[RFC] stowfs translators architecture and parameters

[Gianluca Guida]

Hello,

after a week spent on thinking about the way we could sanely develop
the stowfs translators, I came up with the following idea, which I am
going to implement if people reading this will like it.

In my thoughts, stowfs will be a translator to be set in /stow or any
directory containing links to packages directory.

We could run an example stowfs like this:

# settrans -capfg /stow  /hurd/stowfs -T /hurd/unionfs \
       -d /bin bin  -d /sbin sbin  -d /lib lib local/lib

With this command line, the stowfs translator will set three other 
translators:

1) in /bin: /hurd/unionfs  /stow/pkg1/bin /stow/pkg2/bin ...
2) in /sbin: /hurd/unionfs  /stow/pkg1/sbin /stow/pkg2/sbin ...
3) in /lib: /hurd/unionfs  /stow/pkg1/lib /stow/pkg1/local/lib /stow/pkg...

where pkg1, pkg2, ... are the packages installed on the system.

When a new package is installed, new directories to merge will be
added runtime as argument to the translators in /bin, /sbin, /lib.
When removed we can add the option -r followed by the name of
the directory to be removed. Unionfs support this syntax already.

That's not all. On previous threads there was a talking about the
need to do smarter-than-just-union merge on certain files, like the
ones in /etc.
As Alfred suggested, it would be much better to have a smarter and
specific translator, e.g.  "/hurd/etcfs" than include these rules in a
generic stowfs.
This "etcfs" could accept a syntax compatible with the unionfs one,
so that we could set our stowfs to resolve in a clever way conflicts
on /etc files with this:

# settrans -capfg /stow  /hurd/stowfs -T /hurd/unionfs \
       -d /bin bin  -d /sbin sbin  -d /lib lib \
       -T /hurd/etcfs  -d /etc etc

This will infact set -- together with all the others -- the /hurd/etcfs
translator in /etc, passing it as arguments the etc directory entries
in the packages tree.

What I like of this approach is that makes the stowfs a
not-so-complicated translator with a very specific goal and that
other specific translators can be added to manage special
directories or even single files.

I hope I had been clear enough.

Gianluca

-- 
It was a type of people I did not know, I found them very strange and
they did not inspire confidence at all. Later I learned that I had been
introduced to electronic engineers.
                                                           E. W. Dijkstra