[buildd-tools-devel] Bug#695287: sbuild: synthetic cross-build-dependencies for compilers

[Colin Watson]

Package: sbuild
Version: 0.63.2-1
Severity: wishlist
User: crossbuild at debian.org
Usertags: cross

(This is a bit of an RFC.  I think I understand the general shape of the
problem, but I'd welcome comments on the design.)

One of the problems we have with cross-building packages is with
alternative compilers and other cross-tools.  Generally speaking,
cross-build chroots start with a reasonable set of cross-tools
preinstalled, such as gcc-HOST and pkg-config-HOST.  If somebody
build-depends on pkg-config, pkg-config-HOST is already installed so
it's OK.  However, if somebody build-depends on, say, g++-4.6, then
nothing knows how to install g++-4.6-HOST; or I can easily imagine that
we might want a gobject-introspection-HOST in the future.  It does not
seem correct to me to deal with these problems by adding more and more
packages to the cross-build-essential set.

When I was writing my patch for #695203, it occurred to me that this
offered an opportunity for sbuild to step in and synthesise
build-dependencies in some situations.  A possible design would be as
follows:

  When cross-building, if the package being built build-depends on A
  (taking any [] into account), A is not Multi-Arch: foreign, and A-HOST
  exists in the apt cache, then rewrite the build-dependency to
  "A:native, A-HOST" (including any versioning present).

Wookey suggested that we might also want something for people who don't
use sbuild; I think perhaps a similar algorithm in 'dpkg-checkbuilddeps
-aHOST' would be useful and reasonable.  (CCing debian-dpkg.)  An
alternative to the "exists in the apt cache" rule, which I can imagine
being troublesome, might be to have a hardcoded list of relevant
packages; this would change from time to time but hopefully not too
often.  Or we could have a package control field for "there's a cross
version of this".

Does this sound right to you?  I think I could probably figure out how
to do this on top of my previous patch, but I'd like to make sure that
we agree on the design first.

Thanks,

-- 
Colin Watson                                       [cjwatson at ubuntu.com]