IOLocal propagation for unsafe access

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

@@ -43,4 +43,6 @@ private object IOFiberConstants {
   final val CedeR = 6
   final val AutoCedeR = 7
   final val DoneR = 8
+
+  final val dumpLocals = false
 }

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

@@ -43,4 +43,6 @@ final class IOFiberConstants {
   static final byte CedeR = 6;
   static final byte AutoCedeR = 7;
   static final byte DoneR = 8;
+
+  static final boolean dumpLocals = Boolean.getBoolean("cats.effect.tracing.dumpLocals");
 }

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

@@ -250,6 +250,10 @@ private final class IOFiber[A](
             pushTracingEvent(cur.event)
           }
 
+          if (dumpLocals) {
+            IOLocals.setState(localState)
+          }
+
           var error: Throwable = null
           val r =
             try cur.thunk()
@@ -260,6 +264,10 @@ private final class IOFiber[A](
                 onFatalFailure(t)
             }
 
+          if (dumpLocals) {
+            localState = IOLocals.getAndClearState()
+          }
+
           val next =
             if (error == null) succeeded(r, 0)
             else failed(error, 0)

core/shared/src/main/scala/cats/effect/unsafe/IOLocals.scala

@@ -0,0 +1,26 @@
+package cats.effect
+package unsafe
+
+// TODO handle defaults and lenses. all do-able, just needs refactoring ...
+object IOLocals {
+  def get[A](iol: IOLocal[A]): A = threadLocal.get.apply(iol).asInstanceOf[A]
+
+  def set[A](iol: IOLocal[A], value: A): Unit =
+    threadLocal.set(threadLocal.get() + (iol -> value))
+
+  def reset[A](iol: IOLocal[A]): Unit = threadLocal.set(threadLocal.get() - iol)
+
+  // TODO other ops from IOLocal
+
+  private[this] val empty: IOLocalState = Map.empty
+  private[this] val threadLocal = new ThreadLocal[IOLocalState] {
+    override def initialValue() = empty
+  }
+
+  private[effect] def setState(state: IOLocalState) = threadLocal.set(state)
+  private[effect] def getAndClearState() = {
+    val state = threadLocal.get()
+    threadLocal.set(empty)
+    state
+  }
+}