IOLocal propagation for unsafe access

build.sbt

@@ -676,7 +676,15 @@ lazy val core = crossProject(JSPlatform, JVMPlatform, NativePlatform)
         "cats.effect.unsafe.IORuntimeBuilder.this"),
       // introduced by #3695, which enabled fiber dumps on native
       ProblemFilters.exclude[MissingClassProblem](
-        "cats.effect.unsafe.FiberMonitorCompanionPlatform")
+        "cats.effect.unsafe.FiberMonitorCompanionPlatform"),
+      // introduced by #3636, IOLocal propagation
+      // IOLocal is a sealed trait
+      ProblemFilters.exclude[ReversedMissingMethodProblem]("cats.effect.IOLocal.getOrDefault"),
+      ProblemFilters.exclude[ReversedMissingMethodProblem]("cats.effect.IOLocal.set"),
+      ProblemFilters.exclude[ReversedMissingMethodProblem]("cats.effect.IOLocal.reset"),
+      ProblemFilters.exclude[ReversedMissingMethodProblem]("cats.effect.IOLocal.lens"),
+      // this filter is particulary terrible, because it can also mask real issues :(
+      ProblemFilters.exclude[DirectMissingMethodProblem]("cats.effect.IOLocal.lens")
     ) ++ {
       if (tlIsScala3.value) {
         // Scala 3 specific exclusions
@@ -921,7 +929,8 @@ lazy val tests: CrossProject = crossProject(JSPlatform, JVMPlatform, NativePlatf
     scalacOptions ~= { _.filterNot(_.startsWith("-P:scalajs:mapSourceURI")) }
   )
   .jvmSettings(
-    fork := true
+    fork := true,
+    Test / javaOptions += "-Dcats.effect.ioLocalPropagation=true"
   )
   .nativeSettings(
     Compile / mainClass := Some("catseffect.examples.NativeRunner")

core/js-native/src/main/scala/cats/effect/IOFiberConstants.scala

@@ -46,6 +46,8 @@ private object IOFiberConstants {
   final val AutoCedeR = 7
   final val DoneR = 8
 
+  final val ioLocalPropagation = false
+
   @nowarn212
   @inline def isVirtualThread(t: Thread): Boolean = false
 }

core/js-native/src/main/scala/cats/effect/IOLocalPlatform.scala

@@ -0,0 +1,19 @@
+/*
+ * Copyright 2020-2024 Typelevel
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package cats.effect
+
+private[effect] trait IOLocalPlatform[A]

core/js-native/src/main/scala/cats/effect/unsafe/WorkStealingThreadPool.scala

@@ -43,8 +43,9 @@ private[effect] sealed abstract class WorkStealingThreadPool[P] private ()
       Map[Runnable, Trace])
 }
 
-private[unsafe] sealed abstract class WorkerThread[P] private () extends Thread {
+private[effect] sealed abstract class WorkerThread[P] private () extends Thread {
   private[unsafe] def isOwnedBy(threadPool: WorkStealingThreadPool[_]): Boolean
   private[unsafe] def monitor(fiber: Runnable): WeakBag.Handle
   private[unsafe] def index: Int
+  private[effect] var currentIOFiber: IOFiber[_]
 }

core/jvm/src/main/java/cats/effect/IOFiberConstants.java

@@ -48,6 +48,8 @@ final class IOFiberConstants {
   static final byte AutoCedeR = 7;
   static final byte DoneR = 8;
 
+  static final boolean ioLocalPropagation = Boolean.getBoolean("cats.effect.ioLocalPropagation");
+
   static boolean isVirtualThread(final Thread thread) {
     try {
       return (boolean) THREAD_IS_VIRTUAL_HANDLE.invokeExact(thread);

core/jvm/src/main/scala/cats/effect/IOLocalPlatform.scala

@@ -0,0 +1,57 @@
+/*
+ * Copyright 2020-2024 Typelevel
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package cats.effect
+
+import IOFiberConstants.ioLocalPropagation
+
+private[effect] trait IOLocalPlatform[A] { self: IOLocal[A] =>
+
+  /**
+   * Returns a [[java.lang.ThreadLocal]] view of this [[IOLocal]] that allows to unsafely get,
+   * set, and remove (aka reset) the value in the currently running fiber. The system property
+   * `cats.effect.ioLocalPropagation` must be `true`, otherwise throws an
+   * [[java.lang.UnsupportedOperationException]].
+   */
+  def unsafeThreadLocal(): ThreadLocal[A] = if (ioLocalPropagation)
+    new ThreadLocal[A] {
+      override def get(): A = {
+        val fiber = IOFiber.currentIOFiber()
+        val state = if (fiber ne null) fiber.getLocalState() else IOLocalState.empty
+        self.getOrDefault(state)
+      }
+
+      override def set(value: A): Unit = {
+        val fiber = IOFiber.currentIOFiber()
+        if (fiber ne null) {
+          fiber.setLocalState(self.set(fiber.getLocalState(), value))
+        }
+      }
+
+      override def remove(): Unit = {
+        val fiber = IOFiber.currentIOFiber()
+        if (fiber ne null) {
+          fiber.setLocalState(self.reset(fiber.getLocalState()))
+        }
+      }
+    }
+  else
+    throw new UnsupportedOperationException(
+      "IOLocal-ThreadLocal propagation is disabled.\n" +
+        "Enable by setting cats.effect.ioLocalPropagation=true."
+    )
+
+}

core/jvm/src/main/scala/cats/effect/IOPlatform.scala

@@ -71,7 +71,7 @@ abstract private[effect] class IOPlatform[+A] extends Serializable { self: IO[A]
       implicit runtime: unsafe.IORuntime): Option[A] = {
     val queue = new ArrayBlockingQueue[Either[Throwable, A]](1)
 
-    unsafeRunAsync { r =>
+    val fiber = unsafeRunAsyncImpl { r =>
       queue.offer(r)
       ()
     }
@@ -82,6 +82,9 @@ abstract private[effect] class IOPlatform[+A] extends Serializable { self: IO[A]
     } catch {
       case _: InterruptedException =>
         None
+    } finally {
+      if (IOFiberConstants.ioLocalPropagation)
+        IOLocal.setThreadLocalState(fiber.getLocalState())
     }
   }
 

core/jvm/src/main/scala/cats/effect/unsafe/WorkerThread.scala

@@ -41,7 +41,7 @@ import java.util.concurrent.atomic.AtomicBoolean
  * system when compared to a fixed size thread pool whose worker threads all draw tasks from a
  * single global work queue.
  */
-private final class WorkerThread[P](
+private[effect] final class WorkerThread[P](
     idx: Int,
     // Local queue instance with exclusive write access.
     private[this] var queue: LocalQueue,
@@ -107,6 +107,8 @@ private final class WorkerThread[P](
   private val indexTransfer: LinkedTransferQueue[Integer] = new LinkedTransferQueue()
   private[this] val runtimeBlockingExpiration: Duration = pool.runtimeBlockingExpiration
 
+  private[effect] var currentIOFiber: IOFiber[_] = _
+
   private[this] val RightUnit = Right(())
   private[this] val noop = new Function0[Unit] with Runnable {
     def apply() = ()

core/shared/src/main/scala/cats/effect/IO.scala

@@ -958,6 +958,12 @@ sealed abstract class IO[+A] private () extends IOPlatform[A] {
    */
   def unsafeRunAsync(cb: Either[Throwable, A] => Unit)(
       implicit runtime: unsafe.IORuntime): Unit = {
+    unsafeRunAsyncImpl(cb)
+    ()
+  }
+
+  private[effect] def unsafeRunAsyncImpl(cb: Either[Throwable, A] => Unit)(
+      implicit runtime: unsafe.IORuntime): IOFiber[A @uncheckedVariance] =
     unsafeRunFiber(
       cb(Left(new CancellationException("The fiber was canceled"))),
       t => {
@@ -968,8 +974,6 @@ sealed abstract class IO[+A] private () extends IOPlatform[A] {
       },
       a => cb(Right(a))
     )
-    ()
-  }
 
   def unsafeRunAsyncOutcome(cb: Outcome[Id, Throwable, A @uncheckedVariance] => Unit)(
       implicit runtime: unsafe.IORuntime): Unit = {
@@ -1072,7 +1076,8 @@ sealed abstract class IO[+A] private () extends IOPlatform[A] {
       implicit runtime: unsafe.IORuntime): IOFiber[A @uncheckedVariance] = {
 
     val fiber = new IOFiber[A](
-      Map.empty,
+      if (IOFiberConstants.ioLocalPropagation) IOLocal.getThreadLocalState()
+      else IOLocalState.empty,
       oc =>
         oc.fold(
           {

core/shared/src/main/scala/cats/effect/IOFiber.scala

@@ -107,9 +107,18 @@ private final class IOFiber[A](
   @volatile
   private[this] var outcome: OutcomeIO[A] = _
 
+  def getLocalState(): IOLocalState = localState
+
+  def setLocalState(s: IOLocalState): Unit = localState = s
+
   override def run(): Unit = {
     // insert a read barrier after every async boundary
     readBarrier()
+
+    if (ioLocalPropagation) {
+      IOFiber.setCurrentIOFiber(this)
+    }
+
     (resumeTag: @switch) match {
       case 0 => execR()
       case 1 => asyncContinueSuccessfulR()
@@ -121,6 +130,10 @@ private final class IOFiber[A](
       case 7 => autoCedeR()
       case 8 => () // DoneR
     }
+
+    if (ioLocalPropagation) {
+      IOFiber.setCurrentIOFiber(null)
+    }
   }
 
   /* backing fields for `cancel` and `join` */
@@ -1559,6 +1572,23 @@ private object IOFiber {
   @static private[IOFiber] val OutcomeCanceled = Outcome.Canceled()
   @static private[effect] val RightUnit = Right(())
 
+  @static private[this] val threadLocal = new ThreadLocal[IOFiber[_]]
+  @static def currentIOFiber(): IOFiber[_] = {
+    val thread = Thread.currentThread()
+    if (thread.isInstanceOf[WorkerThread[_]])
+      thread.asInstanceOf[WorkerThread[_]].currentIOFiber
+    else
+      threadLocal.get()
+  }
+
+  @static private def setCurrentIOFiber(f: IOFiber[_]): Unit = {
+    val thread = Thread.currentThread()
+    if (thread.isInstanceOf[WorkerThread[_]])
+      thread.asInstanceOf[WorkerThread[_]].currentIOFiber = f
+    else
+      threadLocal.set(f)
+  }
+
   @static def onFatalFailure(t: Throwable): Nothing = {
     val interrupted = Thread.interrupted()