apt-mirror - snapshotting apt mirror for Ubuntu

apt-mirror is a snapshotting / "time-travelling" mirror for the Ubuntu
archive, using the Launchpad Librarian interface to access old packages.
We (MokaFive) developed it to allow us to effectively freeze the Ubuntu
archive when we had a release branch for our product cycle, so that
further SRUs would not impact our release unless manually overriden.
This also allows us to build old release branches from any point (for
point releases of our product) without incorporating the Ubuntu updates
since that point.

To use the mirror to see the repository as of a certain time, simply
access the web server with the first path component being the UNIX time
stamp of the desired date. The server will serve the most recent
snapshotted metadata as of that date. For instance, to access the Ubuntu
archive as of January 1, 2012, use a sources.list line like

   deb http://apt-mirror:9999/1325376000/ oneiric main universe

The original metadata files are signed by Ubuntu, so this does not
require adding a different apt key.

It consists of two main parts:
 - a cronjob, apt-mirror-snapshot, to copy archive metadata (packages
   and release files) into a git repository, so that they can be
   retrieved later by date
 - a web server, apt-mirror-web, to serve the archive metadata as of a
   certain requested date, and serve the actual packages referenced by
   that metadata by acting as a caching proxy to the Launchpad Librarian.

There is also a cron job to clean up cached packages that have not been
accessed recently.

Note that the git repository of metadata can get very large over a long
period of snapshotting, even with git's compression. MokaFive has five
gigabytes of archives dating from June 2011; if you are interested,
contact opensource@mokafive.com.

apt-mirror requires:
 - python2.6
 - git
 - rsync
 - python-cherrypy3
 - python-pycurl
 - python-routes
 - a filesystem with atime

To install:
 - Setup a cron job to run apt-mirror-snapshot. Should ideally be run hourly
 - Setup a cron job to run apt-mirror-cleanup. Should be run daily or weekly
 - Configure apt-mirror-web to run at boot. It doesn't daemonize. Example Upstart job:

# cat /etc/init/apt-mirror.conf

start on networking and filesystem
stop on runlevel [016]
exec /path/to/apt-mirror-web