Update Text Update #6001 to Batch

Here's a modification of the update to use the built-in batching functionality:

function text_update_6001(&$sandbox) {
  include_once('./'. drupal_get_path('module', 'content') .'/content.install');
  $ret = array();

  if (!isset($sandbox['progress'])) {
    if ($abort = content_check_update('text')) {
      return $abort;
    }

    drupal_load('module', 'content');
    // Get the latest cache values and schema.
    content_clear_type_cache(TRUE, TRUE);

    $types = content_types_install();

    if (empty($types)) {
    	return $ret;
	}

	$sandbox['fields'] = array();
	foreach ($types as $type_name => $fields) {
	  foreach ($fields as $field) {
	    if ($field['type'] == 'text') {
	      $sandbox['fields'][] = $field;
		}
	  }
	}

	if (empty($sandbox['fields'])) {
	  return $ret;
	}

	$sandbox['progress'] = 0;
	$sandbox['visited'] = array();
  }

  $field = $sandbox['fields'][$sandbox['progress']];

  // We only want to process a field once -- if we hit it a second time,
  // that means it's its own table and it should have already been updated.
  if (!in_array($field['field_name'], $sandbox['visited'])) {
    $db_info = content_database_info($field);
    $table = $db_info['table'];
    foreach ($db_info['columns'] as $column => $attributes) {
      $attributes['not null'] = FALSE;
      $column = $attributes['column'];
      db_change_field($ret, $table, $column, $column, $attributes);
      // TODO: errors on text/blob columns: no default value allowed (!)
      db_field_set_no_default($ret, $table, $column);
      if ($attributes['type'] == 'varchar' || $attributes['type'] == 'text') {
        $ret[] = update_sql("UPDATE {". $table ."} SET ". $column ." = NULL WHERE ". $column ." = ''");
      }
      else {
        // TODO: replace format = 0 with format = NULL ?? Is this right ?
        $ret[] = update_sql("UPDATE {". $table ."} SET ". $column ." = NULL WHERE ". $column ." = 0");
      }
    }
    $sandbox['visited'][] = $field['field_name'];
  }

  $sandbox['progress']++;
  $ret['#finished'] = $sandbox['progress'] / count($sandbox['fields']);

  return $ret;
}

Then you also have to update the 6003 update as well:

function text_update_6003(&$sandbox) {
  return text_update_6001($sandbox);
}

With these changes, I've been able to complete the update with my data. There's also an issue where shared fields will have their table updated for each node type that shares it, which is somewhat wasteful. I admit that the $sandbox['visited'] part could probably be done a bit better, but this can hopefully be a starting point for something like this. The real question is whether this is the right approach for the update?