When a large number of channels is added to a mix simultaneously, this error is thrown:
java.lang.AssertionError: Assert failed: No more than 1024 pending puts are allowed on a single channel. Consider using a windowed buffer.
We can reproduce the issue with the following in a REPL:
user> (require '[clojure.core.async :as a])
user> (defn mixtest 
(let [out (a/chan 1024)
mix (a/mix out)]
(dotimes [i 2048]
(let [chan (a/chan)]
(a/admix mix chan)
(a/put! chan i)))))
#'user/mixtestAssertionError Assert failed: No more than 1024 pending puts are allowed on a single channel. Consider using a windowed buffer.
(< (.size puts) impl/MAX-QUEUE-SIZE) clojure.core.async.impl.channels.ManyToManyChannel (channels.clj:150)
This is a consequence of the use of an unbuffered channel in the mix implementation. Since the channel's function is just to update the mix's internal data structure, using a windowed buffered channel appears to clear up this issue. Patch 0001-Change-channel-to-sliding-buffer.patch contains the proposed solution.
Found in core.async version 0.1.346.0-17112a-alpha