Following up this thread: https://groups.google.com/forum/?fromgroups=#!topic/play-framework-dev/58VZD-YXdJw

Using a lot of flatMap calls leaks. Here is a snippet of code that shows the problem:

import scala.concurrent._
import scala.concurrent.duration._
import ExecutionContext.Implicits.global
import scala.util._

object FutureLeak extends App {
val step = 100000
val upper = 1000000

// code that leaks
def loop(future: Future[Int]): Future[Int] = {
future.flatMap { i =>
if (i % step == 0) println(i)
if (i < upper)
loop(Future(i + 1))
else Future(i)
}
}

// code that does not leak
def loop2(future: Future[Int]): Future[Int] = {
val promise = Promise[Int]

def inloop(future: Future[Int]): Unit = {
  future.onComplete {
    case Success(i) if i < upper =>
      if (i % step == 0) println(i)
      inloop(Future(i + 1))
    case Success(i) =>
      println("done with " + i)
      promise.success(i)
  }
}

inloop(future)
promise.future

}

Await.result(loop(Future(0)), 100 seconds) // leads to java.lang.OutOfMemoryError: Java heap space with -Xms32m -Xmx32m

Await.result(loop2(Future(0)), 100 seconds) // runs fine with -Xms32m -Xmx32m
}

It should not leak since the produced futures do not depend on each other.