implement dpkgs vision of interpreting pkg:<arch> dependencies

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.

1 files changed, 14 insertions, 20 deletions

diff --git a/test/integration/test-apt-cache b/test/integration/test-apt-cache
index 97d180a74..7927686fc 100755
--- a/test/integration/test-apt-cache
+++ b/test/integration/test-apt-cache
@@ -120,12 +120,9 @@ bar
   Depends: bar
   Breaks: foo
   Replaces: foo
-  Breaks: <foo:i386>
-  Replaces: <foo:i386>
 <foobar>
 <cool>
-<cooler>
-<foo:i386>' aptcache depends foo --recurse --implicit
+<cooler>' aptcache depends foo --recurse --implicit
 testsuccessequal 'foo
   Depends: bar
 bar
@@ -135,9 +132,7 @@ testsuccessequal 'foo
   Depends: bar
 bar
   Depends: bar
-  Replaces: foo
-  Replaces: <foo:i386>
-<foo:i386>' aptcache depends foo --recurse --important --replaces --implicit
+  Replaces: foo' aptcache depends foo --recurse --important --replaces --implicit
 testsuccessequal 'bar
   Depends: bar
   Breaks: foo
@@ -145,38 +140,37 @@ testsuccessequal 'bar
 testsuccessequal 'bar
   Depends: bar
   Breaks: foo
-  Replaces: foo
-  Breaks: <foo:i386>
-  Replaces: <foo:i386>' aptcache depends bar --implicit
+  Replaces: foo' aptcache depends bar --implicit
+
 testsuccessequal 'specific
   Depends: <bar:i386>
-  Depends: specific:amd64
-  Breaks: foo:amd64
+  Depends: <specific:amd64>
+    specific
+  Breaks: <foo:amd64>
   Replaces: <foo:i386>' aptcache depends specific
 testsuccessequal 'specific
   Depends: <bar:i386>
-  Depends: specific:amd64
-  Breaks: foo:amd64
+  Depends: <specific:amd64>
+    specific
+  Breaks: <foo:amd64>
   Replaces: <foo:i386>' aptcache depends specific --implicit
 
-## rdpends
+## rdepends
 
+# Note that specific does not appear in this list as it doesn't depend on foo,
+# but on an arch-specific foo!
 testsuccessequal 'foo
 Reverse Depends:
   bar
-  specific
   bar' aptcache rdepends foo
 testsuccessequal 'foo
 Reverse Depends:
   Breaks: bar
-  Breaks: specific
   Replaces: bar' aptcache rdepends foo -o APT::Cache::ShowDependencyType=1
 testsuccessequal 'foo
 Reverse Depends:
   Breaks: bar (<< 1)
-  Breaks: specific (<< 1)
   Replaces: bar (<< 1)' aptcache rdepends foo -o APT::Cache::ShowDependencyType=1 -o APT::Cache::ShowVersion=1
 testsuccessequal 'foo
 Reverse Depends:
-  Breaks: bar (<< 1)
-  Breaks: specific (<< 1)' aptcache rdepends foo -o APT::Cache::ShowDependencyType=1 -o APT::Cache::ShowVersion=1 --important --breaks
+  Breaks: bar (<< 1)' aptcache rdepends foo -o APT::Cache::ShowDependencyType=1 -o APT::Cache::ShowVersion=1 --important --breaks