SI-6185 - add "prepare" hook to ExecutionContext

Enables important abstractions to be built on top of futures,
such as Twitter's "Local" for handling data local to a callback chain.

1 files changed, 61 insertions, 0 deletions

diff --git a/test/files/jvm/scala-concurrent-tck.scala b/test/files/jvm/scala-concurrent-tck.scala
index 36ab910593..976d98a337 100644
--- a/test/files/jvm/scala-concurrent-tck.scala
+++ b/test/files/jvm/scala-concurrent-tck.scala
@@ -926,6 +926,66 @@ trait CustomExecutionContext extends TestBase {
   testCallbackChainCustomEC()
 }
 
+trait ExecutionContextPrepare extends TestBase {
+  val theLocal = new ThreadLocal[String] {
+    override protected def initialValue(): String = ""
+  }
+  
+  class PreparingExecutionContext extends ExecutionContext {
+    def delegate = ExecutionContext.global
+    
+    override def execute(runnable: Runnable): Unit =
+      delegate.execute(runnable)
+    
+    override def prepare[T, U](f: Try[T] => U): ExecutionContext = {
+      // save object stored in ThreadLocal storage
+      val localData = theLocal.get
+      new PreparingExecutionContext {
+        override def execute(runnable: Runnable): Unit = {
+          val wrapper = new Runnable {
+            override def run(): Unit = {
+              // now we're on the new thread
+              // put localData into theLocal
+              theLocal.set(localData)
+              runnable.run()
+            }
+          }
+          delegate.execute(wrapper)
+        }
+      }
+    }
+    
+    override def reportFailure(t: Throwable): Unit =
+      delegate.reportFailure(t)
+  }
+  
+  implicit val ec = new PreparingExecutionContext
+  
+  def testOnComplete(): Unit = once {
+    done =>
+    theLocal.set("secret")
+    val fut = future { 42 }
+    fut onComplete {
+      case _ =>
+        assert(theLocal.get == "secret")
+        done()
+    }
+  }
+  
+  def testMap(): Unit = once {
+    done =>
+    theLocal.set("secret2")
+    val fut = future { 42 }
+    fut map { x =>
+      assert(theLocal.get == "secret2")
+      done()
+    }
+  }
+  
+  testOnComplete()
+  testMap()
+}
+
 object Test
 extends App
 with FutureCallbacks
@@ -935,6 +995,7 @@ with Promises
 with BlockContexts
 with Exceptions
 with CustomExecutionContext
+with ExecutionContextPrepare
 {
   System.exit(0)
 }