the help of multiple --force-conf* options:

--force-confold: do not modify the current configuration file, the new version is installed with a .dpkg-dist suffix. With this option alone, even configuration files that you have not modified are left untouched. You need to combine it with --force-confdef to let dpkg overwrite configuration files that you have not modified.
--force-confnew: always install the new version of the configuration file, the current version is kept in a file with the .dpkg-old suffix.
--force-confdef: ask dpkg to decide alone when it can and prompt otherwise. This is the default behavior of dpkg and this option is mainly useful in combination with --force-confold.
--force-confmiss: ask dpkg to install the configuration file if it’s currently missing (for example because you have removed the file by mistake).

If you use Apt, you can pass options to dpkg with a command-line like this:

$ apt-get -o Dpkg::Options::="--force-confdef" -o Dpkg::Options::="--force-confold" dist-upgrade

You can also make those options permanent by creating /etc/apt/apt.conf.d/local:

Dpkg::Options {
   "--force-confdef";
   "--force-confold";
}

Bringing up the conffile prompt at any time

The conffile prompt is only displayed when dpkg detects that the package provides an new version of the conffile. Thus reinstalling the same package will not bring up the prompt. But you can instruct dpkg to ask nevertheless with the --force-confask option. This is a new feature in Debian Squeeze. It will only ask for files that are locally modified.

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Understanding Membership Structures in Debian and Ubuntu

August 30, 2010 by Raphaël Hertzog

Debian and Ubuntu have a set of official membership roles that can be granted to regular contributors. Those roles come with rights that enable the contributors to do their work and to participate in the project governance (elections and other official decision-making processes). It’s also a way for the distributions to acknowledge the work done: most contributors are proud of the status they reached.

The membership structure plays an important role in the development of a distribution: it defines the kind of contributors that are welcome in the project, it sets expectations of the project towards its contributors and defines their rights. In the end, this shapes the project’s ability to recruit new contributors to keep the project alive and kicking. This article introduces the existing statuses in Debian and Ubuntu, and defines the — sometimes confusing — jargon associated with them.

The Debian Case

Debian only has two types of official members: Debian Developers (DD) and Debian Maintainers (DM). The rights of the developers are codified in the Debian Constitution while those of the maintainers have been defined in a general resolution of 2007. The Debian Maintainer status is still mostly documented in a wiki page. The integration of this new status in Debian’s official processes has been slow to come largely because it was introduced — at that time — without enough negotiation with the involved parties. Nowadays, it is preferred that people get the DM status before applying for DD.

DM is a very limited role: maintainers can only upload packages that already have their name on them (either in the Maintainer or Uploaders field) and a specific flag (DM-Upload-Allowed: yes) that only Debian Developers can add. They have no other rights and limited access to Debian’s resources.

Besides those official roles, there are also maintainers of packages that have no official status within Debian except that they are listed in the “Maintainer” field of the package. They are doing the maintenance work but all uploads are done by a Debian Developer after verification of the work done (this is called “sponsorship” and is the only way to start with official packaging work). Once the DD trusts the maintainer, the developer will typically ask the maintainer to apply for DM status in order to be relieved from the sponsorship work.

In the end, that makes three different kind of package maintainers and a lot of confusion when you discuss membership issues… in particular when the New Maintainer process is the path that you follow to become a Debian Developer. Don’t be fooled by the names when reading Debian’s documentation!

The Ubuntu Case

Ubuntu had, from the start, an official Ubuntu Member status that includes all contributors: developers of course, but also documentation writers, artists, translators, etc. This status notably grants the right to vote in elections of the Community Council, the right to participate on Planet Ubuntu, and the @ubuntu.com email alias.

For developers, the situation is more complicated: the wiki page lists no less than five different statuses. Initially, developers were split between Ubuntu Core Developers and the MOTU (Masters Of The Universe). The latter were responsible of the universe/multiverse sections of the archive while the former also had upload rights for the main/restricted sections. But, inspired by the Debian Maintainers concept and facing concrete problems in terms of archive management, they changed their infrastructure to offer more fine-grained control on package uploads.

Ubuntu can now grant upload permissions on a package-per-package basis, but it can also delegate the right to grant upload permissions with the same granularity. This lead to the new Per-Package Uploader status which is simply an Ubuntu Member with upload rights on a limited set of packages where they have a specific expertise. The more generic Ubuntu Developers status now encompasses members of various development teams that have been delegated the right to manage upload permissions on a (usually large) package set (the current teams are Ubuntu Desktop, Mythubuntu, Kubuntu, and Edubuntu). Those teams can define their own policy to add new members provided they follow the basic rules defined by the Developer Membership Board (see this wiki page).

Ubuntu Contributing Developer is an intermediate status for someone who is not yet ready for one of the other developer statuses but who has still shown enough commitment to be an Ubuntu Member.

All those statuses can be obtained in a similar way: you prepare a wiki page listing your past contributions, you collect testimonials from existing members that you have worked with, you add yourself in the agenda of the next meeting of the board (or council) that grants the status that you seek, and you attend the meeting. The members of the board will decide whether you are ready for the status (or not) based on what you provided in the wiki, based on your answers during the meeting (and on a mailing-list for developers), and based on what others have to say about you.

The most important boards are usually elected by the community while others are commonly appointed by the community council. Those governance bodies include Canonical employees but not as many as one would expect: two out of eight in the Developer Membership Board, two out of eight in the Community Council, but all six members of the Technical Board. The last figure, while not intended, is not surprising given the high expectations set on potential members of the technical board. Mark, as the founder, is the only person to have a permanent seat on both the Community Council and the Technical Board.

Comparison of the Statuses Between Debian and Ubuntu

The following table summarizes the rights given to each developer role in the two projects (Put the mouse over the abbreviations to know what they are referring to).

Rights	Debian		Ubuntu
Rights	DM	DD	UM	PPU/UD	MOTU	UCD
Package maintenance via sponsorship	Y	N/A	Y	Y	Y	N/A
Official email alias	–	Y	Y	Y	Y	Y
Participate in votes for members	–	Y	Y	Y	Y	Y
Participate in votes for developers	–	Y	–	Y	Y	Y
Upload rights restricted to pre-approved packages	Y	–	–	Y	–	–
Upload rights restricted to a section of the archive	–	–	–	–	Y	–
Unlimited upload rights	–	Y	–	–	–	Y
Number of contributors (as of 2010-07-27)	117	904	462	27	85	63

Please note that the number of contributors are not 100% accurate for Ubuntu. A contributor can have multiple statuses (direct membership to a launchpad group) granted over time (while gaining experience). The problem has been mostly avoided by calculating differences between number of members of the various groups but it’s not perfect and it can’t be: some MOTU are also PPU for packages in main and it’s legitimate (but I only counted them as MOTU and not as PPU). Another limitation is that members of some administrative teams are included indirectly in many teams and thus appear in the count while they should not.

Anyway, this simple table makes it obvious that Ubuntu’s structure offers a broader choice of statuses. They acknowledge the work of all contributors from the start while still giving the most critical rights only to those who have proven that they deserve them. Despite this difference, Debian still has a significant advantage in terms of number of developers. That number does not tell the whole story though: the Ubuntu contributors include many Canonical employees (e.g. 36 out of 63 core developers have a @canonical.com email registered on their launchpad account) that are likely to spend more time working on the distribution than the average Debian member. But even if comparing person-hours would be a challenging thought experiment, in practice it’s of not much interest if both projects continue to cooperate and if more and more of the contributions flow in both directions.

Debian is aware of the shortcomings of its structure. Changes to better accommodate non-packagers have been discussed several times already. The last efforts in that direction were unfortunately perceived as a solution ready to go rather than a proposal to be discussed, and the project got quickly buried by a general resolution (GR). Even if that resolution invited for further discussion and a new proposal, the truth is that when someone’s initiative is “corrected” by way of GR, it usually kills any motivation to go forward.

Possible Evolution?

On the Ubuntu side, the infrastructural changes were completed recently and they don’t expect any further change in the near future. They do plan, however, to expand usage of those new features so that more teams benefit from the possibility to control upload rights on packages that are relevant to them, and so that more individuals developers apply to become Per-Package Uploaders on packages that they know very well.

On the Debian side, a recent discussion on the debian-project list brought back the topic of the bad terminology and it was agreed that the “New Maintainer process” should be renamed into something else (“New Developer process” has been suggested). But Christoph Berg — Debian Account Manager and hence heavily involved in the New Maintainer Team — suggested that Debian would be better off implementing the long-awaited membership changes before trying to update all the documentation. It would certainly imply some more vocabulary updates. Later in the discussion, he confirmed that membership reform is on the top of the TODO list of the new maintainer team (just after the rewrite of the nm.debian.org website).

What can be expected from this reform? The following answers are my own guesses based on my experience of Debian, but the project hasn’t decided anything yet.

First of all: a new status for contributors that are not packagers. The tricky part will be defining the process to follow and the rights granted.
Changes to the technical implementation of the DM status. The current implementation does not allow to give upload rights to a single DM if two are listed in the Uploaders field of a package (and both might not have the same experience for that package). Furthermore, it suffers from annoying restrictions like the inability to upload new binary packages.
A change of the Debian constitution to integrate those new statuses is almost unavoidable.
Other more invasive changes have been proposed like replacing the NM process by a simple designation by other DD, but it’s unlikely to happen. The NM process can already be greatly simplified by the application manager if the applicant can show good testimonials from other developers and if he has a track record of real contributions (e.g. as witnessed by changelog entries in Debian packages).

Almost two years have elapsed since the previous efforts in that direction, the new maintainer team has recruited new members and is in a general better shape. Hopefully, the next episode of this saga will have a better outcome.

This article was first published in Linux Weekly News. In a comment, Mark Shuttleworth tried to explain how the Ubuntu community is being setup.