[buildd-tools-devel] Bug#610689: Bug#610689: sbuild: cross support

[Roger Leigh]

On Fri, Jan 21, 2011 at 12:09:55PM +0000, Hector Oron wrote:
>   I would love sbuild to gain the ability to handle cross builds.
[…]

Thanks for the informative explanation, it has helped to make the
problem a bit clearer for me.

> It seems that sbuild triggers dpkg --print-architecture to find out chroot architecture, but when doing cross it should try to follow dpkg-architecture's DEB_HOST_ARCH.
> 
> Would it be possible to modify Build.pm in some way, either calling dpkg-architecture or autodetecting -a$arch has been passed and it should follow other architecture changes file?

We can definitely do this.

I think that the several uses of the --arch option are going to need
separating into separate options.  Does the following make sense, or
am I missing out anything major?

1) Selection of the build chroot

   We look for a chroot named $distribution-$arch-sbuild or
   $distribution-$arch

   When we are genuinely building for a different arch, then this works.
   When we're cross-compiling, we want to build on a chroot using the
   host arch (most likely, though you /could/ use a different arch with
   qemu and do an emulated cross-build once we have such support!)

2) Selection of the host architecture

   Linked to (1).  We are assuming that the build and host architectures
   are the same, so --host is implicitly setting both.

I think we should add two additional options:
  --host   select host arch
  --build  select build arch
(or whatever are most commonly used in existing Debian tools; these are
from autoconf configure) and have "--arch=foo" be equivalent to
"--host=foo --build=foo".

The chroot selection will use the host arch only.  So this will
typically be the "real" host arch, but it will also allow "native"
(non-cross) building using qemu as well as kernel personalities (i386
on amd64 etc, which is already supported).  When we come to run
dpkg-buildpackage, we will then pass the build arch which will result
in a cross-build if this differs from the host arch.

Hopefully this will implement support fairly cleanly and flexibly.

Following on from that there's the issue of resolver support and
use of xapt.  Some examples of how xapt would be used would help
here; I'm a bit unsure of if it's used directly as an apt-get
wrapper, or if it's a multi-step process.  Understanding the
workflow would be most useful.


Regards,
Roger

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