From 009dce820c7b53190ec357dd69993f9443a96022 Mon Sep 17 00:00:00 2001
From: Kamil Kloch <kamil.kloch@gmail.com>
Date: Sun, 17 Dec 2023 13:04:42 +0100
Subject: [PATCH] Properly handle short-circuiting.

---
 core/shared/src/main/scala/fs2/Stream.scala   | 18 +++--
 .../test/scala/fs2/StreamParJoinSuite.scala   | 65 +++++++++----------
 2 files changed, 44 insertions(+), 39 deletions(-)

diff --git a/core/shared/src/main/scala/fs2/Stream.scala b/core/shared/src/main/scala/fs2/Stream.scala
index 70240b08c0..679a772b7b 100644
--- a/core/shared/src/main/scala/fs2/Stream.scala
+++ b/core/shared/src/main/scala/fs2/Stream.scala
@@ -4200,11 +4200,19 @@ object Stream extends StreamLowPriority {
   implicit final class ListStreamOps[F[_], O](private val xs: List[Stream[F, O]]) extends AnyVal {
 
     def parJoinUnbounded(implicit F: Concurrent[F]): Stream[F, O] =
-      Stream.eval(Channel.bounded[F, Chunk[O]](64)).flatMap { c =>
-        val producers = xs.parTraverse_(_.chunks.foreach(x => c.send(x).void).compile.drain)
-        c.stream
-          .concurrently(Stream.exec(producers *> c.close.void))
-          .unchunks
+      if (xs.size == 1) xs.head
+      else {
+        Stream.eval((Channel.bounded[F, Chunk[O]](64), F.deferred[Unit]).tupled).flatMap {
+          case (c, stopPublishers) =>
+            val outcomes = xs
+              .parTraverse_(_.chunks.foreach(x => c.send(x).void).compile.drain)
+              .guarantee(c.close.void)
+
+            Stream
+              .bracket(F.start(outcomes.race(stopPublishers.get).void))(fiber =>
+                stopPublishers.complete(()) >> fiber.joinWithUnit
+              ) >> c.stream.unchunks
+        }
       }
   }
 
diff --git a/core/shared/src/test/scala/fs2/StreamParJoinSuite.scala b/core/shared/src/test/scala/fs2/StreamParJoinSuite.scala
index 729462cc50..d62bfb33ea 100644
--- a/core/shared/src/test/scala/fs2/StreamParJoinSuite.scala
+++ b/core/shared/src/test/scala/fs2/StreamParJoinSuite.scala
@@ -254,17 +254,16 @@ class StreamParJoinSuite extends Fs2Suite {
         .value
         .flatMap { actual =>
           IO(assertEquals(actual, Left(TestException)))
-        }
-
-//      List(1, 2, 3)
-//        .map(f)
-//        .parJoinUnbounded
-//        .compile
-//        .toList
-//        .value
-//        .flatMap { actual =>
-//          IO(assertEquals(actual, Left(TestException)))
-//        }
+        } >>
+        List(1, 2, 3)
+          .map(f)
+          .parJoinUnbounded
+          .compile
+          .toList
+          .value
+          .flatMap { actual =>
+            IO(assertEquals(actual, Left(TestException)))
+          }
     }
 
     test(
@@ -322,18 +321,17 @@ class StreamParJoinSuite extends Fs2Suite {
         .value
         .flatMap { actual =>
           IO(assertEquals(actual, Some(Set("1", "2", "3"))))
-        }
-
-      List(1, 2, 3)
-        .map(f)
-        .parJoinUnbounded
-        .compile
-        .toList
-        .map(_.toSet)
-        .value
-        .flatMap { actual =>
-          IO(assertEquals(actual, Some(Set("1", "2", "3"))))
-        }
+        } >>
+        List(1, 2, 3)
+          .map(f)
+          .parJoinUnbounded
+          .compile
+          .toList
+          .map(_.toSet)
+          .value
+          .flatMap { actual =>
+            IO(assertEquals(actual, Some(Set("1", "2", "3"))))
+          }
     }
 
     test(
@@ -351,17 +349,16 @@ class StreamParJoinSuite extends Fs2Suite {
         .value
         .flatMap { actual =>
           IO(assertEquals(actual, None))
-        }
-
-//      List(1, 2, 3)
-//        .map(f)
-//        .parJoinUnbounded
-//        .compile
-//        .toList
-//        .value
-//        .flatMap { actual =>
-//          IO(assertEquals(actual, None))
-//        }
+        } >>
+        List(1, 2, 3)
+          .map(f)
+          .parJoinUnbounded
+          .compile
+          .toList
+          .value
+          .flatMap { actual =>
+            IO(assertEquals(actual, None))
+          }
     }
 
     test("do not block while evaluating an OptionT.none outer stream") {