POC thread-local iolocals

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 +250,12 @@ private final class IOFiber[A](
             pushTracingEvent(cur.event)
           }
 
+          var locals: IOLocals = null
+          if (dumpLocals) {
+            locals = new IOLocals(localState)
+            IOLocals.threadLocal.set(locals)
+          }
+
           var error: Throwable = null
           val r =
             try cur.thunk()
@@ -260,6 +266,11 @@ private final class IOFiber[A](
                 onFatalFailure(t)
             }
 
+          if (dumpLocals) {
+            localState = locals.getState()
+            IOLocals.threadLocal.set(null)
+          }
+
           val next =
             if (error == null) succeeded(r, 0)
             else failed(error, 0)

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

@@ -0,0 +1,21 @@
+package cats.effect
+package unsafe
+
+// TODO handle defaults and lenses. all do-able, just needs refactoring ...
+final class IOLocals private[effect] (private[this] var state: IOLocalState) {
+
+  private[effect] def getState(): IOLocalState = state
+
+  def get[A](iol: IOLocal[A]): A = state(iol).asInstanceOf[A]
+
+  def set[A](iol: IOLocal[A], value: A): Unit = state += (iol -> value)
+
+  def reset[A](iol: IOLocal[A]): Unit = state -= iol
+
+  // TODO other ops from IOLocal
+
+}
+
+object IOLocals {
+  val threadLocal = ThreadLocal.withInitial[IOLocals](() => null)
+}