Bug#749826: Documenting `Multi-Arch: foreign`
Hi Sean,
Thanks for picking up multiarch!
On Sat, Aug 19, 2017 at 09:50:21PM -0700, Sean Whitton wrote:
> I spoke to Russ and we're both of the view that we should document
> multiarch piecemeal. Let's begin by getting a definition of the
> Multi-Arch: field into ch. 5 of policy.
I'm glad you agree to my proposal.
> I have pushed a new branch to the Debian policy repo named
> bug749826-spwhitton. On that branch I've committed a slightly reworked
> form of your draft text.[1] Please review the diff. Here are some
> comments/issues:
Very welcome.
> - I substantially shortened your text. Let me know if you think I went
> too far.
I fear that some important aspects got lost indeed. More on that later.
> - Previously I was worried about defining 'interface' but I've found
> another place where policy uses this word without defining it, and I
> don't think it needs to be changed in either place.
I'm not a friend of vagueness, but I do recognize the difficulty in
expressing the requirements precisely.
> - I couldn't figure out how to include this text, because I didn't
> understand it:
>
> For instance, using dpkg --print-architecture can be used to emit the
> native architecture even though dpkg is marked Multi-Arch:
> foreign. Similarly, calling pkg-config (without a prefix) will behave
> differently on different architectures as its search path is
> architecture-dependent even thoug pkg-config is marked Multi-Arch:
> foreign.
>
> Are you saying that packages that depend or implicitly depend on dpkg
> or pkg-config cannot be Multi-arch: foreign, although dpkg and
> pkg-config themselves are Multi-arch: foreign? Why are dpkg and
> pkg-config Multi-arch: foreign, if they provide these
> architecture-dependent interfaces?
Those are very good questions and clarifying them will lead to a better
understanding of what we have to put into policy. You do understand that
"dpkg --print-architecture" is part of dpkg's interface. Yet its out
varies with its architecture. Taking this strictly would indeed imply
that dpkg is wrongly marked. Similarly, running pkg-config may result in
architecture-dependent paths and thus our strict interpretation would
result in rejecting the foreign marking.
A common theme with such cases is to resort to `Multi-Arch: allowed`
(e.g. make), but that has the downside of requiring most consumers to
attach the :any annotation and that it can never be switched back
(because :any dependencies on packages not marked M-A:allowed are
unsatisfiable).
This is where I thought about README.multiarch:
> - I didn't include your TODO about README.multiarch; let me know whether
> you have a more concrete idea about the purpose of that file
It can document assumptions one makes about users of a package. For
instance, we expect dpkg users to use `dpkg --print-architecture`
diagnostically only. Similarly, we expect that package builds call
pkg-config if they mean the build architecture and they need to call
$(DEB_HOST_GNU_TYPE)-pkg-config if they mean the host architecture.
Indeed that happens automatically for autotools projects that happen to
use PKG_CHECK_MODULES or PKG_PROG_PKG_CONFIG (i.e. most). It also
happens for cmake when built with dh_auto_build.
Let me give a counter example to illustrate more of the point.
haskell-devscripts-minimal is an `Architecture: all` package with some
shell scripts. Sounds like a good candidate for `Multi-Arch: foreign`.
When you look at /usr/share/haskell-devscripts/Dh_Haskell.sh though, you
see that functions such as cpu(), os(), etc. specifically introspect the
build architecture by using the build architecture ghc. Such usage is
not ok for `Multi-Arch: foreign` (#769377).
I believe that policy should encourage some uniform way to document the
intended interface as we have several cases where this is not obvious.
README.multiarch may be that way. In particular, using a package in a
way not permitted by such README.multiarch would need to be a policy
violation on its own. For instance, one could depend on a shared library
and declare it an implementation detail. Relying on the transitive
dependency would then be considered a policy violation.
> - after we've got text documenting the other possible values of the
> Multi-Arch: field, we might want to promote the list of things to
> consider out of the Multi-Arch: foreign subsubsection. It should
> become clear once we've got that other text together.
Indeed, documenting `Multi-Arch: same` may be easier (or not). For the
purpose of defining it, we shall call Debian binary packages for
different architectures with equal binary package name and version
"instances" of a package. I currently see the following requirements:
* It must not be used on `Architecture: all` packages (though I wish
you could ;).
* Given any two instances of a package and any filename, that filename
must be non-existent in at least one package or the type (directory /
regular file / etc.) must match and when the filename refers to a
regular file, the contents must be bitwise equal in both instances.
This point deserves some more thought as it has some pitfalls:
* If there is such a conflict, but the relevant packages are not
coinstallable due to package relations, is that ok? Example: libc6
has such a conflict on /lib/ld.so.1 for mips and mipsel.
(Presently, you get an unpack error here.)
* If you rebuild the source package with a very different
installation set (i.e. much newer Build-Depends), does it still
have to match with older instances? Example: #825146. What
divergence in installation sets is ok?
(A simple way to satisfy this requirement is to use
architecture-dependent paths exclusively. That works except for
/usr/share/doc/$pkg.)
* The maintainer scripts must handle multiple configuration and
multiple deconfiguration correctly. In particular, a package can be
purged for one architecture while being installed for another.
Example: #682420.
(A simple way to satisfy this requirement is to not ship maintainer
scripts.)
* Source packages carrying any binary package marked `Multi-Arch: same`
must always be binNMUed in lock-step. (Presently violated e.g. by
libselinux1)
> Thank you again for your work so far.
Ah right and you asked for a review of the branch. ;)
I think "the files installed by ``Architecture: all`` packages always
provide architecture-independent interfaces." is too broad. The counter
example is haskell-devscripts-minimal. This needs to be weakened
somehow.
Your shortening of my draft has significantly cut precision and improved
readability. It is difficult to strike a useful balance here and I am
certainly biased towards precision. I'm unsure how to proceed here. I
believe that your current form will cause discussion and dispute among
policy readers. For instance, the policy should make it clear that
marking libmdds-dev `Multi-Arch: foreign` (fictional, see #843023) would
be a policy violation.
So the current text falls short on objections e.g. Bill Allombert raised
on the reproducible issue: Can I mechanically check policy requirements?
It is actually worse here: Given a Multi-Arch bug report, it is
difficult to determine whether your draft supports it due to the lack of
precision.
I think having Bill or Russ give more insight on the
precision/readability balance would be useful to me.
The current conversation looks very productive to me. Thanks for your
effort.
Helmut
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