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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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Now that we can dynamically create dependencies and provides as needed
rather than requiring to know with which architectures we will deal
before running we can allow the listparser to parse all records rather
than skipping records of "unknown" architectures.
This can e.g. happen if a user has foreign architecture packages in his
status file without dpkg knowing about this architecture (or apt
configured in this way).
A sideeffect is that now arch:all packages are (correctly) recorded as
available from any Packages file, not just from the native one – which
has its downsides for the resolver as mixed-arch source packages can
appear in different architectures at different times, but that is the
problem of the resolver and dealing with it in the parser is at best a
hack (and also depends on a helpful repository).
Another sideeffect is that his allows :none packages to appear in
Packages files again as we don't do any kind of checks now, but given
that they aren't really supported (anymore) by anyone we can live with
that.
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Now that we deal with provides in a more dynamic fashion the last
remaining problem is explicit dependencies like 'Conflicts: foo' which
have to apply to all architectures, but creating them all at the same
time requires us to know all architectures ending up in the cache which
isn't needed to be the same set as all foreign architectures.
The effect is visible already now through as this prevents the creation
of a bunch of virtual packages for arch:all packages and as such also
many dependencies, just not very visible if you don't look at the stats…
Git-Dch Ignore
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Before MultiArch implicits weren't a thing, so they were hidden by
default by definition. Adding them for MultiArch solved many problems,
but having no reliable way of detecting which dependency (and provides)
is implicit or not causes problems everytime we want to output
dependencies without confusing our observers with unneeded
implementation details.
The really notworthy point here is actually that we keep now a better
record of how a dependency came to be so that we can later reason about
it more easily, but that is hidden so deep down in the library internals
that change is more the problems it solves than the change itself.
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Cache generation needs a way of quickly iterating over the unique potion
of the dependencies to be able to share them. By linking them together
we can reduce the speed penality (~ 80%) with only a small reduction in
saved size (~ 20%).
Git-Dch: Ignore
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DepCache functions are called a lot, so if we can squeeze some drops out
of them for free we should do so. Takes also the opportunity to remove
some whitespace errors from these functions.
Git-Dch: Ignore
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It is a rather strange sight that index items use SiteOnly which strips
the Path, while e.g. deb files are downloaded with NoUserPassword which
does not. Important to note here is that for the file transport Path is
pretty important as there is no Host which would be displayed by Site,
which always resulted in "interesting" unspecific errors for "file:".
Adding a 'middle' ground between the two which does show the Path but
potentially modifies it (it strips a pending / at the end if existing)
solves this "file:" issue, syncs the output and in the end helps to
identify which file is meant exactly in progress output and co as a
single site can have multiple repositories in different paths.
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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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Usually they don't provide a lot in terms of what they test, but they
help in covering many lines from strictly anecdotal commands (stats,
moo) and error messages, so that stuff which really needs to be tested,
but isn't is better visible in coverage reports.
Git-Dch: Ignore
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