Cats Effect Cancellation Testing

example.scala

import cats.effect.Clock
import cats.effect.{IOApp, Resource, IO, ResourceApp, ExitCode}
import scala.concurrent.duration._
object Dummy extends IOApp {
  val resource = Resource.make(IO(System.currentTimeMillis)){ start =>
    IO.sleep(3.seconds) >> IO.println(s"Exiting -- ${System.currentTimeMillis - start}")
  }

  def run(args: List[String]) = resource.use { ctx =>
    val interruptible = IO.never
    for {
      _ <- IO.println(s"Running -- $ctx")
      start <- Clock[IO].monotonic
      // _ <- IO.sleep(10.seconds) // this one exits early: resource cleanup time + elapsed time so far
      _ <- IO.never.race(IO.blocking { Thread.sleep(10000) }) // this one hangs until completion
      finish <- Clock[IO].monotonic
      _ <- IO.println(s"Done running... ${(finish - start).toMillis}")
    } yield ExitCode.Success
  }
}
// SBT setting:
// Compile / run / fork := true
// ctrl-C with SBT: exits to SBT console immediately,

// With sleep:
// [info] running (fork) com.example.Dummy
// [info] Running -- 1630021681166
// ^C
// [warn] Canceling execution...
// [error] Total time: 1 s, completed Aug 26, 2021 4:48:01 PM
// sbt:exporter> [info] Exiting -- 3544

// With blocking:
// [info] running (fork) com.example.Dummy
// [info] Running -- 1630021764778
// ^C
// [warn] Canceling execution...
// [error] Total time: 2 s, completed Aug 26, 2021 4:49:26 PM
// sbt:exporter> [info] Exiting -- 13052


// without "fork" enabled, you _also_ get the "Done running" output,
// signaling that nothing is actually getting interrupted