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Lossless cancelation and joinOrCancel #4641

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e1a5679
Reduce MutexSuite iterations to fix flakey CI
reardonj Jul 17, 2026
d78638a
Suspend crash in IO
reardonj Jul 17, 2026
c6304c4
Replace unsafe joins with joinOrCancel
reardonj Jul 18, 2026
ba3ec52
make cancelable a primitive in IO
reardonj Jul 18, 2026
2a9e3b6
optimize IO.racePair
reardonj Jul 18, 2026
e9b89e5
separate onCancelRequested from cancelable
reardonj Jul 31, 2026
9203754
GenConcurrent doesn't need onCancelRequested
reardonj Jul 31, 2026
006483e
return cancelable syntax for IO
reardonj Jul 31, 2026
f7e220a
Code cleanup of onCancelRequested
reardonj Jul 31, 2026
7600193
Fix scaladoc
reardonj Jul 31, 2026
baaf6e9
Unmask onCancelRequested on failure
reardonj Aug 23, 2026
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1 change: 1 addition & 0 deletions core/js-native/src/main/scala/cats/effect/IOFiberConstants.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -34,6 +34,7 @@ private object IOFiberConstants {
final val UncancelableK = 7
final val UnmaskK = 8
final val AttemptK = 9
final val OnCancelRequestedK = 10

// resume ids
final val ExecR = 0
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2 changes: 1 addition & 1 deletion core/js/src/main/scala/cats/effect/IOApp.scala
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Expand Up @@ -270,7 +270,7 @@ trait IOApp {
case Left(Outcome.Errored(t)) => IO.raiseError(t)
case Left(Outcome.Succeeded(code)) => code
case Right(Outcome.Errored(t)) => IO.raiseError(t)
case Right(_) => sys.error("impossible")
case Right(_) => IO.delay(sys.error("impossible"))

@stasimus stasimus Aug 23, 2026

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I think this statement is not true anymore and reachable.

survives only because shouldFinalize() discards the suspended delay first

@stasimus stasimus Aug 23, 2026

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If needed I can spend time crafting the test - I may be wrong.

@reardonj reardonj Aug 23, 2026

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It's been a while since I looked at this, but iirc, it ends up being canceled before the sys.error executes.

@stasimus stasimus Aug 23, 2026

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Please look into test
reardonj#2

}
.unsafeRunFiber(
hardExit(cancelCode),
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1 change: 1 addition & 0 deletions core/jvm/src/main/java/cats/effect/IOFiberConstants.java
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Expand Up @@ -36,6 +36,7 @@ final class IOFiberConstants {
static final byte UncancelableK = 7;
static final byte UnmaskK = 8;
static final byte AttemptK = 9;
static final byte OnCancelRequestedK = 10;

// resume ids
static final byte ExecR = 0;
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34 changes: 34 additions & 0 deletions core/shared/src/main/scala/cats/effect/IO.scala
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Expand Up @@ -449,6 +449,19 @@ sealed abstract class IO[+A] private () extends IOPlatform[A] {
def cancelable(fin: IO[Unit]): IO[A] =
Spawn[IO].cancelable(this, fin)

/**
* Run the given effect when cancelation is requested. Unlike [[onCancel]], this will run
* before cancelation is observed, on a separate fiber, and will always allow `fa` to complete
* before cancelation is observed.
*
* @param ack
* an effect which orchestrates some external state which terminates `fa`
* @see
* [[onCancel]]
*/
def onCancelRequested(ack: IO[Unit]): IO[A] =
IO.OnCancelRequested(this, ack)

def forceR[B](that: IO[B]): IO[B] =
// cast is needed here to trick the compiler into avoiding the IO[Any]
asInstanceOf[IO[Unit]].handleError(_ => ()).productR(that)
Expand Down Expand Up @@ -2059,6 +2072,23 @@ object IO extends IOCompanionPlatform with IOLowPriorityImplicits with TuplePara
def onCancel[A](ioa: IO[A], fin: IO[Unit]): IO[A] =
ioa.onCancel(fin)

/**
* Run the given effect when cancelation is requested. Unlike [[onCancel]], this will run
* before cancelation is observed, on a separate fiber, and will always allow `fa` to
* complete before cancelation is observed.
*
* @param fa
* the effect to be canceled
* @param ack
* an effect which orchestrates some external state which terminates `fa`
* @see
* [[cancelable]]
* @see
* [[onCancel]]
*/
override def onCancelRequested[A](fa: IO[A], ack: IO[Unit]): IO[A] =
fa.onCancelRequested(ack)

override def bracketFull[A, B](acquire: Poll[IO] => IO[A])(use: A => IO[B])(
release: (A, OutcomeIO[B]) => IO[Unit]): IO[B] =
IO.bracketFull(acquire)(use)(release)
Expand Down Expand Up @@ -2328,6 +2358,10 @@ object IO extends IOCompanionPlatform with IOLowPriorityImplicits with TuplePara
def tag = 24
}

private[effect] final case class OnCancelRequested[A](f: IO[A], ack: IO[Unit]) extends IO[A] {
def tag = 25
}

// INTERNAL, only created by the runloop itself as the terminal state of several operations
private[effect] case object EndFiber extends IO[Nothing] {
def tag = -1
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