Zap's HttpListener is currently a singleton as indicated by the the_one_and_only_listener field. I wonder what the reasoning behind this decision was - I think it would be useful to support multiple listener instances, if possible. From what I can tell, facil.io already supports such use-cases. To make this work, we'd also need to use facil.io's per-connection udata to allow distinguishing between different instances. Btw, the current listener already accepts udata, however it leaves it unused (it's not passed to facil.io nor exposed in any other way).

If there's no argument against it, I'd be happy to implement this.