def upgrade_environment():
    """Actually perform an environment upgrade.

        Implementations of this method don't need to commit any
        database transactions. This is done implicitly for each
        participant if the upgrade succeeds without an error being
        raised.

        However, if the `upgrade_environment` consists of small,
        restartable, steps of upgrade, it can decide to commit on its
        own after each successful step.
        """
