Cross-compiling a flatpak

[Corentin Noël]

Hi everyone,

I'm playing a little bit with flatpak and wanted to experiment cross-
compilation with it.
I haven't seen anything regarding cross-compilation right now, but I'm
wanting to further investigate this issue.

I wanted to know if you had some thoughts about it, or if there was
some plans documented somewhere (or in someone's mind)

Here are my thoughts when I look at this issue:

Let's imagine that I'm using a x86_64 system and want to build a
`bar.foo.HelloWorld` application targetting the aarch64 architecture.
 * my application will target the `org.freedesktop.Platform//1.6`
runtime and require the `org.freedesktop.Sdk//1.6` sdk
 * I start a build with `flatpak build` using a new --target=aarch64
flag showing my intention to cross-build
   * `org.freedesktop.Sdk/x86_64/1.6` and
`org.freedesktop.Platform/x86_64/1.6` are mounted as usual to start the
compilation.
   * the target flag makes flatpak mount a
`org.freedesktop.Sdk.Extension.Cross-aarch64/x86_64/1.6` SDK extension
(mounted under /usr/lib/sdk/Cross-aarch64), this extension contains all
the toolchain for building to the aarch64 architecture. (and it might
also contain some useful things for cross-compilation like a meson
crossfile definition)
   * the target flag makes flatpak mount
`org.freedesktop.Platform/aarch64/1.6` and
`org.freedesktop.Sdk/aarch64/1.6` (and their dependencies) under
/sysroots/aarch64/ as it is doing for the native architecture in /
   * the build happens targetting the sysroot in /sysroots/aarch64/
   * the resulting flatpak is define to be aarch64

So with the current flatpak implementation I'm mostly missing two
parts:
 * Be able to mount at build time a Sdk & Runtime of a foreign
architecture in a well-defined location
 * Define the resulting flatpak as being of another architecture

Do you think that it's the right way to do it and that it's something
that might be implemented in flatpak?