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How the Multi-Arch field and pkg:<arch> dependencies interact was
discussed at DebConf15 in the "MultiArch BoF". dpkg and apt (among other
tools like dose) had a different interpretation in certain scenarios
which we resolved by agreeing on dpkg view – and this commit realizes
this agreement in code.
As was the case so far libapt sticks to the idea of trying to hide
MultiArch as much as possible from individual frontends and instead
translates it to good old SingleArch. There are certainly situations
which can be improved in frontends if they know that MultiArch is upon
them, but these are improvements – not necessary changes needed
to unbreak a frontend.
The implementation idea is simple: If we parse a dependency on foo:amd64
the dependency is formed on a package 'foo:amd64' of arch 'any'. This
package is provided by package 'foo' of arch 'amd64', but not by 'foo'
of arch 'i386'. Both of those foo packages provide each other through
(assuming foo is M-A:foreign) to allow a dependency on 'foo' to be
satisfied by either foo of amd64 or i386. Packages can also declare to
provide 'foo:amd64' which is translated to providing 'foo:amd64:any' as
well.
This indirection over provides was chosen as the alternative would be to
teach dependency resolvers how to deal with architecture specific
dependencies – which violates the design idea of avoiding resolver
changes, especially as architecture-specific dependencies are a
cornercase with quite a few subtil rules. Handling it all over versioned
provides as we already did for M-A in general seems much simpler as it
just works for them.
This switch to :any has actually a "surprising" benefit as well: Even
frontends showing a package name via .Name() [which doesn't show the
architecture] will display the "architecture" for dependencies in which
it was explicitely requested, while we will not show the 'strange' :any
arch in FullName(true) [= pretty-print] either. Before you had to
specialcase these and by default you wouldn't get these details shown.
The only identifiable disadvantage is that this complicates error
reporting and handling. apt-get's ShowBroken has existing problems with
virtual packages [it just shows the name without any reason], so that
has to be worked on eventually. The other case is that detecting if a
package is completely unknown or if it was at least referenced somewhere
needs to acount for this "split" – not that it makes a practical
difference which error is shown… but its one of the improvements
possible.
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Gbp-Dch: ignore
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Expecting the worst is easy to code, but has its disadvantages e.g.
by creating package structures which otherwise would have never
existed. By creating the provides instead at the time a package
structure is added we are well prepared for the introduction of partial
architectures, massive amounts of M-A:foreign (and :allowed) and co as
far as provides are concerned at least. We have something relatively
similar for dependencies already.
Many tests are added for both M-A states and the code cleaned to
properly support implicit provides for foreign architectures and
architectures we 'just' happen to parse.
Git-Dch: Ignore
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We use test{success,failure} now all over the place in the framework, so
its only consequencial to do this in the situations in which we test for
a specific output as well.
Git-Dch: Ignore
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Previously, we had a start and a done of the calculation printed by
higher-level code, but this got intermixed by progress reporting from an
external solver or the output of autoremove code…
The higherlevel code is now only responsible for instantiating a
progress object of its choosing (if it wants progress after all) and the
rest will be handled by the upgrade code. Either it is used to show the
progress of the external solver or the internal solver will give some
hints about its overall progress. The later isn't really a proper
progress as it will jump forward after each substep, but that is at
least a bit better than before without any progress indication.
Fixes also the 'strange' non-display of this progress line in -q=1, while
all others are shown, which is reflected by all testcase changes.
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APT supported versioned provides for a long while in an attempt to get
it working with rpm. While this support is old, we can be relatively
sure that it works as versioned provides are used internally to make
Multi-Arch:foreign work.
Previous versions of apt will print a warning indicating that the
versioned provides is ignored, so that something which "Provides: foo (=
2)" doesn't provide anything.
Note that dpkg does allow only a equals-relation in the provides line
as anything else is deemed too complex. apt doesn't support anything
else either and such a support would require potentially big changes.
Closes: 758153
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