def find_staff_lock_source(xblock):
    """
    Returns the xblock responsible for setting this xblock's staff lock, or None if the xblock is not staff locked.
    If this xblock is explicitly locked, return it, otherwise find the ancestor which sets this xblock's staff lock.
    """

    # Stop searching if this xblock has explicitly set its own staff lock
    if xblock.fields['visible_to_staff_only'].is_set_on(xblock):
        return xblock

    # Stop searching at the section level
    if xblock.category == 'chapter':
        return None

    parent_location = modulestore().get_parent_location(xblock.location,
                                                        revision=ModuleStoreEnum.RevisionOption.draft_preferred)
    # Orphaned xblocks set their own staff lock
    if not parent_location:
        return None

    parent = modulestore().get_item(parent_location)
    return find_staff_lock_source(parent)