[pacman-dev] delta support in libalpm

[Brendan Hide]

Xavier wrote:
> There has never been any real official interests for delta. This seems
> to make a requirement the ability to make a separate delta server.
> This seems to require a separate delta database. This implies a new
> level of complexity and code bloat in pacman. Now maybe it is worth
> it, I don't know, it still makes me wondering why we put all this
> delta stuff in pacman to begin with. What was the problem with
> XferCommand, it seemed like it was a great idea. Now that
> wget-xdelta.sh script is just a toy, but a much more powerful python
> script could be written that has basically the same logic as pacman
> currently has + the ability to fetch and parse a separate delta
> database.  
Unless the server is out of disk space, I'm not too sure exactly why 
there's a requirement for a separate server. If pacman is distributed 
with the delta option turned on by default, the server doing the actual 
"serving" of the updates is probably going to have 60 to 85% less work 
to do.

I will grant that there would be a new level of complexity involved, for 
example, if I've missed 4 updates, we'd have to "chain link" the tar.gz 
in my cache via 4 delta patches to get the current tar.gz.

I believe that the following would be the simplest implementation both 
in terms of how much implementation work is needed and the probable 
effectiveness:
Put delta files into a separate folder (thus also avoiding a snapshot 
from containing the deltas):
http://archlinux.mirror.ac.za/delta/core/os/x86_64/kernel26-2.6.28.4-1-x86_64.kernel26-2.6.28.5-1.pkg.xd3.tar.gz
Thus, I could do the following (bash pseudocode)
curl http://archlinux.mirror.ac.za/delta/core/os/x86_64/ > tmpfile
grep $pkgname < tmpfile > listing
failed=false
cat listing | while read delta
 do
  [ $pkgname-$currentpkgversion-$pkgarch.xd3.tar.gz *within* $delta ] && 
start=true
 if [ start=true ]
 then while read delta
  do
   wget http://archlinux.mirror.ac.za/delta/core/os/x86_64/$delta && 
applydelta $delta $curfile
   [ $output=$pkgname-$newpkgversion-$pkgarch.tar.gz ] && break
   curfile=`ls -rt | tail -n 1`
  done
 fi
 [ $output=$pkgname-$newpkgversion-$pkgarch.tar.gz ] && break
done

The above requires no db implementation at all and can work well even 
using the above very simple logic.
And yes, by my own standards, the above is very bad bash pseudo-code. :P

Of the above, what is already implemented in pacman?

__________
Brendan Hide