TLDR: multiarch, multiarch, multiarch, cross buildability, staged build dependencies, wiki page, corrections/hints/requests to debian-bootstrap at lists.mister-muffin.de
This summer (and this year's GSoC) is nearing its end and to make it easier for people to make use of the information my tools produced so far, I created a page in the Debian wiki. It lists not only the open issues I see but also statistics that I gathered using the output of my GSoC project. I want to use this blog post to make people aware of that page as well as to get some feedback on it and anything related to it.
The biggest blocker my tools face, is that many packages are still missing multiarch information. As long as at least the basic packages do not have their cross build dependencies satisfied via multiarch for an existing foreign architecture, automated tools can not properly analyze the dependency situation in the bootstrapping case, when many packages of the new foreign architecture do not even exist yet.
If Debian is supposed to be bootstrappable, then the first stage is to make a set of basic packages cross compile for an existing foreign architecture. Once this is possible, a tool of mine can analyze the cyclic build dependency situation that might occur when cross compiling for an architecture that does not exist yet. Then, staged cross builds can be used to cross compile a minimal foreign system. Due to missing multiarch classification, it is not known yet how big the cyclic build dependency situation is for the base packages.
It is not only the conversion of packages to multiarch that is needed but also the adding of the :any (and rare cases :native) qualifier to build dependencies on M-A: allowed packages. Prominent build dependencies that should (but are not yet) be M-A: allowed are python and gettext. Both are needed as a build dependency by many packages of the base system.
Unfortunately wanna-build does not understand qualifiers like :any and :native yet. Until it does, no package can be marked :any or :native and cross compilation of many base packages can not succeed.
Once the point is reached, where a base system can be cross compiled from nothing, native compilation can start. Since native compilation doesnt depend on multiarch, the dependency situation when trying to natively compiling all of Debian from nothing is understood much better. Unfortunately, the cyclic build dependency situation is also much worse in the native case and there exists a big 1000 node strongly connected component of binary and source packages that all interdepend on each other.
This dependency mess can be solved using three approaches:
The wiki page gives many hints on how to find packages that each method can be applied to.
Stage building is a tool that might be useful for cross building (we dont know for sure yet) but is definitely needed for native compilation. It is needed for native compilation because after all possible dependencies are moved to Build-Depends-Indep, the only other alternative to stage building for breaking dependency cycles is to cross build source packages. Since building a package without one of its build dependencies "staged" is often much easier than making the package in question cross compile, it is a preferred alternative. Once more packages have been made multiarch, it might be possible to prove that there is no alternative to introducing a notion of staged builds.
Some people (wookey, Patrick McDermott, Guillem Jover, myself) decided that the following format to mark staged build dependencies would be preferred over others:
Build-Depends: huge (>= 1.0) [i386 arm] <!embedded !bootstrap>, tiny
The <> format was proposed by Guillem Jover in bug#661538. Patches for dpkg and dose3 are done. More people need to discuss about this format for a final decision on how to indicate staged build dependencies.
For more information on the topic, have a look at the corresponding wiki page. Feel free to direct any comments/critique/hints to debian-bootstrap at lists.mister-muffin.de or directly to me.