def tarball_name(spec, ext):
    """
    Return the name of the tarfile according to the convention
    <os>-<architecture>-<package>-<dag_hash><ext>
    """
    return "%s-%s-%s-%s-%s%s" % (spec.architecture,
                                 str(spec.compiler).replace("@", "-"),
                                 spec.name,
                                 spec.version,
                                 spec.dag_hash(),
                                 ext)