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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def wal_restore(self, wal_name, wal_destination, prefetch_max):
 """ Downloads a WAL file from S3 or Windows Azure Blob Service This code is intended to typically be called from Postgres's restore_command feature. NB: Postgres doesn't guarantee that wal_name == basename(wal_path), so both are required. """

        url = '{0}://{1}/{2}'.format(
            self.layout.scheme, self.layout.store_name(),
            self.layout.wal_path(wal_name))

        if prefetch_max > 0:
            # Check for prefetch-hit.
            base = os.path.dirname(os.path.realpath(wal_destination))
            pd = prefetch.Dirs(base)
            seg = WalSegment(wal_name)

            started = start_prefetches(seg, pd, prefetch_max)
            last_size = 0

            while True:
                if pd.contains(seg):
                    pd.promote(seg, wal_destination)
                    logger.info(
                        msg='promoted prefetched wal segment',
                        structured={'action': 'wal-fetch',
                                    'key': url,
                                    'seg': wal_name,
                                    'prefix': self.layout.path_prefix})

                    pd.clear_except(started)
                    return True

                # If there is a 'running' download, wait a bit for it
                # to make progress or finish.  However, if it doesn't
                # make progress in some amount of time, assume that
                # the prefetch process has died and go on with the
                # in-band downloading code.
                sz = pd.running_size(seg)
                if sz <= last_size:
                    break

                last_size = sz
                gevent.sleep(0.5)

            pd.clear_except(started)

        logger.info(
            msg='begin wal restore',
            structured={'action': 'wal-fetch',
                        'key': url,
                        'seg': wal_name,
                        'prefix': self.layout.path_prefix,
                        'state': 'begin'})

        ret = do_lzop_get(self.creds, url, wal_destination,
                          self.gpg_key_id is not None)

        logger.info(
            msg='complete wal restore',
            structured={'action': 'wal-fetch',
                        'key': url,
                        'seg': wal_name,
                        'prefix': self.layout.path_prefix,
                        'state': 'complete'})

        return ret