Building everything from source vs self-hosting

In this post, I’m going to answer a seemingly simple question: Why do neither Debian or Fedora have a well defined and reliable process to rebuild everything from source?

I’m often surprised by how many people I encounter in the FOSS world, even experienced developers, that are either intimidated by the idea of building “everything” from source, or think it’s crazy, or just not worth it. Let’s just assume for the purposes of this discussion that rebuilding from source is valuable. I mean, after all it’s Free Software, not Free Binaries Wrapped With Some Metadata.

First, let me define “everything”: the goal here is to construct a basic Linux-based system, bootable in qemu, and you can log in as root. A perfect example of our goal here is to build the source code that comprises JS/Linux. That means the kernel, bash, glibc, gcc, etc. If you read the tech notes, you’ll see it’s built using Buildroot.

The first thing to observe here is not only do multiple projects to accomplish this goal exist, they do so with a high degree of reliably, and solve real-world needs. For example, the Yocto project’s “core-image-minimal” target gets you basically this same thing, and you just run bitbake core-image-minimal, and everything else is done for you. Likewise, a quick read of the Buildroot manual will show you just how little needs configuration or manual intervention.

The second thing to note about these systems like Buildroot, Yocto, and others – they are not (by default), self hosting. The host and target systems need not be the same. For example, you can use Yocto to build “core-image-minimal” from a Red Hat Enterprise Linux 6 system, an Ubuntu 12.04 system, and a variety of others. In fact, you can even do full cross builds from x86_64 to ARM. Now, interestingly Yocto can generate self-hosting systems, but it’s not the default.

We’re getting closer to answering our original question. Let’s further observe that both Debian and Fedora are defined to be self hosting systems. Why is self hosting a problem? It’s because of circular build dependencies. The classic example of this is gcc, which is written in the C programming language. In order to build it, you need a C compiler already. The Yocto/Buildroot type build systems get out of this problem in a simple way – they assume you already have a functioning gcc on the host system.

But in Debian and Fedora, in order to build the gcc package, you need gcc already built as a package – the build system won’t accept just having a “gcc” binary in the $PATH. That’s how the build systems work because again, that’s how the projects are defined.

If you haven’t done this recently, grab a mirror you can hold in your hand, and go into your bathroom, and point the hand mirror at the wall mirror. You’ll get an infinite recursion. It’s really quite beautiful and fun to do, but since I’m sure many of you won’t, there’s a good picture here.

This infinite recursion resulting from self-hosting is the reason there isn’t one reliable command to rebuild all of Debian or Fedora from source.

One question you might have – would it make sense to have a well-defined process for bootstrapping a self-hosting system like Debian? Some of the developers think so, and the DebianBootstrap wiki page describes the thoughts so far. Personally though, I think it’s both too complex and too vague. A much simpler, and ultimately more reliable goal would be to ensure that version N of the system can be built by N-1. So Fedora 17 can be built on a Fedora 16 system, Debian Wheezy can be built from Squeeze, Red Hat Enterprise Linux 6 can be built from Red Hat Entrerprise Linux 5, etc. Eventually this is a goal I’d like to achieve for Red Hat Enterprise Linux at least. There’d be some cost to packages with circular build dependencies, but having a well defined, reliable process for building from source: priceless.

GNOME Summit 2012 – Friday & Saturday

Friday: Newcomers pre-event

The Newcomers pre-event on Friday evening was very successful! A number of students came from nearby universities like Tufts and MIT, and were able to dive straight in to tools for GNOME development like Git, Jhbuild, and of course GTK+ and Gjs.

You can see the bugs they reported (with patches!) in bugzilla. Not everyone reported a bug – creating an account was a hurdle. Still though, the event was a success and likely something to replicate in the future.

Saturday opening

Saturday morning started with coffee and bagels – thanks to SUSE for sponsoring! As the summit is an “un-conference”, we made up a schedule on the fly, which you can see here.

GTK+

Benjamin Otte demoed some new CSS features that are implemented in GTK+ – notably a use of animated transition for background of active menu items. He also started a discussion on the challenges in maintaining the 750,000 lines of code in GTK+. We brainstormed about how we can attract more contributors to infrastructure such as GTK+.

Owen Taylor repeated his demo of client-compositor synchronization, covered by LWN previously.

GObject Introspection

Colin Walters discussed the state of introspection and the goals for the 3.7 development cycle – in particular, generating documentation from GIR files is the primary goal. There was an obligatory debate about which programming languages to use/promote in GNOME, along with some questions about how we can improve “bindability” of APIs in GNOME.

GNOME OS

This was a followup from GUADEC, and ended up taking up several hours. It opened with a status update on OSTree, also on LWN previously. The system has been doing successful continuous integration on over 200 git repositories, and notably all of GNOME up to gnome-shell, plus some application dependencies like gtksourceview.

While the current OSTree builder is automated, there was a lot of interest around making it better, and ensuring that when the build does break, that both the responsible party knows about it, as well as other people. This led into some comparisons with the WebKitGtk+ development process. Concrete action items resulting from this were an IRC bot and an improved web page.

There was general consensus that the composition of the modulesets is best handled by the release-team. A lot of time was spent on the topic of Jhbuild, and its strengths and weaknesses for application authors, GTK+ developers, and core OS hackers. Ryan Lortie described how we could make jhbuild better by having GNOME builders provide distribution-specific binaries, and later in the day could be seen furiously hacking on implementing it.

After lunch, the topic of applications came up. Allan discussed application stores, and how that can provide a good experience around things like centralized updates, and generating revenue for authors.

Following application stores, the topic of application sandboxes came up. Chris Ball mentioned difficulties OLPC had encountered in getting application authors to adapt to their sandboxing scheme. Strengths and weaknesses of the Android model came up, and at this
point we agreed to break out sandboxing as a standalone topic for the next day.

Developer toolset

Benjamin Otte started a discussion about our developer tools, and we talked about different kinds of application authors:

  • ourselves (GNOME Documents, Rhythmbox, etc.)
  • iphone/android type app
  • libreoffice-size application
  • enterprise apps (e.g. proprietary creative apps)
  • sysadmin apps

There are challenges in meeting the needs of all of these different
kinds of authors.

A11Y

PiƱeiro started with a summary of the state of accessibility. 3.6 was a significant achievement in that a11y is always-on by default, and only impacts the system when an accessibilty tool is running.

He then talked about 3.7 features, such as improving the mangifier. There was also some plans to improve configurability, and improve discoverability of existing options.

The ongoing cost of maintaining fallback mode was mentioned briefly. Then he talked about allowing third parties to extend accessibility support when using custom widgets in GTK+.

Finally, two other points that were touched on were that some new applications (Documents, Clocks) have inaccessible features. Second, touch accessibility (for blind users).

3.7 planning

While there are a few features listed for GNOME 3.7 development, pretty much the entire hour of discussion was on fallback mode. As the feature page says, dropping fallback mode would allow us to significantly clean up some GNOME internals, it turned out on the other hand that Ubuntu Unity is likely relying on some of those as well.

The fallback mode users fall into two general categories; those who wanted a more GNOME 2 like experience, and those who were unable to run GNOME 3.

In general, the discussion seemed to follow along with what the feature page had already, and opinions varied. No hard decisions were made now; this will be an ongoing discussion.