Multi Thread remove from ConcurrentHashMap will throw NPE use Scala 2.13.8

[LuciferYang]

Reproduction steps

Scala version: 2.13.8

import scala.jdk.CollectionConverters._
    import java.util.concurrent.ConcurrentHashMap
    val map = new ConcurrentHashMap[Int, Integer]().asScala
    val dataRange = 0 until 100
    dataRange.foreach { i => map.put(i, i) }
    val threads = new Array[Thread](10)
    threads.indices.foreach { i =>
      threads(i) = new Thread() {
        override def run(): Unit = {
          dataRange.foreach { i =>
            map.remove(i).foreach(_.intValue())
          }
        }
      }
    }
    threads.foreach(_.start())
    threads.foreach(_.join())

Running the above program with Scala 2.13 may throw the following exceptions

Exception in thread "Thread-3" Exception in thread "Thread-2" java.lang.NullPointerException
  at scala213.test.MultiThreadRemoveSuite$$anon$1.$anonfun$run$2(MultiThreadRemoveSuite.scala:33)
  at scala213.test.MultiThreadRemoveSuite$$anon$1.$anonfun$run$2$adapted(MultiThreadRemoveSuite.scala:33)
  at scala.Option.foreach(Option.scala:437)
  at scala213.test.MultiThreadRemoveSuite$$anon$1.$anonfun$run$1(MultiThreadRemoveSuite.scala:33)
  at scala.collection.immutable.Range.foreach$mVc$sp(Range.scala:190)
  at scala213.test.MultiThreadRemoveSuite$$anon$1.run(MultiThreadRemoveSuite.scala:32)
java.lang.NullPointerException
  at scala213.test.MultiThreadRemoveSuite$$anon$1.$anonfun$run$2(MultiThreadRemoveSuite.scala:33)
  at scala213.test.MultiThreadRemoveSuite$$anon$1.$anonfun$run$2$adapted(MultiThreadRemoveSuite.scala:33)
  at scala.Option.foreach(Option.scala:437)
  at scala213.test.MultiThreadRemoveSuite$$anon$1.$anonfun$run$1(MultiThreadRemoveSuite.scala:33)
  at scala.collection.immutable.Range.foreach$mVc$sp(Range.scala:190)
  at scala213.test.MultiThreadRemoveSuite$$anon$1.run(MultiThreadRemoveSuite.scala:32)

Problem

NPE should not be thrown

[LuciferYang]

cc @SethTisue

[som-snytt]

This seems to be resolved by scala/scala#9957 which changes naive remove to compute.

[LuciferYang]

This seems to be resolved by scala/scala#9957 which changes naive remove to compute.

2.13.9 solved?

[LuciferYang]

This seems to be resolved by scala/scala#9957 which changes naive remove to compute.

Or is there a snapshot version that I can test?

[lrytz]

See here: https://stackoverflow.com/questions/40622878/how-do-i-tell-sbt-or-scala-cli-to-use-a-nightly-build-of-scala-2-12-or-2-13

[som-snytt]

Recap: the previous change was

// support Some(null) if currently bound to null

but the condition test was not atomic. We allow that in the adaptors.

It was improved just for efficiency, but with the additional happy effect that it helps atomicity for this case.

ConcurrentHashMap doesn't benefit from the previous change because null values are not supported. IIRC.