Inadequate behavior of agent await?

[Ernesto Garcia]

I have discovered that, in the case of an agent action exception, the behavior of agent await depends on timing:

- If your await happens to execute after the action is completed, it will happily throw an exception that the agent is in a failed state.

- If your await happens to execute before the action is completed, it will dangerously block. (!!!)

Shouldn't await behave the same in both cases?

Code example:

(defn await-agent-after-fail []

  (let [agt (agent nil)]

    (send-off agt (fn [_] 

                    (println "Agent throws exception now")

                    (throw (ex-info "fail!" {}))))

    (Thread/sleep 1000)

    (println "Agent is awaited now...")

    (println "Await timed out?" (not (await-for 10000 agt)))))

(comment

  (await-agent-after-fail))

(defn await-agent-before-fail []

  (let [agt (agent nil)]

    (send-off agt (fn [_]

                    (Thread/sleep 1000)

                    (println "Agent throws exception now")

                    (throw (ex-info "fail!" {}))))

    (println "Agent is awaited now...")

    (println "Await timed out?" (not (await-for 10000 agt)))))

(comment

  (await-agent-before-fail))

[Sam Griffith]

You may want to ask this on ClojureVerse or on the official Q/A site at https://ask.clojure.org/index.php/

[Sean Corfield]

If you provide an error handler on the agent -- or just specify the error mode as :continue -- it does not hang. That makes me think that what you're seeing is "expected behavior" although it does seem a bit strange...?

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Sean A Corfield -- (904) 302-SEAN
An Architect's View -- https://corfield.org/
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[Ernesto Garcia]

Hey, thanks for the suggestions. ClojureVerse seems to be the most active forum?

[Ernesto Garcia]

On Thursday, June 11, 2020 at 6:20:19 AM UTC+2, Sean Corfield wrote:

If you provide an error handler on the agent -- or just specify the error mode as :continue -- it does not hang.

Thanks for pointing out those solutions.

 

That makes me think that what you're seeing is "expected behavior" although it does seem a bit strange...?

Yes, `await` doc says: "Will block on failed agents". We should infer that we should only use `await` when we ensure that the agent continues on exceptions. Looking into the implementation of `await`, it creates a latch, sends a `latch.countDown()` action to the agent, and awaits that latch. It makes sense that it blocks forever if the agent stops processing actions.

However, there's a thing I don't understand. `await` doc says: "Blocks the current thread (indefinitely!) until all actions dispatched thus far, from this thread or agent, to the agent(s) have occurred." But, considering its implementation, we seem to be waiting for all actions to occur, not only the actions dispatched by this thread. Also, we are not allowed to call `await` from inside an agent action; why does it say "from this thread or agent" then?

Am I misinterpreting something?

Thanks,

Ernesto

[Matching Socks]

await takes many steps and is not synchronized, so it would be messy to guarantee how it relates to other threads.