Scalaz-stream bracket closes a resource early

Question

Consider this code:

import java.time.Instant
import scalaz.concurrent.Task
import scalaz.stream._

  class MyResource {

    println("resource obtained")

    @volatile var v = 1

    def read() = v

    def close() {
      v = 0
      println("resource closed")
    }
  }

  val expensiveCalculation = (i: Int, rc: MyResource) => {
    Thread.sleep(1000)
    val r = rc.read()
    println(s"calculating ($i, $r) on ${Thread.currentThread} at ${Instant.now}")
    i / r
  }

I'd like to use Process.bracket to manage the resource:

val resource = Process.bracket(Task.delay(new MyResource))(r => Process.eval_(Task.now(r.close())))(rc => Process.repeatEval(Task.now(rc)))

And run the calculation for a range of numbers:

val range = Process.range(0, 5).toSource

val s1 = range.tee(resource)(tee.zip).map {
    case (i, rc) => expensiveCalculation(i, rc)
}

println(s"start s1 at ${Instant.now}")
val res1 = s1.runLog.attemptRun
println(s"done at ${Instant.now}: $res1")

So far so good ... I get this result:

start s1 at 2016-04-04T13:44:26.450Z
resource obtained
calculating (0, 1) on Thread[main,5,main] at 2016-04-04T13:44:27.565Z
calculating (1, 1) on Thread[main,5,main] at 2016-04-04T13:44:28.573Z
calculating (2, 1) on Thread[main,5,main] at 2016-04-04T13:44:29.575Z
calculating (3, 1) on Thread[main,5,main] at 2016-04-04T13:44:30.577Z
calculating (4, 1) on Thread[main,5,main] at 2016-04-04T13:44:31.578Z
resource closed
done at 2016-04-04T13:44:31.597Z: \/-(Vector(0, 1, 2, 3, 4))

Now, I'd like to run the expensive calculation in parallel:

val s2 = merge.mergeN(range.zipWith(resource){
    case (i, rc) => Process.eval(Task.delay(expensiveCalculation(i, rc)))
})

println(s"start s2 at ${Instant.now}")
val res2 = s2.runLog.attemptRun
println(s"done at ${Instant.now}: $res2")

And I get this result:

start s2 at 2016-04-04T13:44:31.601Z
resource obtained
resource closed
calculating (0, 0) on Thread[pool-1-thread-3,5,main] at 2016-04-04T13:44:32.717Z
calculating (1, 0) on Thread[pool-1-thread-8,5,main] at 2016-04-04T13:44:32.717Z
calculating (4, 0) on Thread[pool-1-thread-7,5,main] at 2016-04-04T13:44:32.717Z
calculating (2, 0) on Thread[pool-1-thread-2,5,main] at 2016-04-04T13:44:32.717Z
calculating (3, 0) on Thread[pool-1-thread-6,5,main] at 2016-04-04T13:44:32.717Z
done at 2016-04-04T13:44:32.779Z: -\/(java.lang.ArithmeticException: / by zero)

Apparently the bracket's release function is called too early and closing the resource prematurely. What am I doing wrong? Is there a way to close the resource after the parallel calculation finishes?

Thanks.

Jan · Answer 1 · 2016-04-25T20:34:54.533

After reading some more articles and experimenting with scalaz-stream I came to this solution on how to release a resource after a parallel execution:

val p: Process[Task, Int] = Process.await(Task.delay(new MyResource)) { rc =>

    val range = Process.range(0, 5).toSource

    merge.mergeN(range.map { i =>
      Process.eval(Task.delay(expensiveCalculation(i, rc)))
    }).onComplete(Process eval_ Task.delay {
      rc.close()
    })
}

p.runLog.attemptRun

Scalaz-stream bracket closes a resource early

1 Answers1