App + cats-effect hangs on 3.1.0 but works fine in 2.13.7

[jmgimeno]

This code

import cats.effect._
import cats.implicits._
import cats.effect.unsafe.implicits.global

object IOComposition extends App {
  val hello = IO(println(s"[${Thread.currentThread.getName}] Hello"))
  val world = IO(println(s"[${Thread.currentThread.getName}] World"))
  val hw1: IO[Unit] = for {
    _ <- hello
    _ <- world
  } yield ()
  val hw2: IO[Unit] =
    (hello, world).mapN((_, _) => ())
  hw1.unsafeRunSync()
  hw2.unsafeRunSync()
}

hangs with Scala 3.1.0 after printing only "Hello"

Compiling project (Scala 3.1.0, JVM)
Compiled project (Scala 3.1.0, JVM)
[io-compute-2] Hello

but works perfecty with Scala 2.13.7

Compiling project (Scala 2.13.7, JVM)
Compiled project (Scala 2.13.7, JVM)
[io-compute-2] Hello
[io-compute-4] World

[djspiewak]

Try using defs. I'd be willing to bet this is caused by the issues with MethodHandle linking to null during class load.

[SethTisue]

I'd also be interested in knowing if the problem goes away if you avoid using App. App is not recommended in Scala 3, as per https://docs.scala-lang.org/scala3/reference/changed-features/main-functions.html

[djspiewak]

I suspect that, if you avoid using App, the source of the problem will actually become a hair more obvious. Or you can just push the vals into the body of the main method (which is effectively what Scala 2 is doing with the same code but Scala 3 is not), and that should also resolve the deadlock.

[jmgimeno]

I'd also be interested in knowing if the problem goes away if you avoid using App

Yes, if I create a regular main method instead of extending App, it works perfectly

Changing the definitions of hello and world from val instead to def also solves the problem in Scala 3.1.0 even when using App.

[djspiewak]

Sweet! So this is definitely the MethodHandle concurrent initialization issue then. It's actually part of the JVM itself and not really anything that either Scala or Cats Effect can address. In fact, you can reproduce this issue using bare Java.