def failFastZip[T, U]()

in scalding-base/src/main/scala/com/twitter/scalding/Execution.scala [557:612]
48 lines of code
19 McCabe index (conditional complexity)

  def failFastZip[T, U](ft: Future[T], fu: Future[U])(implicit
      cec: ConcurrentExecutionContext
  ): Future[(T, U)] = {
    type State = Either[(T, Promise[U]), (U, Promise[T])]
    val middleState = Promise[State]()

    ft.onComplete {
      case f @ Failure(err) =>
        if (!middleState.tryFailure(err)) {
          // the right has already succeeded
          middleState.future.foreach {
            case Right((_, pt)) => pt.complete(f)
            case Left((t1, _)) => // This should never happen
              sys.error(s"Logic error: tried to set Failure($err) but Left($t1) already set")
          }
        }
      case Success(t) =>
        // Create the next promise:
        val pu = Promise[U]()
        if (!middleState.trySuccess(Left((t, pu)))) {
          // we can't set, so the other promise beat us here.
          middleState.future.foreach {
            case Right((_, pt)) => pt.success(t)
            case Left((t1, _)) => // This should never happen
              sys.error(s"Logic error: tried to set Left($t) but Left($t1) already set")
          }
        }
    }
    fu.onComplete {
      case f @ Failure(err) =>
        if (!middleState.tryFailure(err)) {
          // we can't set, so the other promise beat us here.
          middleState.future.foreach {
            case Left((_, pu)) => pu.complete(f)
            case Right((u1, _)) => // This should never happen
              sys.error(s"Logic error: tried to set Failure($err) but Right($u1) already set")
          }
        }
      case Success(u) =>
        // Create the next promise:
        val pt = Promise[T]()
        if (!middleState.trySuccess(Right((u, pt)))) {
          // we can't set, so the other promise beat us here.
          middleState.future.foreach {
            case Left((_, pu)) => pu.success(u)
            case Right((u1, _)) => // This should never happen
              sys.error(s"Logic error: tried to set Right($u) but Right($u1) already set")
          }
        }
    }

    middleState.future.flatMap {
      case Left((t, pu))  => pu.future.map((t, _))
      case Right((u, pt)) => pt.future.map((_, u))
    }
  }