Error on async job

I was able to get rid of this exception in a controller by removing the onComplete and onError calls. I guess the exception happens because the parent thread ended when you called render.

So your:

Promise p = task {
    complexAsyncMethodCall(); // (1) do stuff
}
.onComplete { result -> println result } // (2) on success
.onError { Throwable t -> System.err.println("Error: " + t) } // (3) on error

Becomes:

Promise p = task {
    try {
        def result = complexAsyncMethodCall(); // (1) do stuff
        println result // (2) on success
    } catch(Throwable t) {
        System.err.println("Error: " + t) // (3) on error
    }
}

This adds coupling between your work (1) and the result processing (2 and 3) but you could overcome this by writing your own Closure wrapper that takes extra Closures as arguments. Something like this:

// may not work! written off the top of my head
class ProcessableClosure<V> extends Closure<V> {
    Closure<V> work;
    Closure<?> onError;
    Closure<?> onComplete;

    @Override
    public V call(Object... args) {
        try {
            def result = work.call(args); // (1) do stuff
            onComplete.call(result); // (2) on complete
        } catch(Exception e) {
            onError.call(result); // (3) on error
        }
    }
}

That makes your code more readable:

Closure doWork = { complexAsyncMethodCall(); } // (1) do stuff
Closure printResult = { println it } // (2) on complete
Closure logError = { Throwable t -> log.error t } // (3) on error
Closure runEverythingNicely = new ProcessableClosure(work: doWork, onComplete: printResult, onError: logError)
Promise p = task { runEverythingNicely }

I ended using the executor framework with the grails-executor plugin. I uploaded a very basic example here: https://github.com/agusl88/grails-async-job-queuqe

That code is using a "custom" version of the grails-executor plugin, i merged some PR's from the plugin repo and packaged as jar just for testing propuses. The repo of the plugin is this: https://github.com/basejump/grails-executor