human
Solve the following python programming problem given the following code:
@ skip_backend (unsupported = { backends.LOOPBACK }, reason = "Does not maintain compute_instance_id across restarting flocker (and didn't as of most recent release).") 
@ skip_backend (unsupported = { backends.GCE }, reason = 'GCE was not available during the most recent release.') 
@ run_test_with (async_runner (timeout = timedelta (minutes = 6))) 
@ require_cluster (1) 
def test_upgrade(self, cluster) : 
    '
        Given a dataset created and used with the previously installable
        version of flocker, uninstalling the previous version of flocker and
        installing HEAD does not destroy the data on the dataset.
        ' 
    node = cluster.nodes [0] 
    SAMPLE_STR = ('123456' * 100) 
    upgrade_from_version = get_installable_version (HEAD_FLOCKER_VERSION) 
    d = cluster.client.version () 
    original_package_source = [None] 
    def setup_restore_original_flocker(version) : 
        version_bytes = version.get ('flocker', '').encode ('ascii') 
        original_package_source [0] = self._get_package_source (default_version = (version_bytes or None)) 
        self.addCleanup ((lambda : cluster.install_flocker_version (original_package_source [0]))) 
        return version 
d.addCallback (setup_restore_original_flocker) 
    d.addCallback ((lambda _ : cluster.clean_nodes ())) 
    d.addCallback ((lambda _ : cluster.install_flocker_version (PackageSource (version = upgrade_from_version), destroy_persisted_state = True))) 
    d.addCallback ((lambda _ : create_dataset (self, cluster, node = node))) 
    first_dataset = [None] 
    def write_to_file(dataset) : 
        first_dataset [0] = dataset 
        return node.run_as_root (['bash', '-c', ('echo "%s" > %s' % (SAMPLE_STR, os.path.join (dataset.path.path, 'test.txt')))]) 
d.addCallback (write_to_file) 
    d.addCallback ((lambda _ : cluster.install_flocker_version (original_package_source [0]))) 
    d.addCallback ((lambda _ : create_dataset (self, cluster, node = node))) 
    d.addCallback ((lambda _ : cluster.wait_for_dataset...
    
    Given the code above, what is a proper replacement for <MASK>? Choose among: original_package_source, self, setup_restore_original_flocker, cluster, node, write_to_file, SAMPLE_STR, first_dataset, upgrade_from_version, d
assistant
#Here is a proper replacement for <MASK>.
def cat_and_verify_file(