Clarify timeout doc to mention that close! should not be called on a timeout channel

Question

After running into a race-condition involving closed timeout channels, it seems like it would be appropriate to mention that close! should never be called on a timeout channel in its docstring. The attached patch tweaks the doc string to that effect. Please advise if you'd like the wording changed a bit.

Answer 1

Comment made by: alexmiller

Yeah, I think it would be better to fix this than doc it.

Answer 2

Reference: https://clojure.atlassian.net/browse/ASYNC-109 (reported by alex+import)

Comments

> Yeah, I think it would be better to fix this than doc it.

I for one would very much welcome the patch for turning this into an error. Would have saved us around 3 days of intense debugging of very strange timeout behavior in a busy server.

---

As an alternative it might be considered to revert https://github.com/clojure/core.async/commit/ca80990bdcef6c46a93b93b82a3337fff22432d7
Another strong reason to revert it is that you can easily run into the 1024 taker limit unexpectedly. Check the following repro, assuming your machine is fast enough:
```
(let [err (promise)
      chs (into [] (repeatedly 32 #(a/timeout 1000)))]
  (doseq [ch chs]
    (dotimes [_ 33]
      (try (a/take! ch (fn [_]))
           (catch Error e
             (deliver err e)))))
  @err)
```

This may also be a sleeper in existing core.async deployments that will only surface once machines become faster.

Answer 3

Comment made by: hlewisship

Alternately/additionally, it would be nice if close! on a timeout channel would throw an exception.

Answer 4

Comment made by: slipset

or alternately, make it a no-op?