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Lossless cancelable 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
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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 CancelableK = 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"))
}
.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 CancelableK = 10;

// resume ids
static final byte ExecR = 0;
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12 changes: 11 additions & 1 deletion core/shared/src/main/scala/cats/effect/IO.scala
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Expand Up @@ -447,7 +447,7 @@ sealed abstract class IO[+A] private () extends IOPlatform[A] {
* [[onCancel]]
*/
def cancelable(fin: IO[Unit]): IO[A] =
Spawn[IO].cancelable(this, fin)
IO.Cancelable(this, fin)

def forceR[B](that: IO[B]): IO[B] =
// cast is needed here to trick the compiler into avoiding the IO[Any]
Expand Down Expand Up @@ -2059,6 +2059,12 @@ object IO extends IOCompanionPlatform with IOLowPriorityImplicits with TuplePara
def onCancel[A](ioa: IO[A], fin: IO[Unit]): IO[A] =
ioa.onCancel(fin)

override def cancelable[A](poll: Poll[IO], ioa: IO[A], ack: IO[Unit]): IO[A] =
ioa.cancelable(ack)

override def cancelable[A](ioa: IO[A], ack: IO[Unit]): IO[A] =
ioa.cancelable(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 +2334,10 @@ object IO extends IOCompanionPlatform with IOLowPriorityImplicits with TuplePara
def tag = 24
}

private[effect] final case class Cancelable[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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