groupBy does not invoke decider

groupBy does not invoke a decider when the maximum number of substreams has been exceeded. This commit fixes that.

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

@@ -336,6 +336,18 @@ class FlowGroupBySpec extends StreamSpec {
       s1.expectError(ex)
     }
 
+    "resume when exceeding maxSubstreams" in {
+      val (up, down) = Flow[Int]
+        .groupBy(0, identity).mergeSubstreams
+        .withAttributes(ActorAttributes.supervisionStrategy(resumingDecider))
+        .runWith(TestSource.probe[Int], TestSink.probe)
+
+      down.request(1)
+
+      up.sendNext(1)
+      down.expectNoMessage(1.second)
+    }
+
     "emit subscribe before completed" in assertAllStagesStopped {
       val futureGroupSource =
         Source.single(0)

akka-stream/src/main/scala/akka/stream/TooManySubstreamsOpenException.scala

@@ -0,0 +1,16 @@
+/**
+ * Copyright (C) 2015-2018 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package akka.stream
+
+import scala.util.control.NoStackTrace
+
+/**
+ * This exception signals that the maximum number of substreams declared has been exceeded.
+ * A finite limit is imposed so that memory usage is controlled.
+ */
+final class TooManySubstreamsOpenException
+  extends IllegalStateException("Cannot open a new substream as there are too many substreams open")
+  with NoStackTrace {
+}

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

@@ -223,6 +223,7 @@ import scala.collection.JavaConverters._
 
   override def createLogic(inheritedAttributes: Attributes): GraphStageLogic = new TimerGraphStageLogic(shape) with OutHandler with InHandler {
     parent ⇒
+
     lazy val decider = inheritedAttributes.mandatoryAttribute[SupervisionStrategy].decider
     private val activeSubstreamsMap = new java.util.HashMap[Any, SubstreamSource]()
     private val closedSubstreams = if (allowClosedSubstreamRecreation) Collections.unmodifiableSet(Collections.emptySet[Any]) else new java.util.HashSet[Any]()
@@ -234,6 +235,8 @@ import scala.collection.JavaConverters._
     private val substreamsJustStared = new java.util.HashSet[Any]()
     private var firstPushCounter: Int = 0
 
+    private val tooManySubstreamsOpenException = new TooManySubstreamsOpenException
+
     private def nextId(): Long = { _nextId += 1; _nextId }
 
     private def hasNextElement = nextElementKey != null
@@ -304,7 +307,7 @@ import scala.collection.JavaConverters._
         }
       } else {
         if (activeSubstreamsMap.size == maxSubstreams)
-          fail(new IllegalStateException(s"Cannot open substream for key '$key': too many substreams open"))
+          throw tooManySubstreamsOpenException
         else if (closedSubstreams.contains(key) && !hasBeenPulled(in))
           pull(in)
         else runSubstream(key, elem)