Splitting a polygon by a polyline

[dabreegster]

I'm trying to implement something like https://postgis.net/docs/ST_Split.html. But I'm stuck, because the boolean operations require both inputs to be closed. I have one closed input, and I'm trying to perform operations against an open polyline. I might be able to hack around this by artificially closing the polyline (carefully, outside the first polygon), but I'm wondering if there's a proper way to do this. Thanks!