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Implement a safer Resource.attempt which releases acquired resource(s) in case of error #4128

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Implement a safer Resource.attempt which releases acquired resource(s) in case of error #4128

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8d16f92
Implement a safer Resource.attempt, fix #3757
lenguyenthanh Sep 2, 2024
07fa387
Add test cases for safe attempt
lenguyenthanh Sep 5, 2024
2839446
Make allocatedCase interupible
lenguyenthanh Sep 5, 2024
3c2d6af
Implement handleErrorWith with MonadCancelThrow
lenguyenthanh Sep 5, 2024
cb1a293
Deprecate ResourceMonadError
lenguyenthanh Sep 5, 2024
3b7d0ee
Best effort to improve deprecated attempt
lenguyenthanh Sep 7, 2024
206020c
Deprecate catsEffectMonadErrorForResource
lenguyenthanh Sep 7, 2024
cd729a5
Apply suggestions from code review
lenguyenthanh Sep 8, 2024
f00b9cd
Revert implicit for Resource MonadError instance
lenguyenthanh Sep 8, 2024
40cb12a
Verify attempt returns Left in case of error
lenguyenthanh Sep 8, 2024
840514d
Fix compilation error
lenguyenthanh Sep 8, 2024
6bda660
Better deprecated message
lenguyenthanh Sep 8, 2024
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2 changes: 1 addition & 1 deletion core/jvm/src/main/scala/cats/effect/metrics/JvmCpuStarvationMetrics.scala
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Expand Up @@ -66,7 +66,7 @@ private[effect] object JvmCpuStarvationMetrics {
case (mbeanServer, _) => IO.blocking(mbeanServer.unregisterMBean(mBeanObjectName))
}
.map(_._2)
.handleErrorWith[CpuStarvationMetrics, Throwable] { th =>
.handleErrorWith[CpuStarvationMetrics] { th =>
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This change because handleErrorWith[E, A] now requires explicit ApplicativeError

Resource.eval(Console[IO].errorln(warning(th))).map(_ => new NoOpCpuStarvationMetrics)
}
}
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45 changes: 37 additions & 8 deletions kernel/shared/src/main/scala/cats/effect/kernel/Resource.scala
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Expand Up @@ -700,7 +700,9 @@ sealed abstract class Resource[F[_], +A] extends Serializable {
}
}

def attempt[E](implicit F: ApplicativeError[F, E]): Resource[F, Either[E, A]] =
@deprecated("Use overload with MonadCancelThrow", "3.6.0")
def attempt[E](F: ApplicativeError[F, E]): Resource[F, Either[E, A]] = {
implicit val x: ApplicativeError[F, E] = F
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We can do a trick here to automatically fix more callsites without requirng them to recompile: we can pattern match on F and if it is Sync or Async then we can use the fixed implementation instead.

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I'm not sure how to implement your suggestion.

I tried:

  @deprecated("Use overload with MonadCancelThrow", "3.6.0")
  def attempt[E](F: ApplicativeError[F, E]): Resource[F, Either[E, A]] =
    F match {
      case x: Async[_] =>
        implicit val x: MonadCancelThrow[F] = x
        attempt
      case x: Sync[_] => attempt(x)
        implicit val x: MonadCancelThrow[F] = x
        attempt
      case _ => ???
   }

But it doesn't work as attempt with MonadCancelThrow returns Resource[F, Either[Throwable, A]] but not Resource[F, Either[E, A]]

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You can cast unsafely. If F is Sync then E must be Throwable. Also I just realized since Async <: Sync we only need to match for Sync.

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Fixed it 3b7d0ee

this match {
case Allocate(resource) =>
Resource.applyFull { poll =>
Expand All @@ -710,19 +712,36 @@ sealed abstract class Resource[F[_], +A] extends Serializable {
}
}
case Bind(source, f) =>
Resource.unit.flatMap(_ => source.attempt).flatMap {
Resource.unit.flatMap(_ => source.attempt(F)).flatMap {
case Left(error) => Resource.pure(error.asLeft)
case Right(s) => f(s).attempt
case Right(s) => f(s).attempt(F)
}
case p @ Pure(_) =>
Resource.pure(p.a.asRight)
case e @ Eval(_) =>
Resource.eval(e.fa.attempt)
}
}

def attempt(implicit F: MonadCancelThrow[F]): Resource[F, Either[Throwable, A]] =
Resource.applyFull[F, Either[Throwable, A]] { poll =>
poll(allocatedCase).attempt.map {
case Left(error) => (error.asLeft[A], _ => ().pure[F])
case Right((a, r)) => (a.asRight[Throwable], r)
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}
}

@deprecated("Use overload with MonadCancelThrow", "3.6.0")
def handleErrorWith[B >: A, E](f: E => Resource[F, B])(
implicit F: ApplicativeError[F, E]): Resource[F, B] =
attempt.flatMap {
F: ApplicativeError[F, E]): Resource[F, B] =
attempt(F).flatMap {
case Right(a) => Resource.pure(a)
case Left(e) => f(e)
}

def handleErrorWith[B >: A](f: Throwable => Resource[F, B])(
implicit F: MonadCancelThrow[F]): Resource[F, B] =
attempt(F).flatMap {
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imo, We should add a "safe" variant of handleErrorWith as well to avoid inconsistent behaviors between this and attempt.

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Good catch. You are right, we definitely should do that.

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fixed in 3c2d6af

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case Right(a) => Resource.pure(a)
case Left(e) => f(e)
}
Expand Down Expand Up @@ -1330,10 +1349,19 @@ abstract private[effect] class ResourceFOInstances1 {
}

abstract private[effect] class ResourceMonadCancel[F[_]]
extends ResourceMonadError[F, Throwable]
extends ResourceMonad[F]
with MonadCancel[Resource[F, *], Throwable] {
implicit protected def F: MonadCancel[F, Throwable]

override def attempt[A](fa: Resource[F, A]): Resource[F, Either[Throwable, A]] =
fa.attempt(F)
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def handleErrorWith[A](fa: Resource[F, A])(f: Throwable => Resource[F, A]): Resource[F, A] =
fa.handleErrorWith(f)

def raiseError[A](e: Throwable): Resource[F, A] =
Resource.raiseError[F, A, Throwable](e)

def canceled: Resource[F, Unit] = Resource.canceled

def forceR[A, B](fa: Resource[F, A])(fb: Resource[F, B]): Resource[F, B] =
Expand Down Expand Up @@ -1455,17 +1483,18 @@ abstract private[effect] class ResourceAsync[F[_]]
Resource.executionContext
}

@deprecated("Use ResourceMonadCancel", "3.6.0")
abstract private[effect] class ResourceMonadError[F[_], E]
extends ResourceMonad[F]
with MonadError[Resource[F, *], E] {

implicit protected def F: MonadError[F, E]

override def attempt[A](fa: Resource[F, A]): Resource[F, Either[E, A]] =
fa.attempt
fa.attempt(F)
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this still used the old attempt but We can't use the new one because the error type is not fixed to Throwable.

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We should mark this instance as deprecated and create a new instance that requires MonadCancelThrow.

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@lenguyenthanh lenguyenthanh Sep 5, 2024 •
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fixed in cb1a293. But now

cats-effect/kernel/shared/src/main/scala/cats/effect/kernel/Resource.scala

Lines 1300 to 1306 in cb1a293

private[effect] trait ResourceHOInstances4 extends ResourceHOInstances5 {
implicit def catsEffectMonadErrorForResource[F[_], E](
implicit F0: MonadError[F, E]): MonadError[Resource[F, *], E] =
new ResourceMonadError[F, E] {
def F = F0
}
}
complains because it use the deprecated ResourceMonadError.

I'm not sure what the best way to fix this. I feel like We can remove it completely. But maybe it broke client code.

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We deprecate this one too :)

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We deprecate this one but keep the trait hierarchy?

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That's a good question. Maybe we should try to deprioritize this instance by moving it to the bottom of the hierarchy. Alternatively, we could just remove the implicit modifier.

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I kept the hierachy the same but removed implicit modifier and deprecated catsEffectMonadErrorForResource function.

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I thought about it more and actually I think it's okay to leave the implicit there. We don't need to change any prioritization, because we aren't introducing any new instances, and the instance derived from MonadCancelThrow (which is required for the fixed attempt) is already higher priority than this one.

By leaving it implicit, users will get an actionable deprecation instead of non-compiling code.


def handleErrorWith[A](fa: Resource[F, A])(f: E => Resource[F, A]): Resource[F, A] =
fa.handleErrorWith(f)
fa.handleErrorWith(f)(F)

def raiseError[A](e: E): Resource[F, A] =
Resource.raiseError[F, A, E](e)
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23 changes: 23 additions & 0 deletions tests/shared/src/test/scala/cats/effect/ResourceSpec.scala
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Expand Up @@ -1172,6 +1172,29 @@ class ResourceSpec extends BaseSpec with ScalaCheck with Discipline {
}
}

"attempt" >> {

"releases resource on error" in ticked { implicit ticker =>
IO.ref(0)
.flatMap { ref =>
val resource = Resource.make(ref.update(_ + 1))(_ => ref.update(_ + 1))
val error = Resource.raiseError[IO, Unit, Throwable](new Exception)
(resource *> error).attempt.use { _ => ref.get.map { _ must be_==(2) } }
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instead of ignoring the value _ we can check that it's a Left.

}
.void must completeAs(())
}

"acquire is interruptible" in ticked { implicit ticker =>
val sleep = IO.sleep(1.second)
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val timeout = 500.millis
IO.ref(false).flatMap { ref =>
val r = Resource.makeFull[IO, Unit] { poll => poll(sleep).onCancel(ref.set(true)) }(_ =>
IO.unit)
r.attempt.timeout(timeout).attempt.use_ *> ref.get
} must completeAs(true)
}
}

"uncancelable" >> {
"does not suppress errors within use" in real {
case object TestException extends RuntimeException
Expand Down
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