stream: fail eagerly during SubSink/SubSource materialization (#28492)

This way the stack trace will be more helpful because it contains the stage
that actually triggered the materialization.

Otherwise, we will only fail during `preStart` in the interpreter where the
stage will be failed and the error be propagated through the stream where
it can be hard to figure out what happened.

Also improve the message itself to contain the user provided name of the
sink/source.

akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowPrefixAndTailSpec.scala

@@ -100,8 +100,9 @@ class FlowPrefixAndTailSpec extends StreamSpec("""
 
       val subscriber2 = TestSubscriber.probe[Int]()
       tail.to(Sink.fromSubscriber(subscriber2)).run()
-      subscriber2.expectSubscriptionAndError().getMessage should ===(
+      val ex = subscriber2.expectSubscriptionAndError()
-        "Substream Source cannot be materialized more than once")
+      ex.getMessage should ===("Substream Source(TailSource) cannot be materialized more than once")
+      ex.getStackTrace.exists(_.getClassName contains "FlowPrefixAndTailSpec") shouldBe true
 
       subscriber1.requestNext(2).expectComplete()
 
@@ -126,7 +127,7 @@ class FlowPrefixAndTailSpec extends StreamSpec("""
 
       tail.to(Sink.fromSubscriber(subscriber)).run()(tightTimeoutMaterializer)
       subscriber.expectSubscriptionAndError().getMessage should ===(
-        s"Substream Source has not been materialized in ${ms} milliseconds")
+        s"Substream Source(TailSource) has not been materialized in ${ms} milliseconds")
     }
     "not fail the stream if substream has not been subscribed in time and configured subscription timeout is noop" in assertAllStagesStopped {
       @silent("deprecated")

akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowSplitWhenSpec.scala

@@ -9,6 +9,7 @@ import akka.NotUsed
 import akka.stream.Supervision.resumingDecider
 import akka.stream._
 import akka.stream.impl.SubscriptionTimeoutException
+import akka.stream.impl.fusing.Split
 import akka.stream.testkit.Utils._
 import akka.stream.testkit._
 import akka.stream.testkit.scaladsl.StreamTestKit._
@@ -263,21 +264,25 @@ class FlowSplitWhenSpec extends StreamSpec("""
     }
 
     "fail substream if materialized twice" in assertAllStagesStopped {
-
       import system.dispatcher
-      val stream = Source(1 to 5)
+      val stream =
-        .splitWhen(_ => true)
+        Source(1 to 5)
-        .lift
+        // Need to drop to internal API to get a plain Source[Source[Int]] instead of a SubFlow.
-        .map { src =>
+        // `lift` doesn't cut here because it will prevent the behavior we'd like to see.
-          src.runWith(Sink.ignore).flatMap(_ => src.runWith(Sink.ignore))
+        // In fact, this test is somewhat useless, as a user cannot trigger double materialization using
-        }
+        // the public splitWhen => SubFlow API.
-        .toMat(TestSink.probe[Future[Done]])(Keep.right)
+          .via(Split.when(_ => true, SubstreamCancelStrategy.drain))
+          .map { source =>
+            // run twice, but make sure we return the result of the materialization that ran second
+            source.runWith(Sink.ignore).flatMap(_ => source.runWith(Sink.ignore))
+          }
+          .toMat(TestSink.probe[Future[Done]])(Keep.right)
       val probe = stream.withAttributes(Attributes.inputBuffer(1, 1)).run()
-      probe.request(1)
       val future = probe.requestNext()
-      an[IllegalStateException] mustBe thrownBy {
+      val ex = the[IllegalStateException] thrownBy Await.result(future, 3.seconds)
-        Await.result(future, 3.seconds)
+      ex.getMessage should ===("Substream Source(SplitSource) cannot be materialized more than once")
-      }
+      ex.printStackTrace
+      ex.getStackTrace.exists(_.getClassName contains "FlowSplitWhenSpec") shouldBe true
       probe.cancel()
     }
 

akka-stream/src/main/scala/akka/stream/impl/fusing/StreamOfStreams.scala

@@ -700,10 +700,17 @@ import scala.util.control.NonFatal
 
       case cmd: CommandScheduledBeforeMaterialization =>
         throw new IllegalStateException(
-          s"${newState.command} on subsink is illegal when ${cmd.command} is still pending")
+          s"${newState.command} on subsink($name) is illegal when ${cmd.command} is still pending")
     }
 
   override def createLogic(attr: Attributes) = new GraphStageLogic(shape) with InHandler {
+    // check for previous materialization eagerly so we fail with a more useful stacktrace
+    private[this] val materializationException: OptionVal[IllegalStateException] =
+      if (status.get.isInstanceOf[AsyncCallback[_]])
+        OptionVal.Some(createMaterializedTwiceException())
+      else
+        OptionVal.None
+
     setHandler(in, this)
 
     override def onPush(): Unit = externalCallback(ActorSubscriberMessage.OnNext(grab(in)))
@@ -726,7 +733,7 @@ import scala.util.control.NonFatal
             setCallback(callback)
 
         case _: /* Materialized */ AsyncCallback[Command @unchecked] =>
-          failStage(new IllegalStateException("Substream Source cannot be materialized more than once"))
+          failStage(materializationException.getOrElse(createMaterializedTwiceException()))
       }
 
     override def preStart(): Unit =
@@ -734,6 +741,9 @@ import scala.util.control.NonFatal
         case RequestOne    => tryPull(in)
         case Cancel(cause) => cancelStage(cause)
       }
+
+    def createMaterializedTwiceException(): IllegalStateException =
+      new IllegalStateException(s"Substream Sink($name) cannot be materialized more than once")
   }
 
   override def toString: String = name
@@ -778,9 +788,16 @@ import scala.util.control.NonFatal
     status.compareAndSet(
       null,
       ActorSubscriberMessage.OnError(
-        new SubscriptionTimeoutException(s"Substream Source has not been materialized in $d")))
+        new SubscriptionTimeoutException(s"Substream Source($name) has not been materialized in $d")))
 
   override def createLogic(inheritedAttributes: Attributes) = new GraphStageLogic(shape) with OutHandler {
+    // check for previous materialization eagerly so we fail with a more useful stacktrace
+    private[this] val materializationException: OptionVal[IllegalStateException] =
+      if (status.get.isInstanceOf[AsyncCallback[_]])
+        OptionVal.Some(createMaterializedTwiceException())
+      else
+        OptionVal.None
+
     setHandler(out, this)
 
     @tailrec private def setCB(cb: AsyncCallback[ActorSubscriberMessage]): Unit = {
@@ -789,7 +806,7 @@ import scala.util.control.NonFatal
         case ActorSubscriberMessage.OnComplete  => completeStage()
         case ActorSubscriberMessage.OnError(ex) => failStage(ex)
         case _: AsyncCallback[_] =>
-          failStage(new IllegalStateException("Substream Source cannot be materialized more than once"))
+          failStage(materializationException.getOrElse(createMaterializedTwiceException()))
       }
     }
 
@@ -804,6 +821,9 @@ import scala.util.control.NonFatal
 
     override def onPull(): Unit = externalCallback.invoke(RequestOne)
     override def onDownstreamFinish(cause: Throwable): Unit = externalCallback.invoke(Cancel(cause))
+
+    def createMaterializedTwiceException(): IllegalStateException =
+      new IllegalStateException(s"Substream Source($name) cannot be materialized more than once")
   }
 
   override def toString: String = name