I understand it is not good tagging but please help me to tell Potlatch they are wrong: https://trac.openstreetmap.org/ticket/4510 If I change it to correct tagging there will be someone else who notice this as "Lake" and again tag it to waterway=canal. There are dozens of mappers who are making this mistake and I'm tired of writing them or correcting them all.
In general maybe it would also help to add the way-id to problematic_polygons list. So mkgmap has the hole way and there is no need to autoclose a way.
That is not the point. The point is that correct waterway=canals which are single lines are being closed too if I render it as polygons. I can think of other forms of lines that could either be polygons or lines. Think of highway=pedestrian or man_made=pier so I think this patch can help.