Future's Flatmap is not stacksafe

[He-Pin]

@RunWith(classOf[JUnit4])
class Stacksafe {

  @Test
  def testFib(): Unit = {
    implicit val ex: ExecutionContext = new ExecutionContext {
      override def execute(runnable: Runnable): Unit = runnable.run()
      override def reportFailure(cause: Throwable): Unit = cause.printStackTrace()
    }

    def fib(n: Int, a: Long = 0, b: Long = 1): Future[Long] =
      Future(a + b).flatMap { b2 =>
        if (n > 0)
          fib(n - 1, b, b2)
        else
          Future(b2)
      }

    val future = fib(1000)
    println(Await.result(future,Duration.Inf))
  }
}

It's ok with 100,but not 1000

[He-Pin]

#6932

[viktorklang]

This is not a bug. Your Execution context is not stack safe.

[viktorklang]

Think of ExecutionContext as the Interpreter, where the "next actions" enter through execute and execution of the Runnable can lead to more calls to execute. Therefor it is the responsibility of the implementor of ExecutionContext to make sure that it is stack safe / reentrant.

[He-Pin]

@viktorklang I try this because of Task or IO claims they are, so I take a try. I think to fix this will change so much, may not worth it.

[He-Pin]

This is not a bug. Your Execution context is not stack safe.

Seems it's by design:)

[viktorklang]

Task/IO are only stacksafe when their interpreters are stack safe. This is also true for Future. For some examples on how to make stacksafe ExecutionContexts, look at: https://github.com/scala/scala/blob/2.13.x/src/library/scala/concurrent/BatchingExecutor.scala which is used by this. :)

[He-Pin]

thanks, I was expecting it will be safe in any way.

[He-Pin]

@viktorklang So,now anyone want to make it safe then will need to take care of the Batchable:)

[viktorklang]

@hepin1989 Not necessarily :) Most thread pools are stack safe by definition since they add new Runnables to a queue. (just like the BatchingExecutor does).

[He-Pin]

@viktorklang But I do expect even I call runnable.run directly, it works too.

[viktorklang]

@hepin1989

A general purpose ExecutionContext must be asynchronous in executing any Runnable that is passed into its execute-method. A special purpose ExecutionContext may be synchronous but must only be passed to code that is explicitly safe to be run using a synchronously executing ExecutionContext.
--https://www.scala-lang.org/api/current/scala/concurrent/ExecutionContext.html