Bug#1120799: Using python-apt's apt_pkg with a chroot

[Sebastiaan Couwenberg <sebastic@xs4all.nl>]

On 12/2/25 7:44 PM, Sebastiaan Couwenberg wrote:
> On 12/2/25 7:17 PM, Johannes Schauer Marin Rodrigues wrote:
> > Quoting Sebastiaan Couwenberg (2025-12-02 18:36:00)
> > > Using python-apt's apt_pkg with a chroot turns out to be non-trivial unlike apt.cache.
> > >
> > > I'm running a script [0] on a trixie system and need to interact with the cache in a testing chroot.
> > >
> > > The chroot is created with mmdebstrap --mode=unshare, and the script is run via unshare as well.
> >
> > maybe this is an XY problem.
> >
> > Why do you need a full chroot for something that is as far as I get from the
> > script just a "I want apt to resolve the dependencies with my custom
> > sources.list"? This sounds like something like chdist is doing?
>
> I want to use apt to resolve dependencies for packages (both source & binary) in testing (and unstable), but the code won't be running on a testing (or unstable) system.
>
> See #1120799 for context.
>
> > Maybe the mmdebstrap man page has an example which helps you. It is a snippet
> > which is the very bare-bones of what mmdebstrap does with a few thousand more
> > lines of code:
> >
> >      mkdir -p "$2/etc/apt" "$2/var/cache" "$2/var/lib"
> >      cat << END > "$2/apt.conf"
> >      Apt::Architecture "$(dpkg --print-architecture)";
> >      Apt::Architectures "$(dpkg --print-architecture)";
> >      Dir "$(cd "$2" && pwd)";
> >      Dir::Etc::Trusted "$(eval "$(apt-config shell v Dir::Etc::Trusted/f)"; printf %s "$v")";
> >      Dir::Etc::TrustedParts "$(eval "$(apt-config shell v Dir::Etc::TrustedParts/d)"; printf %s "$v"
> >
> >      END
> >      echo "deb http://deb.debian.org/debian/ $1 main" > "$2/etc/apt/sources.list"
> >      APT_CONFIG="$2/apt.conf" apt-get update
> >      APT_CONFIG="$2/apt.conf" apt-get --yes --download-only install '?essential'
> >      for f in "$2"/var/cache/apt/archives/*.deb; do dpkg-deb --extract "$f" "$2"; done
> >      chroot "$2" sh -c "dpkg --install --force-depends /var/cache/apt/archives/*.deb"
>
> How would python-apt's apt_pkg use this?
>
> > > Setting RootDir in the config does not result in the chroot paths to be used:
> >
> > I think you need Dir.
>
> Not according to the apt.conf(5):
>
> "
>    The configuration item RootDir has a special meaning. If set, all paths will be relative to RootDir, even paths that are specified absolutely. So, for instance, if RootDir is set to /tmp/staging and Dir::State::status is set to /var/lib/dpkg/status, then the status file will be looked up in /tmp/staging/var/lib/dpkg/status. If you want to prefix only relative paths, set Dir instead.
> "
>
> Using Dir fails in the same way:
>
>   [...]

python-apt fails to use the chdist the same way it fails to use the chroot, Perl AptPkg uses both as expected.

You can reproduce this with the scripts from my transition-autopkgtest branch:

 https://salsa.debian.org/sebastic/release.debian.org/-/commit/1d872c5a1d30ce5bea2064de75747ea4b6f0e73b

Perl AptPkg initializes the system via the config:

 $_config->init;
 if ($cfg{use_chdist}) {
     $_config->set('Dir', $cfg{chdist_path});
 }
 else {
     $_config->set('RootDir', $cfg{chroot_path});
 }
 $_system = $_config->system;

Whereas python-apt initializes them separately:

 apt_pkg.init_config()
 apt_pkg.init_system()

 config = apt_pkg.Configuration()
 if args.use_chdist:
     config.set('Dir', args.chdist_path)
 else:
     config.set('RootDir', args.chroot_path)

The modified config is not used by the python-apt classes.

The config dump for Perl AptPkg shows a mix of the system apt.conf and the config set in the code, the config dump for python-apt shows only the config set in the code.

Kind Regards,

Bas

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