Deadlock with toEffect

[steinybot]

cats-effect 3.5.4, fs2 3.10.2, and interop-cats 23.1.0.2 and Scala 2.13

This will hang:

package com.goodcover

import cats.effect.IO
import cats.effect.unsafe.IORuntime
import com.typesafe.scalalogging.StrictLogging
import org.scalatest.flatspec.AnyFlatSpec
import zio.ZIO
import zio.interop._
import zio.interop.catz.implicits._

final class IOSpec extends AnyFlatSpec with StrictLogging {

  behavior of "ZIO to IO"

  it should "not deadlock" in {
    var count = 0
    while (!Thread.interrupted()) {
      count += 1
      logger.debug("Running test " + count)
      ZIO.succeed(()).toEffect[IO].unsafeRunSync()(IORuntime.global)
    }
  }
}

[steinybot]

What's weird is that there are no live cats fibers when it deadlocks.

[steinybot]

This problem looks more fundamental. Something wrong with fiber observers.

This also hangs:

package com.goodcover

import com.typesafe.scalalogging.StrictLogging
import org.scalatest.flatspec.AnyFlatSpec
import zio.interop._
import zio.interop.catz.implicits._
import zio.{Unsafe, ZIO}

import java.util.concurrent.CountDownLatch

final class ZSpec extends AnyFlatSpec with StrictLogging {

  behavior of "ZIO to IO"

  it should "not deadlock" in {
    var count = 0
    while (!Thread.interrupted() && count < 100_000) {
      count += 1
      logger.debug("Running test " + count)
      val done = new CountDownLatch(1)
      Unsafe.unsafe { implicit unsafe =>
        val fiber = rts.unsafe.fork(ZIO.succeed(()))
        fiber.unsafe.addObserver { exit =>
          logger.debug(s"Done: $exit")
          done.countDown()
        }
      }
      done.await()
    }
  }
}

[steinybot]

Ahh I think the fiber is completing before the call to addObserver happens and it doesn't handle that case.

[steinybot]

Nope that's not it. It does handle that. However observers is not volatile so there is no happens-before relationship. I bet that is it.

[steinybot]

Caused by zio/zio#9040

[steinybot]

Actually reopening this so as to ensure this is updated on release and/or worked around.

I think a workaround is possible using onExit rather than a fiber observer.

[calvinlfer]

hey @steinybot is this still happening with the issue in core being fixed?

[steinybot]

We tried the snapshot build and it fixed it so I assume it is fixed. I've just updated to 2.1.7 so will see if it is still gone.