intentional back-pressure (or a poor man's side-input)

[Derek VerLee]

I was just thinking about about letting a coprocessfunction "block" or cause back pressure on one of it's streams? Has this been discussed as an option? Does anyone know a way to effectively accomplish this? I think I could get a lot of mileage out of something like that without needing a full implementation of FLIP-17 (which I would eagerly await still). As mentioned on another thread, one could use a liststate to buffer the main input until the "side input" was sufficiently processed.  However the downside of this is that I have no way to control the size of those buffers, whereas with backpressure, the system will naturally take care of it.