[pacman-dev] delta support in libalpm

[Xavier]

On Fri, Feb 27, 2009 at 4:55 PM, Aaron Griffin <aaronmgriffin at gmail.com> wrote:
>
> Hmm, good questions. I was thinking that we would simply run "repo-add
> --deltas" or something, but it still requires us to figure out the old
> package file and copy that as well.
>
> That's the main issue here - determining the old/existing package
> file. I guess it's not TOO hard, the more I think about it. I'll just
> have to extract the DB and parse through it, unless someone else can
> figure out a cleaner way to get the filename for an old package from a
> DB knowing the new package's name (and version and all the other meta
> info)
>
>>> To do this in the db-scripts, we need to open the existing DB, find
>>> the entry for this pkgname, and get the filename. Does pkgdelta
>>> correctly construct deltas in different directories, or do we also
>>> need to copy the file next to the new package before running pkgdelta
>>> need to be next to each other?
>>
>> The old package and new package can be anywhere. But the new delta
>> will be put next to the new package.
>> What would be the best output location for the new delta?
>
> That works fine. The way the dbscripts do operations is by:
> a) locking the existing DB (with a lock file)
> b) copying the db file to a temp dir
> c) copying all staging files to that temp dir
> d) adding everything to the db
> e) copying the entire temp dir to the existing db location
> f) unlocking the db
>
> So putting deltas in-place works fine - I was just hoping we wouldn't
> have to figure out additional files to copy to the temp dir for deltas

So, even if repo-add finds the old filename, which is easy to do, it
is not going to know in which directory that old package actually
sits. That's a big problem, I started to add delta generation to
repo-add but we have a show stopper here.

Here is the code I used to find the old filename :
      pkgentry=$(find_pkgentry $pkgname)
      if [ -n "$pkgentry" ]; then
        oldfilename=$(grep -A1 FILENAME $pkgentry/desc | tail -n1)
        oldfile="$(dirname $1)/$oldfilename"
      fi

So here I assumed that the old package file sits in the same directory
that the new package file (dirname $1), which is not the case in
dbscripts.

find_pkgentry was already defined in repo-add so that was practical to re-use.

find_pkgentry()
{
  local pkgname=$1
  local pkgentry
  for pkgentry in $gstmpdir/$pkgname*; do
    name=${pkgentry##*/}
    if [ "${name%-*-*}" = "$pkgname" ]; then
      echo $pkgentry
      return 0
    fi
  done
  return 1
}