=htc #19834 Replaced PushStage based captureTermination with GraphStage

* Move promise completion success into postStop to simplify impl

akka-http-core/src/main/scala/akka/http/impl/util/StreamUtils.scala

@@ -63,17 +63,25 @@ private[http] object StreamUtils {
 
   def captureTermination[T, Mat](source: Source[T, Mat]): (Source[T, Mat], Future[Unit]) = {
     val promise = Promise[Unit]()
-    val transformer = new PushStage[T, T] {
-      def onPush(element: T, ctx: Context[T]) = ctx.push(element)
-      override def onUpstreamFailure(cause: Throwable, ctx: Context[T]) = {
-        promise.failure(cause)
-        ctx.fail(cause)
-      }
-      override def postStop(): Unit = {
-        promise.trySuccess(())
+    val transformer = new SimpleLinearGraphStage[T] {
+      override def createLogic(inheritedAttributes: Attributes): GraphStageLogic = new GraphStageLogic(shape) with InHandler with OutHandler {
+        override def onPush(): Unit = push(out, grab(in))
+
+        override def onPull(): Unit = pull(in)
+
+        override def onUpstreamFailure(ex: Throwable): Unit = {
+          promise.failure(ex)
+          failStage(ex)
+        }
+
+        override def postStop(): Unit = {
+          promise.trySuccess(())
+        }
+
+        setHandlers(in, out, this)
       }
     }
-    source.transform(() ⇒ transformer) -> promise.future
+    source.via(transformer) -> promise.future
   }
 
   def sliceBytesTransformer(start: Long, length: Long): Flow[ByteString, ByteString, NotUsed] = {