concurrent Future failed when function called

[conderls]

reproduction steps

using Scala (2.11.12, 2.12),

import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.{ Await, Future }
import scala.concurrent.duration._

object FutureTest {
  def printIdInfo(id: Long): String = {
    if (id % 2 == 0) {
      "even:%s".format(id)
    } else {
      "odd:%s".format(id)
    }
  }

  println(s"Current thread: ${printIdInfo(Thread.currentThread().getId)}")
  val future: Future[Unit] = Future(println {
    s"Thread id in future: ${printIdInfo(Thread.currentThread().getId)}."
  })

  Await.result(future, 2.seconds)
  println("Future has been executed")
}

FutureTest

problem

(explain how the above behavior isn't what you expected)

Current thread: odd:1
java.lang.NoClassDefFoundError: Could not initialize class $line6.$read$$iw$$iw$$iw$$iw$FutureTest$
        at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
        at scala.concurrent.Future$.$anonfun$apply$1(Future.scala:659)
        at scala.util.Success.$anonfun$map$1(Try.scala:255)
        at scala.util.Success.map(Try.scala:213)
        at scala.concurrent.Future.$anonfun$map$1(Future.scala:292)
        at scala.concurrent.impl.Promise.liftedTree1$1(Promise.scala:33)
        at scala.concurrent.impl.Promise.$anonfun$transform$1(Promise.scala:33)
        at scala.concurrent.impl.CallbackRunnable.run(Promise.scala:64)
        at java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1402)
        at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
        at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
        at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
        at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)
java.util.concurrent.TimeoutException: Futures timed out after [2 seconds]
  at scala.concurrent.impl.Promise$DefaultPromise.ready(Promise.scala:259)
  at scala.concurrent.impl.Promise$DefaultPromise.result(Promise.scala:263)
  at scala.concurrent.Await$.$anonfun$result$1(package.scala:223)
  at scala.concurrent.BlockContext$DefaultBlockContext$.blockOn(BlockContext.scala:57)
  at scala.concurrent.Await$.result(package.scala:146)
  ... 30 elided

[SethTisue]

is the problem reproducible in Scala 2.13.4?

[som-snytt]

This is "don't kick off threads from object initializer because it can lead to deadlock," which is attested by many tickets.

Here, the deadlock is explicit: the future is blocked on object initialization, which can't complete because of Await.

import concurrent.{Await => `Oh, wait!`}

[som-snytt]

As a refresher, "fixed in dotty" because bootstrap methods are non-static so the future lambda does not block for static init of module. However, fields are static, so any data references will deadlock again.

The more exact duplicate is #10782

#9076