Add a more or less useful README file for everything related to MultiArch

and install it in the apt-doc package.

1 files changed, 106 insertions, 0 deletions

diff --git a/README.MultiArch b/README.MultiArch
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+Before we start with this topic: Note that MultiArch is not yet ready for
+prime time and/or for the casual user. The implementation is so far widely
+untested and only useful for developers of packagemanagment tools which
+use APT and his friends and maintainers of (upcoming) MultiArch packages.
+This README is especially NOT written for the casual user and is NOT a
+usage guide - you have been warned. It is assumed that the reader has
+at least a bit of knowledge about APT internals, dependency relations
+and the MultiArch spec [0].
+
+
+The implementation is focused on NOT breaking existing singleArch-only
+applications and/or systems as this is the current status-quo for all
+systems. Also, many systems don't need (or can't make use of) MultiArch,
+so APT will proceed in thinking SingleArch as long as it is not explicitly
+told to handle MultiArch:
+To activate MultiArch handling you need to specify architectures you
+want to be considered by APT with the config list APT::Architectures
+(Insert architectures in order of preference).
+APT will download Packages files for all these architectures in the
+update step. Exception: In the sourcelist is the optionfield used:
+deb [ arch=amd64,i386 ] http://example.org/ experimental main
+(This optionfield is a NOP in previous apt versions)
+
+Internally in APT a package is represented as a PkgIterator -
+before MultiArch this PkgIterator was architecture unaware,
+only VerIterators include the architecture they came from.
+This is/was a big problem as all versions in a package are
+considered for dependency resolution, so pinning will not work in all cases.
+
+The problem is solved by a conceptional change:
+A PkgIterator is now architecture aware, so the packages
+of foobar for amd64 and for i386 are now for apt internal totally
+different packages. That is a good thing for e.g. pinning, but
+sometimes you need the information that such packages are belonging together:
+All these foobar packages therefore form a Group accessible with GrpIterators.
+Note that the GrpIterator has the same name as all the packages in this group,
+so e.g. apt-cache pkgnames iterates over GrpIterator to get the package names:
+This is compatible to SingleArch as a Group consists only of a single package
+and also to MultiArch as a Group consists of possible many packages which
+all have the same name and are therefore out of interest for pkgnames.
+
+
+Caused by the paragraph "Dependencies involving Architecture: all packages"
+in the MultiArch spec we have a second major conceptional change
+which could even break existing applications, but we hope for the best…
+An Architecture: all package is internally split into pseudo packages
+for all MultiArch Architectures and additional a package with the
+architecture "all" with no dependencies which is a dependency of all
+these architecture depending packages. While the architecture depending
+packages are mainly used for dependency resolution (a package of arch A which
+depends on an arch all package assumes that the dependencies of this package
+are also from arch A. Packages also sometimes change from any to all or v.v.)
+the arch "all" package is used for scheduling download/installation of the
+underlying "real" package. Note that the architecture depending packages can
+be detected with Pseudo() while the "all" package reports exactly this arch
+as package architecture and as pseudo architecture of the versions of this pkg.
+Beware: All versions of a "real" architecture all package will be report "all"
+as their architecture if asked with Arch() regardless if they are the "all" or
+the architecture depending packages. If you want to know the architecture this
+pseudo package was created for call Arch(true). Also, while the spec say that
+arch:all packages are not allowed to have a MultiArch flag APT assigns a
+special value to them: MultiArch: all.
+
+
+As you might guess this arch:all handling has a few problems (but we think so
+far that the problems are minor compared to the problems we would have with
+other implementations.)
+APT doesn't know which pseudo packages of such an arch all package are
+"installed" (to satisfy dependencies), so APT will generate a Cache in which
+all these pseudo packages are installed (e.g. apt-cache policy will display
+them all as installed). Later in the DepCache step it will "remove"
+all pseudo packages whose dependencies are not satisfied.
+The expense is that if the package state is broken APT could come to the
+conclusion to "remove" too many pseudo packages, but in a stable environment
+APT should never end up in a broken system state…
+
+
+Given all these internal changes it is quite interesting that the actual
+implementation of MultiArch is trivial: Some implicit dependencies and a few
+more provides are all changes needed to get it working. Especially noteworthy
+is that it wasn't needed to change the resolver in any way and other parts only
+need to be told about ignoring pseudo packages or using GrpIterator instead of
+PkgIterator, so chances are good that libapt-applications will proceed to work
+without or at least only require minor changes, but your mileage may vary…
+
+
+Known Issues and/or noteworthy stuff:
+* The implementation is mostly untested, so it is very likely that APT will
+  eat your kids if you aren't as lucky as the author of these patches.
+* the (install)size of a pseudo package is always NULL - if you want to know
+  the (install)size you need to get the info from the arch "all" package.
+* It is maybe confusing, but the arch "all" package does have the same versions
+  and in general roughly the same information with one subtil difference:
+  It doesn't have any dependency, regardless of the type. The pseudo packages
+  depend on this package.
+* apt-cache policy foobar on installed architecture all package foobar will
+  report all architecture depending packages as installed. Displaying here the
+  correct information would require to build the complete DepCache…
+* [BUG] An installed package which changes the architecture from any to all
+  (and v.v.) shows up in the NEW packages section instead of UPGRADE.
+* [TODO] Investigate the DepCache pseudo-package killer heuristic:
+  e.g. add more safety guards…
+* [FIXME] a few corner cases/missing features marked as FIXME in the code
+
+
+[0] https://wiki.ubuntu.com/MultiarchSpec