Merge pull request #16006 from volkerstampa/wip-16001-foldsink-error-handling-volkerstampa

Propagate error to FoldSink's future. Fixes #16001

akka-stream-tests/src/test/scala/akka/stream/scaladsl2/FlowFoldSpec.scala

@@ -4,12 +4,12 @@
 package akka.stream.scaladsl2
 
 import scala.concurrent.Await
-import scala.concurrent.duration._
+import scala.util.control.NoStackTrace
 import akka.stream.testkit.AkkaSpec
+import akka.testkit.DefaultTimeout
 
-class FlowFoldSpec extends AkkaSpec {
+class FlowFoldSpec extends AkkaSpec with DefaultTimeout {
   implicit val mat = FlowMaterializer()
-  import system.dispatcher
 
   "A Fold" must {
 
@@ -19,7 +19,15 @@ class FlowFoldSpec extends AkkaSpec {
       val mf = FlowFrom(input).withSink(foldSink).run()
       val future = foldSink.future(mf)
       val expected = input.fold(0)(_ + _)
-      Await.result(future, 5.seconds) should be(expected)
+      Await.result(future, timeout.duration) should be(expected)
+    }
+
+    "propagate an error" in {
+      val error = new Exception with NoStackTrace
+      val foldSink = FoldSink[Unit, Unit](())((_, _) ⇒ ())
+      val mf = FlowFrom[Unit](() ⇒ throw error).withSink(foldSink).run()
+      val future = foldSink.future(mf)
+      the[Exception] thrownBy Await.result(future, timeout.duration) should be(error)
     }
 
   }

akka-stream/src/main/scala/akka/stream/scaladsl2/Sink.scala

@@ -199,12 +199,12 @@ final case class OnCompleteSink[Out](callback: Try[Unit] ⇒ Unit) extends Simpl
   override def attach(flowPublisher: Publisher[Out], materializer: ActorBasedFlowMaterializer, flowName: String): Unit =
     FlowFrom(flowPublisher).transform("onCompleteSink", () ⇒ new Transformer[Out, Unit] {
       override def onNext(in: Out) = Nil
-      override def onError(e: Throwable) = {
-        callback(Failure(e))
-        throw e
-      }
+      override def onError(e: Throwable) = ()
       override def onTermination(e: Option[Throwable]) = {
-        callback(OnCompleteSink.SuccessUnit)
+        e match {
+          case None    ⇒ callback(OnCompleteSink.SuccessUnit)
+          case Some(e) ⇒ callback(Failure(e))
+        }
         Nil
       }
     }).consume()(materializer.withNamePrefix(flowName))
@@ -248,6 +248,7 @@ final case class FoldSink[U, Out](zero: U)(f: (U, Out) ⇒ U) extends SinkWithKe
     FlowFrom(flowPublisher).transform("fold", () ⇒ new Transformer[Out, U] {
       var state: U = zero
       override def onNext(in: Out): immutable.Seq[U] = { state = f(state, in); Nil }
+      override def onError(cause: Throwable) = ()
       override def onTermination(e: Option[Throwable]) = {
         e match {
           case None    ⇒ promise.success(state)