Add Storage.CrossThreadLocal

core/kamon-core-bench/src/main/scala/kamon/bench/ThreadLocalStorageBenchmark.scala

@@ -16,9 +16,9 @@
 
 package kamon.bench
 
-import java.util.concurrent.TimeUnit
+import kamon.context.Storage.CrossThreadLocal
 
-import kamon.context.Storage.Scope
+import java.util.concurrent.TimeUnit
 import kamon.context.{Context, Storage}
 import org.openjdk.jmh.annotations._
 
@@ -28,19 +28,19 @@ class ThreadLocalStorageBenchmark {
   val TestKey: Context.Key[Int] = Context.key("test-key", 0)
   val ContextWithKey: Context = Context.of(TestKey, 43)
 
-  val TLS: Storage =  new OldThreadLocal
+  val CrossTLS: Storage =  new CrossThreadLocal
   val FTLS: Storage =  new Storage.ThreadLocal
 
 
   @Benchmark
   @BenchmarkMode(Array(Mode.AverageTime))
   @OutputTimeUnit(TimeUnit.NANOSECONDS)
   @Fork
-  def currentThreadLocal: Context = {
-    val scope = TLS.store(ContextWithKey)
-    TLS.current()
+  def crossThreadLocal: Context = {
+    val scope = CrossTLS.store(ContextWithKey)
+    CrossTLS.current()
     scope.close()
-    TLS.current()
+    CrossTLS.current()
   }
 
   @Benchmark
@@ -54,28 +54,3 @@ class ThreadLocalStorageBenchmark {
     FTLS.current()
   }
 }
-
-
-class OldThreadLocal extends Storage {
-  private val tls = new java.lang.ThreadLocal[Context]() {
-    override def initialValue(): Context = Context.Empty
-  }
-
-  override def current(): Context =
-    tls.get()
-
-  override def store(context: Context): Scope = {
-    val newContext = context
-    val previousContext = tls.get()
-    tls.set(newContext)
-
-    new Scope {
-      override def context: Context = newContext
-      override def close(): Unit = tls.set(previousContext)
-    }
-  }
-}
-
-object OldThreadLocal {
-  def apply(): OldThreadLocal = new OldThreadLocal()
-}

core/kamon-core-tests/src/test/scala/kamon/context/ThreadLocalStorageSpec.scala

@@ -16,9 +16,17 @@
 package kamon.context
 
 
-import org.scalatest.{Matchers, WordSpec}
+import kamon.context.Storage.Scope
+import org.scalatest.{WordSpec, BeforeAndAfterAll, AsyncWordSpec, Matchers, Assertion}
 
-class ThreadLocalStorageSpec extends WordSpec with Matchers {
+import java.util.concurrent.Executors
+import scala.concurrent.{Promise, ExecutionContext, Future}
+import scala.util.Try
+import org.scalatest.concurrent.ScalaFutures._
+
+class ThreadLocalStorageSpec extends WordSpec with Matchers with BeforeAndAfterAll {
+
+  private val ec = ExecutionContext.fromExecutorService(Executors.newFixedThreadPool(2))
 
   "the Storage.ThreadLocal implementation of Context storage" should {
     "return a empty context when no context has been set" in {
@@ -47,9 +55,64 @@ class ThreadLocalStorageSpec extends WordSpec with Matchers {
 
   }
 
-  val TLS: Storage = new Storage.ThreadLocal
+  "the Storage.CrossThreadLocal implementation of Context storage" should {
+    "return a empty context when no context has been set" in {
+      CrossTLS.current() shouldBe Context.Empty
+    }
+
+    "return the empty value for keys that have not been set in the context" in {
+      CrossTLS.current().get(TestKey) shouldBe 42
+      CrossTLS.current().get(AnotherKey) shouldBe 99
+      CrossTLS.current().get(BroadcastKey) shouldBe "i travel around"
+
+      ScopeWithKey.get(TestKey) shouldBe 43
+      ScopeWithKey.get(AnotherKey) shouldBe 99
+      ScopeWithKey.get(BroadcastKey) shouldBe "i travel around"
+    }
+
+    "allow setting a context as current and remove it when closing the Scope" in {
+      CrossTLS.current() shouldBe Context.Empty
+
+      val scope = CrossTLS.store(ScopeWithKey)
+      CrossTLS.current() shouldBe theSameInstanceAs(ScopeWithKey)
+      scope.close()
+
+      CrossTLS.current() shouldBe Context.Empty
+    }
+
+    "Allow closing the scope in a different thread than the original" in {
+      var scope: Scope = null
+
+      val f1 = Future {
+        // previous context
+        CrossTLS.store(ContextWithAnotherKey)
+        scope = CrossTLS.store(ScopeWithKey)
+        Thread.sleep(10)
+        CrossTLS.current() shouldBe theSameInstanceAs(ScopeWithKey)
+      }(ec)
+
+      val f2 = Future {
+        while (scope == null) {} // wait for scope to be created in the other thread
+        CrossTLS.current() shouldBe Context.Empty
+        scope.close()
+        CrossTLS.current() shouldBe theSameInstanceAs(ContextWithAnotherKey)
+      }(ec)
+
+      f1.flatMap(_ => f2)(ec).futureValue
+    }
+
+  }
+
+  override protected def afterAll(): Unit = {
+    ec.shutdown()
+    super.afterAll()
+  }
+
+  val TLS: Storage = Storage.ThreadLocal()
+  val CrossTLS: Storage = Storage.CrossThreadLocal()
   val TestKey = Context.key("test-key", 42)
   val AnotherKey = Context.key("another-key", 99)
   val BroadcastKey = Context.key("broadcast", "i travel around")
   val ScopeWithKey = Context.of(TestKey, 43)
+  val ContextWithAnotherKey = Context.of(AnotherKey, 98)
 }

core/kamon-core/src/main/scala/kamon/ContextStorage.scala

@@ -141,8 +141,10 @@ object ContextStorage {
     * instrumentation follows them around.
     */
   private val _contextStorage: Storage = {
-    if(sys.props("kamon.context.debug") == "true")
+    if (sys.props("kamon.context.debug") == "true")
       Storage.Debug()
+    else if (sys.props("kamon.context.crossThread") == "true")
+      Storage.CrossThreadLocal()
     else
       Storage.ThreadLocal()
   }

core/kamon-core/src/main/scala/kamon/context/Storage.scala

@@ -57,11 +57,43 @@ object Storage {
     def close(): Unit
   }
 
+  /**
+    * A ThreadLocal context storage that allows the scope to be closed in a different
+    * thread than the thread where store(..) was called.
+    * This is roughly 25% slower than [[kamon.context.Storage.ThreadLocal]] but is required for certain
+    * library integrations such as cats-effect IO or Monix.
+    * Turn this on by setting the System Property "kamon.context.crossThread" to "true".
+    */
+  class CrossThreadLocal extends Storage {
+    private val tls = new java.lang.ThreadLocal[Context]() {
+      override def initialValue(): Context = Context.Empty
+    }
+
+    override def current(): Context =
+      tls.get()
+
+    override def store(newContext: Context): Scope = {
+      val previousContext = tls.get()
+      tls.set(newContext)
+
+      new Scope {
+        override def context: Context = newContext
+        override def close(): Unit = tls.set(previousContext)
+      }
+    }
+  }
+
+  object CrossThreadLocal {
+    def apply(): Storage.CrossThreadLocal =
+      new Storage.CrossThreadLocal()
+  }
+
   /**
     * Wrapper that implements an optimized ThreadLocal access pattern ideal for heavily used ThreadLocals. It is faster
     * to use a mutable holder object and always perform ThreadLocal.get() and never use ThreadLocal.set(), because the
     * value is more likely to be found in the ThreadLocalMap direct hash slot and avoid the slow path of
-    * ThreadLocalMap.getEntryAfterMiss().
+    * ThreadLocalMap.getEntryAfterMiss(). Closing of the returned Scope **MUST** be called in the same thread as
+    * store(..) was originally called.
     *
     * Credit to @trask from the FastThreadLocal in glowroot. One small change is that we don't use an kamon-defined
     * holder object as that would prevent class unloading.
@@ -104,6 +136,8 @@ object Storage {
     * different stack traces for every store/close operation pair. Do not use this for any reason other than debugging
     * Context propagation issues (like, dirty Threads) in a controlled environment.
     *
+    * Note: Similar to the default ThreadLocal storage, the scope must be closed in the same thread as where store(..)
+    * was called, otherwise you are potentially modifying the context of another thread.
     */
   class Debug extends Storage {
     import Debug._