DragonFly kernel List (threaded) for 2003-10
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Re: variant symlinks vs VFS, and microkernels vs error kernels
:On Fri, Oct 03, 2003 at 03:36:49PM -0700, Matthew Dillon wrote:
:> We aren't going to be doing anything esoteric when we implement
:> variant symlinks... as far as programs like 'ls' are concerned, they
:> will simply be symlinks.
:
:Perhaps these symlinks should have a special prefix like AFS mount
:points have? That way namei can easily identify variant symlinks
:and skip the expansion process for normal symlinks. It is also
:important to provide a way to access a file with '$' or other
:special symbols. With a prefix character most applications will
:continue to work without any change.
I think simply embedding something like ${VARNAME} in the link,
e.g.
ln -s '/usr/${A1}${A2}/${A3}' fubar
Would be sufficient. We could still have a prefix character (by
having the 'ln' program prefix it automatically), but the namei/lookup
code copies the symlink into a buffer anyway and checking for the
expansion on the fly would not represent a noticeable cost.
Also, if a particular variable cannot be parsed or does not exist,
we could skip that variable. So if there is no 'A2', then the
system might resolve the above symlink to "/usr/charlie${A2}/chan".
And note that readlink() is unaffected. It is only when the system
itself is asked to traverse a symlink where the variables are replaced.
So an ls -la will still show '/usr/${A1}${A2}/${A3}' (and thus programs
that store binary data in symlinks will still work as expected).
:> The VFS environments serve a larger purpose. For example, the only way
:> you can be sure that you have specified the correct dependancies in
:> a port config file is to do the build in an environment which heavily
:> restricts what the build system actually sees. build, installation, and
:> execution stages would all need different verification environments.
:
:With packages of the base system we can deploy chroot() for package
:building. That way only the dependencies are in place and the problem
:of base packages like gcc or perl vanishes. One thing to keep in mind
:with the compiler is the C++ incompatibility between 2.95.x and 3.x.
:Having C++ libraries in base other then libstdc++ is problematic.
Precisely, or a jailed VFS, which would be better.
:>
:> Another example where a custom VFS environment would be useful is in
:> a jailed or emulation environment.
:
:VFS environment are the ideal solution for jails ;-) No need for a
:second system installed, just a PLIST which files are there.
:
:Joerg
Yes, and because the namecache topology will be fixed it will be
possible to alias directories without creating ".." confusion in
the system.
-Matt
Matthew Dillon
<dillon@xxxxxxxxxxxxx>
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