This function will decide whether to update the version document with forward pointers to segments.
    It detects cases where no prior writes/appends have been performed with FW pointers, and extracts the segment IDs.
    It also sets the metadata which indicate the mode of operation at the time of the version creation.

def _update_fw_pointers(collection, symbol, version, previous_version, is_append, shas_to_add=None):
    """
    This function will decide whether to update the version document with forward pointers to segments.
    It detects cases where no prior writes/appends have been performed with FW pointers, and extracts the segment IDs.
    It also sets the metadata which indicate the mode of operation at the time of the version creation.
    """
    version[FW_POINTERS_CONFIG_KEY] = ARCTIC_FORWARD_POINTERS_CFG.name  # get the str as enum is not BSON serializable

    if ARCTIC_FORWARD_POINTERS_CFG is FwPointersCfg.DISABLED:
        return

    version_shas = set()

    if is_append:
        # Appends are tricky, as we extract the SHAs from the previous version (assuming it has FW pointers info)
        prev_fw_cfg = get_fwptr_config(previous_version)
        if prev_fw_cfg is FwPointersCfg.DISABLED.name:
            version_shas.update(Binary(sha) for sha in collection.find(
                {'symbol': symbol,
                 'parent': version_base_or_id(previous_version),
                 'segment': {'$lt': previous_version['up_to']}},
                {'sha': 1}))
        else:
            version_shas.update(previous_version[FW_POINTERS_REFS_KEY])

    # It is a write (we always get the all-inclusive set of SHAs), so no need to obtain previous SHAs
    version_shas.update(shas_to_add)

    # Verify here the number of seen segments vs expected ones
    if len(version_shas) != version['segment_count']:
        raise pymongo.errors.OperationFailure("Mismatched number of forward pointers to segments for {}: {} != {})"
                                              "Is append: {}. Previous version: {}. "
                                              "Gathered forward pointers segment shas: {}.".format(
            symbol, len(version_shas), version['segment_count'], is_append, previous_version['_id'], version_shas))

    version[FW_POINTERS_REFS_KEY] = list(version_shas)