async.queue: throw error if called with a concurrency of 0

[LinusU]

I recently had this code:

var concurrency = os.cpus.length;

function doWork(opts, cb) {
  cb(null);
}

var queue = async.queue(doWork, concurrency);

Things queued never called the callback because os.cpus is a function that takes 0 arguments. Thus os.cpus.length returns 0 and the queue never does anything. The correct code would be os.cpus().length, notice the ().

Maybe it would be a good idea to throw an error to alert the user that the queue was initialised with 0 concurrency and thus never will run anything.

[VitoNordloh]

PR #762

[VitoNordloh]

Ah, i see that it's possible to change the concurrency after the initalisation.
I will update my PR later.

Edit: it's not possible to throw an error after the initalisation because we can't assert that the concurrency isn't zero since it's just a variable.
Especially in browser it would cause problems.

Further on we should assert that the concurrency isn't a negative value.

[aearly]

Thanks!