How to cancel execution when client is disconnected?

[kachayev]

Right now there's no obvious way to cancel request processing as we do not have access to the information about connection status in the handler. With this change I can do something like (pseudo-code):

(defn handler [req]
  (let [s' (manifold.stream/stream)
        d' (process-request req)]
     (manifold.stream/on-close s' (fn [] (manifold.deferred/error! d' ::closed)))
     (manifold.deferred/chain'
        d'
        (fn [r]
           (manifold.stream/put! s' r)
           (manifold.stream/close! s')))
     {:status 200
      :body s'}))

But this would change semantic in many cases as it forces me to use chunked transfer and it sends status/headers right away. I think the good approach to tackle this would be to inject into the request map something like :aleph/termination-future that I could use to setup a listener when necessary. Or even provide such functionality as a configurable "cancellation" behavior. @ztellman what do you think?

P.S. I'm not also sure about using manifold.deferred/error!, but as far as I see there's not "cancel" state in manifold's deferred, so setting error state seems the most reasonable approach here.