停止生产者/消费者循环的惯用方法？

Question

In an effort to get some good concurrent programming practice I'm trying to implement the producer/consumer pattern using Clojure's core.async library. All is working well but I wanted to be able to stop both the producer and consumer at some point in time.

My current code looks something like this...

(def c (a/chan 5))
(def alive (atom true))

(def producer (a/go-loop []
                (Thread/sleep 1000)
                (when @alive 
                  (a/>! c 1)
                  (recur))))

(def consumer (a/go-loop []
                (Thread/sleep 3000)
                (when @alive 
                  (println (a/<! c))
                  (recur))))
(do
  (reset! alive false)
  (a/<!! producer)
  (a/<!! consumer))

Unfortunately it appears that the 'do' block occasionally blocks indefinitely. I essentially want to be able to stop both go-loops from continuing and block until both loops have exited. The Thread/sleep code is there to simulate performing some unit of work.

I suspect that stopping the producer causes the consumer to park, hence the hanging, though I'm not sure of an alternative approach, any ideas?

Answer 1

Please see ClojureDocs for details. Example:

(let [c (chan 2) ]
  (>!! c 1)
  (>!! c 2)
  (close! c)
  (println (<!! c)) ; 1
  (println (<!! c)) ; 2
  ;; since we closed the channel this will return false(we can no longer add values)
  (>!! c 1))

For your problem something like:

  (let [c        (a/chan 5)
        producer (a/go-loop [cnt 0]
                   (Thread/sleep 1000)
                   (let [put-result (a/>! c cnt)]
                     (println "put: " cnt put-result)
                     (when put-result
                       (recur (inc cnt)))))

        consumer (a/go-loop []
                   (Thread/sleep 3000)
                   (let [result (a/<! c)]
                     (when result
                       (println "take: " result)
                       (recur))))]
    (Thread/sleep 5000)
    (println "closing chan...")
    (a/close! c))

with result

put:  0 true
put:  1 true
take:  0
put:  2 true
put:  3 true
closing chan...
put:  4 false
take:  1
take:  2
take:  3