TestDispatcher.scala

import cats.effect.std.{Dispatcher, Queue}
import cats.effect.{IO, IOApp}
import com.typesafe.scalalogging.Logger
import fs2.{Pipe, Stream}

import java.util.concurrent.atomic.AtomicInteger
import scala.collection.mutable
import scala.concurrent.duration.DurationInt

object Subscriptions {
  private object lock

  private val cnt = new AtomicInteger(0)

  private val subscriptions = mutable.Map.empty[Int, String => Unit]

  def subscribe(cb: String => Unit, token: Int): Unit = lock.synchronized {
    subscriptions.update(token, cb)
  }

  def unsubscribe(token: Int): Unit = lock.synchronized {
    subscriptions.remove(token)
  }

  def notifyListeners: Unit = {
    val listeners = lock.synchronized(subscriptions.values.toList)
    listeners.foreach(_.apply(s"from callback: ${cnt.incrementAndGet().toString}"))
  }
}


object TestDispatcher extends IOApp.Simple {
  private val log = Logger[this.type]

  private val requestStream = Stream.iterate(1)(_ + 1).covary[IO].evalTap(_ => IO.sleep(10.millis))

  private val tokenFactory = new AtomicInteger(0)

  def onInput(dispatcher: Dispatcher[IO], q: Queue[IO, String]): Unit = {

    val token = tokenFactory.incrementAndGet()

    def cb(s: String): Unit = {
      dispatcher.unsafeRunSync(q.offer(s).delayBy(1.milli))
      Subscriptions.unsubscribe(token)
    }

    Subscriptions.subscribe(cb, token)
  }

  val pipe: Pipe[IO, Int, String] = { input =>
    Stream.eval(Queue.unbounded[IO, String]).flatMap { output =>
      Stream.resource(Dispatcher[IO].onFinalize(IO(log.info("Releasing Dispatcher")))).flatMap { dispatcher =>
        val processInput = input
          .map(_ => onInput(dispatcher, output))
          .drain
        Stream.fromQueueUnterminated(output).concurrently(processInput)
      }
    }
  }


  def run: IO[Unit] = {
    val consumer = requestStream
      .through(pipe)
      .evalTap(x => IO(log.info(s"consumer: $x")))
      .timeout(200.millis)
      .compile
      .drain

    val producer = IO.interruptible(true) {
      while (true) {
        Thread.sleep(10)
        Subscriptions.notifyListeners
      }
    }.guarantee(IO(log.info("Canceling producer")))

    IO.race(consumer, producer).void
  }
}

Example output with RTE:

[2021-09-16 18:38:08,919] INFO  [io-compute-6] TestDispatcher:66 - consumer: from callback: 1
[2021-09-16 18:38:08,924] INFO  [io-compute-6] TestDispatcher:66 - consumer: from callback: 2
[2021-09-16 18:38:08,924] INFO  [io-compute-6] TestDispatcher:66 - consumer: from callback: 3
[2021-09-16 18:38:08,926] INFO  [io-compute-6] TestDispatcher:66 - consumer: from callback: 4
[2021-09-16 18:38:08,927] INFO  [io-compute-6] TestDispatcher:66 - consumer: from callback: 5
[2021-09-16 18:38:08,929] INFO  [io-compute-2] TestDispatcher:66 - consumer: from callback: 6
[2021-09-16 18:38:08,941] INFO  [io-compute-0] TestDispatcher:66 - consumer: from callback: 7
[2021-09-16 18:38:08,953] INFO  [io-compute-1] TestDispatcher:66 - consumer: from callback: 8
[2021-09-16 18:38:08,964] INFO  [io-compute-4] TestDispatcher:66 - consumer: from callback: 9
[2021-09-16 18:38:08,999] INFO  [io-compute-5] TestDispatcher:53 - Releasing Dispatcher
[2021-09-16 18:38:09,014] INFO  [io-compute-6] TestDispatcher:76 - Canceling producer
Exception in thread "main" java.lang.IllegalStateException: dispatcher already shutdown
	at interruptible @ TestDispatcher$.run(TestDispatcher.scala:72)