<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def ensure_benchmark_data(symbol, first_date, last_date, now, trading_day, environ=None):
 """ Ensure we have benchmark data for `symbol` from `first_date` to `last_date` Parameters symbol : str The symbol for the benchmark to load. first_date : pd.Timestamp First required date for the cache. last_date : pd.Timestamp Last required date for the cache. now : pd.Timestamp The current time. This is used to prevent repeated attempts to re-download data that isn't available due to scheduling quirks or other failures. trading_day : pd.CustomBusinessDay A trading day delta. Used to find the day before first_date so we can get the close of the day prior to first_date. We attempt to download data unless we already have data stored at the data cache for `symbol` whose first entry is before or on `first_date` and whose last entry is on or after `last_date`. If we perform a download and the cache criteria are not satisfied, we wait at least one hour before attempting a redownload. This is determined by comparing the current time to the result of os.path.getmtime on the cache path. """

    filename = get_benchmark_filename(symbol)
    data = _load_cached_data(filename, first_date, last_date, now, 'benchmark',
                             environ)
    if data is not None:
        return data

    # If no cached data was found or it was missing any dates then download the
    # necessary data.
    logger.info(
        ('Downloading benchmark data for {symbol!r} '
            'from {first_date} to {last_date}'),
        symbol=symbol,
        first_date=first_date - trading_day,
        last_date=last_date
    )

    try:
        data = get_benchmark_returns(symbol)
        data.to_csv(get_data_filepath(filename, environ))
    except (OSError, IOError, HTTPError):
        logger.exception('Failed to cache the new benchmark returns')
        raise
    if not has_data_for_dates(data, first_date, last_date):
        logger.warn(
            ("Still don't have expected benchmark data for {symbol!r} "
                "from {first_date} to {last_date} after redownload!"),
            symbol=symbol,
            first_date=first_date - trading_day,
            last_date=last_date
        )
    return data