“unbecoming” blog post races

theVar.scala

var isDownloading = false

// set up a scheduler to sync the registry
system.scheduler.schedule(0 millis, 3 hours) {
  if(!isDownloading) {
    system.actorOf(Props[Downloader]) ! Download
  }
}

/*
 * Since isDownloading is not volatile any write performed by another thread
 * might not ever be visible to the code above, in principle it would be
 * a correct implementation to always execute the conditional branch.
 */

unbecome.scala

class Downloader extends Actor {

  import context._

  def receive = {
    case _ =>
      download onComplete unbecome
      /*
       * The line above is really problematic, since you are calling
       * context.unbecome from a different thread; ActorContext must
       * not ever leave the actor, and you are throwing it out there.
       * The resulting race condition is that either the become or the
       * unbecome can be lost when they happen “at the same time”.
       */
      become {
        case _ => // do nothing
      }
  }

  def download = future {
    // perform download
  }
}