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fix: ensure Topic#publish1 returns accurate result during concurrent closure #3665

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fix: ensure Topic#publish1 returns accurate result during concurrent closure #3665

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a30644f
fix: ensure Topic#publish1 returns accurate result during concurrent …
guptapratykshh Jan 17, 2026
29e4b62
Fix race condition between Topic.publish1 and Topic.close
guptapratykshh Feb 2, 2026
20a44ae
Apply scalafmt
guptapratykshh Feb 2, 2026
eb774c0
Fix unused variable warning in Scala 2.12
guptapratykshh Feb 2, 2026
eb30b4e
Fix: revert uncancelable publish1 and make close non-blocking, relax …
guptapratykshh Mar 15, 2026
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17 changes: 15 additions & 2 deletions core/shared/src/main/scala/fs2/concurrent/Topic.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -164,8 +164,21 @@ object Topic {
case State.Closed() =>
Topic.closed.pure[F]
case State.Active(subs, _) =>
foreach(subs)(_.send(a).void)
.as(Topic.rightUnit)
subs.foldLeft(F.pure(Topic.rightUnit)) { case (acc, (_, chan)) =>
acc.flatMap {
case Left(Topic.Closed) => Topic.closed.pure[F]
case Right(_) =>
chan.send(a).flatMap {
case Right(_) => Topic.rightUnit.pure[F]
case Left(_) =>
// Channel send failed, check if topic was closed
state.get.map {
case State.Closed() => Topic.closed
case State.Active(_, _) => Topic.rightUnit
}
}
}
}
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@TomasMikula TomasMikula Jan 30, 2026

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Minor thing: I think constructing the effect in a right-associative way will lead to slightly faster execution for an IO-like F.

}

def subscribeAwait(maxQueued: Int): Resource[F, Stream[F, A]] =
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2 changes: 1 addition & 1 deletion core/shared/src/test/scala/fs2/concurrent/TopicSuite.scala
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Expand Up @@ -218,7 +218,7 @@ class TopicSuite extends Fs2Suite {

// https://github.com/typelevel/fs2/issues/3644
test(
"when publish1 returns success, subscribers must receive the event, even if the publish1 races with close".fail
"when publish1 returns success, subscribers must receive the event, even if the publish1 races with close"
) {
val check: IO[Unit] =
Topic[IO, String]
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