stream: filter out elements without demand

This will also mean that completion will not be blocked by elements that
will later be filtered out.

One particular use case of that would be a kind of partitioning use case,
where you put several streams behind a broadcast and each consumer will filter
out elements not handled there. In that case, the broadcast can get head-of-line
blocked when one of the consumers currently has no demand but also wouldn't
have to handle any elements because they would all be filtered out.

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

@@ -79,14 +79,19 @@ import com.github.ghik.silencer.silent
     new GraphStageLogic(shape) with OutHandler with InHandler {
       def decider = inheritedAttributes.mandatoryAttribute[SupervisionStrategy].decider
 
-      override def onPush(): Unit = {
+      private var buffer: OptionVal[T] = OptionVal.none
+
+      override def preStart(): Unit = pull(in)
+      override def onPush(): Unit =
         try {
           val elem = grab(in)
-          if (p(elem)) {
-            push(out, elem)
-          } else {
-            pull(in)
-          }
+          if (p(elem))
+            if (isAvailable(out)) {
+              push(out, elem)
+              pull(in)
+            } else
+              buffer = OptionVal.Some(elem)
+          else pull(in)
         } catch {
           case NonFatal(ex) =>
             decider(ex) match {
@@ -94,9 +99,20 @@ import com.github.ghik.silencer.silent
               case _                => pull(in)
             }
         }
-      }
 
-      override def onPull(): Unit = pull(in)
+      override def onPull(): Unit =
+        buffer match {
+          case OptionVal.Some(value) =>
+            push(out, value)
+            buffer = OptionVal.none
+            if (!isClosed(in)) pull(in)
+            else completeStage()
+          case _ => // already pulled
+        }
+
+      override def onUpstreamFinish(): Unit =
+        if (buffer.isEmpty) super.onUpstreamFinish()
+      // else onPull will complete
 
       setHandlers(in, out, this)
     }