Merge pull request #2653 from vasilmkd/concurrently-transformers-25x

Fix `Stream#concurrently` for short-circuiting monad transformers - `2.5.x`

Author: mpilquist

core/shared/src/main/scala/fs2/Stream.scala

@@ -545,25 +545,24 @@ final class Stream[+F[_], +O] private[fs2] (private val free: FreeC[F, O, Unit])
   )(implicit F: Concurrent[F2]): Stream[F2, O] = {
     val fstream: F2[Stream[F2, O]] = for {
       interrupt <- Deferred[F2, Unit]
-      doneR <- Deferred[F2, Either[Throwable, Unit]]
+      backResult <- Deferred[F2, Either[Throwable, Unit]]
     } yield {
-      def runR: F2[Unit] =
-        that.interruptWhen(interrupt.get.attempt).compile.drain.attempt.flatMap { r =>
-          doneR.complete(r) >> {
-            if (r.isLeft)
-              interrupt
-                .complete(())
-                .attempt
-                .void // interrupt only if this failed otherwise give change to `this` to finalize
-            else F.unit
-          }
-        }
+      def watch[A](str: Stream[F2, A]) = str.interruptWhen(interrupt.get.attempt)
+
+      val compileBack: F2[Unit] = watch(that).compile.drain.guaranteeCase {
+        // Pass the result of backstream completion in the backResult deferred.
+        // If result of back-stream was failed, interrupt fore. Otherwise, let it be
+        case ExitCase.Error(t) =>
+          backResult.complete(Left(t)).attempt >> interrupt.complete(()).handleError(_ => ())
+        case _ => backResult.complete(Right(())).handleError(_ => ())
+      }
 
       // stop background process but await for it to finalise with a result
-      val stopBack: F2[Unit] = interrupt.complete(()).attempt >> doneR.get.flatMap(F.fromEither)
+      // We use F.fromEither to bring errors from the back into the fore
+      val stopBack: F2[Unit] =
+        interrupt.complete(()).attempt >> backResult.get.flatMap(F.fromEither)
 
-      Stream.bracket(F.start(runR))(_ => stopBack) >>
-        this.interruptWhen(interrupt.get.attempt)
+      Stream.bracket(F.start(compileBack))(_ => stopBack) >> watch(this)
     }
 
     Stream.eval(fstream).flatten

core/shared/src/test/scala/fs2/StreamConcurrentlySuite.scala

@@ -23,6 +23,7 @@ package fs2
 
 import scala.concurrent.duration._
 
+import cats.data.EitherT
 import cats.effect.IO
 import cats.effect.concurrent.{Deferred, Ref, Semaphore}
 import cats.syntax.all._
@@ -146,4 +147,22 @@ class StreamConcurrentlySuite extends Fs2Suite {
     }
   }
 
+  test("background stream completes with short-circuiting transformers") {
+    Stream(1, 2, 3)
+      .concurrently(Stream.eval(EitherT.leftT[IO, Int]("left")))
+      .compile
+      .lastOrError
+      .value
+      .assertEquals(Right(3))
+  }
+
+  test("foreground stream short-circuits") {
+    Stream(1, 2, 3)
+      .evalMap(n => EitherT.cond[IO](n % 2 == 0, n, "left"))
+      .concurrently(Stream.eval(EitherT.rightT[IO, String](42)))
+      .compile
+      .lastOrError
+      .value
+      .assertEquals(Left("left"))
+  }
 }

io/src/test/scala/fs2/io/IoSuite.scala

@@ -22,6 +22,7 @@
 package fs2
 package io
 
+import cats.data.EitherT
 import cats.effect.{Blocker, IO, Resource}
 import fs2.Fs2Suite
 import org.scalacheck.{Arbitrary, Gen, Shrink}
@@ -215,7 +216,8 @@ class IoSuite extends Fs2Suite {
     }
 
     test("can copy more than Int.MaxValue bytes") {
-      // Unit test adapted from the original issue reproduction at https://github.com/mrdziuban/fs2-writeOutputStream.
+      // Unit test adapted from the original issue reproduction at
+      // https://github.com/mrdziuban/fs2-writeOutputStream.
 
       val byteStream =
         Stream
@@ -238,6 +240,16 @@ class IoSuite extends Fs2Suite {
           .drain
       }
     }
+
+    test("works with short-circuiting monad transformers") {
+      // Unit test adapted from the original issue reproduction at
+      // https://github.com/mrdziuban/fs2-readOutputStream-EitherT.
+
+      Blocker[IO].use { blocker =>
+        readOutputStream(blocker, 1)(_ => EitherT.left[Unit](IO.unit)).compile.drain.value
+          .timeout(5.seconds)
+      }
+    }
   }
 
   group("unsafeReadInputStream") {