def METHOD_NAME(path_rgb_msk_byte_tif):
    """Boundless read of a source with .msk succeeds
    Success in this case means that we read a mask that has
    invalid pixels around the edges, is appropriately padded,
    and has valid data pixels in the center.
    """
    with rasterio.open(path_rgb_msk_byte_tif) as src:
        msk = src.read_masks(1, boundless=True, window=Window(-200, -200, 1000, 1000), out_shape=((600, 600)))
        # Invalid region is padded correctly.
        assert not msk[0:195,0:195].any()
        # We have the valid data expected in the center.
        assert msk.mean() > 90