Missing information about failing background fibers

[ghostbuster91]

Tested against 3.3.0 and 3.3.5

Following code doesn't print any information about the fact that an exception was thrown in one of the fibers.

object Main extends IOApp {
  override def run(args: List[String]): IO[ExitCode] = {
    Thread.setDefaultUncaughtExceptionHandler((t, e) => println("Uncaught exception in thread: " + t, e))
    for {
      _ <- IO
        .sleep(5.seconds)
        .flatMap(_ => IO.delay(println("after sleep")))
        .flatMap(_ => IO.raiseError(new RuntimeException("something went wrong")))
        .start
      _ <- IO.delay(println("started"))
      _ <- IO.never[Unit]
    } yield ExitCode.Success
  }
}

I would expect it to print some information to stdout/stderr. It would be even better if there was a way to customize that information.

For example in zio there is a Plaform object that contains an error reporter which can be customized by the user.

In the Platform that is a part of every runtime, there is an error reporter that will be called by ZIO to report every unhandled error

Link to the original discussion on discord: https://discord.com/channels/632277896739946517/632278585700384799/946443755286855702

[armanbilge]

Btw, I wondered if there's some way to make a Supervisor-like thing to help with this situation as well (possibly in addition to dealing with it at the runtime-level). It seems like this is exactly the kind of thing to supervise 😉

trait ErrorSupervisor[F[_]] {
  def supervise[A](fa: F[A]): F[Fiber[F, Throwable, A]]
  def error: F[Throwable] // semantically blocks until the first error
}

[ghostbuster91]

I must admit that it crossed my mind to check whether the Supervisor doesn't have any kind of option for that. I like that direction :)

[djspiewak]

Btw I realized that we do have an obvious place to put this: the compute ExecutionContext. In fact, it's already there. We don't have a good way of modifying it, but it does exist and we simply don't use it on fiber termination. This should be fixed, and we should add a better way of adjusting the handler.

[ghostbuster91]

Somehow it didn't get closed automatically 🤔 Closing as it was resolved.