flushing the content of a core.async channel - clojure

Question

Consider a core.async channel which is created like so:
(def c (chan))
And let's assume values are put and taken to this channel from different places (eg. in go-loops).
How would one flush all the items on the channel at a certain time?
For instance one could make the channel an atom and then have an event like this:
(def c (atom (chan))
(defn reset []
(close! #c)
(reset! c (chan)))
Is there another way to do so?

Answer 1

Read everything to a vector with into and don't use it.
(go (async/into [] c))

Answer 2

Let's define a little more clearly what you seem to want to do: you have code running in several go-loops, each of them putting data on the same channel. You want to be able to tell them all: "the channel you're putting values on is no good anymore; from now on, put your values on some other channel." If that's not what you want to do, then your original question doesn't make much sense, as there's no "flushing" to be done -- you either take the values being put on the channel, or you don't.
First, understand the reason your approach won't work, which the comments to your question touch on: if you deref an atom c, you get a channel, and that value is always the same channel. You have code in go-loops that have called >! and are currently parked, waiting for takers. When you close #c, those parked threads stay parked (anyone parked while taking from a channel (<!) will immediately get the value nil when the channel closes, but parked >!s will simply stay parked). You can reset! c all day long, but the parked threads are still parked on a previous value they got from derefing.
So, how do you do it? Here's one approach.
(require '[clojure.core.async :as a
:refer [>! <! >!! <!! alt! take! go-loop chan close! mult tap]])
(def rand-int-chan (chan))
(def control-chan (chan))
(def control-chan-mult (mult control-chan))
(defn create-worker
[put-chan control-chan worker-num]
(go-loop [put-chan put-chan]
(alt!
[[put-chan (rand-int 10)]]
([_ _] (println (str "Worker" worker-num " generated value."))
(recur put-chan))
control-chan
([new-chan] (recur new-chan)))))
(defn create-workers
[n c cc]
(dotimes [n n]
(let [tap-chan (chan)]
(a/tap cc tap-chan)
(create-worker c tap-chan n))))
(create-workers 5 rand-int-chan control-chan-mult)
So we are going to create 5 worker loops that will put their result on rand-int-chan, and we will give them a "control channel." I will let you explore mult and tap on your own, but in short, we are creating a single channel which we can put values on, and that value is then broadcast to all channels which tap it.
In our worker loop, we do one of two things: put a value onto the rand-int-chan that we use when we create it, or we will take a value off of this control channel. We can cleverly let the worker thread know that the channel to put its values on has changed by actually handing it the new channel, which it will then bind on the next time through the loop. So, to see it in action:
(<!! rand-int-chan)
=> 6
Worker2 generated value.
This will take random ints from the channel, and the worker thread will print that it has generated a value, to see that indeed multiple threads are participating here.
Now, let's say we want to change the channel to put the random integers on. No problem, we do:
(def new-rand-int-chan (chan))
(>!! control-chan new-rand-int-chan)
(close! rand-int-chan) ;; for good measure, may not be necessary
We create the channel, and then we put that channel onto our control-chan. When we do this, ever worker thread will have the second portion of its alt! executed, which simply loops back to the top of the go-loop, except this time, the put-chan will be bound to the new-rand-int-chan we just received. So now:
(<!! new-rand-int-chan)
=> 3
Worker1 generated value.
This gives us our integers, which is exactly what we want. Any attempt to <!! from the old channel will give nil, since we closed the channel:
(<!! rand-int-chan)
; nil