CFEngine: upgrade Debian packages

say you use CFEngine to install Debian packages on your server, so it's likely you'll have a bundle looking like this:

bundle agent agentname
{
    vars:

        "packages" slist => {
                             "pkg1",
                             "pkg2",
                             "pkg3"
                            };

    packages:

        "$(packages)"
            package_policy => "addupdate",
            package_method => apt_get;

}

this works great to guarantee those packages are installed, but if a newer version is available in the repositories, that wont be installed. If you want CFEngine to do that too, then the web suggests this trick:

    packages:

        "$(packages)"
            package_policy => "addupdate",
            package_version => "999999999",
            package_method => apt_get;

which tweak the install system declaring that you want to install version 999999999 of each package, so if you have available a higher version than the one installed, CFEngine will happily upgrade it for you. It works great.. but sometimes it doesn't. why oh why?

That's because Debian versions can have a epoch: every plain version (like 1.0-1) has an implicit epoch of 0, and same goes for the 999999999 above, that means if any of the installed packages has an epoch, that version will sort higher than 999999999 and the package wont be upgraded. If you want to be sure to upgrade every package, then the right solution is:

    packages:

        "$(packages)"
            package_policy => "addupdate",
            package_version => "9:999999999",
            package_method => apt_get;