Re: Filesystem Hierarchy Standard 2.0 (fwd)
Fabrizio:
> Anyway [a] proposal was harshly rejected with the assertion that "dpkg
> have problems with symlinks to directories", argument that I can't
> debate. Maybe now that Klee and Ian have time again for Debian, we
> could hear a more informed opinion.
I am here, honest. Technical details:
Basically, dpkg will _follow_ symlinks when installing packages. So
if a package contains /usr/share/doc/foo/bar and /usr/share/doc is a
link to /usr/doc then dpkg will install the file in /usr/doc/foo/bar
(but record it as being installed as /usr/share/doc/foo/bar).
Likewise, if /usr/doc is a link to /usr/share/doc and a package
contains /usr/doc/foo/bar then dpkg will install it in
/usr/share/doc/foo/bar (but record it as /usr/doc/foo/bar).
dpkg will not replace a directory with a symlink, nor will it replace
a symlink to a directory with a directory.
The intent is that packages with both locations for a directory can
coexist, provided the right symlinks are in place; dpkg won't handle
moving the directory - that has to be done by scripts - but it will
`do as it is told' by the filesystem.
The best first thing to do is probably to make a version of base-files
that has links from the _new_ names to the _old_ ones, once we decide
what things we're going to change and how.
Then packages can start containing the new layout provided that they
say `Pre-Depends: base-files (>= whatever)'. dpkg will then install
the stuff in the old locations.
When most of our packages are converted we can release a version of
base-files which contains the opposite situation: the new directories
and old links. That will make newly-installed systems have the new
layout and old systems have the old, and both old and new packages
will work on either.
Later we can have a version of base-files whose postinst switches the
directories and links around. This script has to be written with some
care, and will inevitably leave the system in a very bad state for a
few instants while it's running because of the lack of an atomic
`change all of this' operation in the filesystem. However, if written
with a good deal of care it could minimise the time during which the
system is in this bad state, and it can probably be made very-nearly
idempotent, so that only users whose systems crash during a very small
interval will suffer problems.
In the far future we can remove the compatibility links from
base-files. This has to be considered carefully, because after those
links are gone installing an old package will make `duplicate'
instances of the moved directories.
Ian.
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