Multi-arch all-architecture plugins

[Ian Jackson <ijackson@chiark.greenend.org.uk>]

Goswin von Brederlow writes ("Re: Please test gzip -9n - related to dpkg with multiarch support"):
> Steve Langasek <vorlon@debian.org> writes:
> >  the [pam] module packages should be installed
> > for all archs, not just a subset[1].
> 
> Ok, that is acceptable. We just lack any technical means to ensure this
> so far. Same problem as for input method plugins for example.

So we need a new mechanism for this.

Where should this fact be declared ?  Is it a property of a package
that it makes sense to install it only on all configured architectures
or none ?  Or is it a property of the dependency from the depending
package ?

I'm inclined to the former view.  After all if you think about
installing some plugin, there may not even /be/ a dependency in
question.  And I'm finding it difficult to imagine a package which has
this all-arches-needed multiarch property for some of its purposes but
not others.

This situation can only arise for a m-a:same package, since only those
are coinstallable.  I would suggest that the right answer is a new
value for the Multi-Arch field, let's call it "all".  It would work
like Multi-Arch: same except that:

 * Dependencies on multiarch:all package are not satisfied unless
   the package is suitably installed and configured on all configured 
   architectures.  Ie with

      Package: plugin
      Multi-Arch: all
      Architecture: i386

      Package: plugin
      Multi-Arch: all
      Architecture: amd64

   this dependency

      Depends: plugin

   is read as

      Depends: plugin:i386, plugin:amd64

 * The higher-level package manager, when it is asked to install a
   multiarch:all package, will install it for all configured
   architectures.

Ian.