[DONOTMERGE] Demonstrates issue #2330

core/jvm/src/main/scala/UnreliableInterruption.scala

@@ -0,0 +1,67 @@
+import cats.effect.concurrent.{Deferred, Ref}
+import cats.effect.{ExitCode, IO, IOApp}
+import fs2.{Pipe, Pull, Stream}
+
+object UnreliableInterruption extends IOApp {
+  private def resume[A, B](mk: A => Stream[IO, B], checkpoint: B => A)(start: A): Stream[IO, B] = {
+    def go(s: Stream[IO, Either[Throwable, B]], watermark: A): Pull[IO, B, Unit] = s.pull.uncons1.flatMap {
+      case Some((Right(b), rest)) => Pull.output1(b) >> go(rest, checkpoint(b))
+      case Some((Left(_), _)) => go(mk(watermark).attempt, watermark)
+      case None => go(mk(watermark).attempt, watermark)
+    }
+
+    go(mk(start).attempt, start).stream
+  }
+
+  // Interrupt the stream after a five items, up to a max number of times
+  private def interrupter[A](deferred: Deferred[IO, Unit], maxInterrupts: Int, interruptCount: Ref[IO, Int]): Pipe[IO, A, A] = {
+    input: Stream[IO, A] =>
+      input.zipWithIndex
+        .evalTap {
+          case (_, 5) => interruptCount.getAndUpdate(_ + 1).flatMap { i =>
+            if (i < maxInterrupts) {
+              deferred.complete(())
+            } else IO.unit
+          }
+          case _ => IO.unit
+        }
+        .map(_._1)
+        .interruptWhen(deferred.get.attempt)
+  }
+
+
+  def run(args: List[String]): IO[ExitCode] = {
+    val stream: Int => Stream[IO, Int] = Stream.iterate(_)(_ + 1)
+
+    val usuallyWorks = for {
+      interruptCount <- Ref.of[IO, Int](0)
+      msg <- resume[Int, Int](
+        start => Stream.eval(Deferred[IO, Unit]).flatMap(d => stream(start).through(interrupter(d, 1, interruptCount))),
+        _ + 1
+      )(0)
+        .take(1000)
+        .compile
+        .toList
+        .map(lst => s"${lst.size} should be 1000, meaning it restarted once")
+    } yield msg
+
+    val usuallyDoesNot = for {
+      interruptCount <- Ref.of[IO, Int](0)
+      msg <- resume[Int, Int](
+        start => Stream.eval(Deferred[IO, Unit]).flatMap(d => stream(start).through(interrupter(d, 10, interruptCount))),
+        _ + 1
+      )(0)
+        .take(1000)
+        .compile
+        .toList
+        .map(lst => s"${lst.size} should be 1000, meaning it restarted 10 times (but it's not)")
+    } yield msg
+
+    val output = for {
+      a <- usuallyWorks
+      b <- usuallyDoesNot
+    } yield List(a, b).mkString("\n")
+
+    output.map(println).as(ExitCode.Success)
+  }
+}