=htc slight renaming, refs #19052

akka-http-core/src/main/scala/akka/http/impl/engine/server/HttpServerBluePrint.scala

@@ -51,14 +51,15 @@ private[http] object HttpServerBluePrint {
     stack(settings, remoteAddress, log)
 
   def stack(settings: ServerSettings, remoteAddress: Option[InetSocketAddress], log: LoggingAdapter)(implicit mat: Materializer): Http.ServerLayer = {
-    One2OneBidiFlow[HttpRequest, HttpResponse](settings.pipeliningLimit).reversed atop
+    userHandlerGuard(settings.pipeliningLimit) atop
       requestPreparation(settings, remoteAddress) atop
       controller(settings, log) atop
       parsingRendering(settings, log) atop
       websocketSupport(settings, log) atop
       unwrapTls
   }
-  def unwrapTls: BidiFlow[ByteString, SslTlsOutbound, SslTlsInbound, ByteString, Unit] =
+
+  val unwrapTls: BidiFlow[ByteString, SslTlsOutbound, SslTlsInbound, ByteString, Unit] =
     BidiFlow.fromFlows(Flow[ByteString].map(SendBytes), Flow[SslTlsInbound].collect { case x: SessionBytes ⇒ x.bytes })
 
   /** Wrap an HTTP implementation with support for switching to Websocket */
@@ -324,6 +325,14 @@ private[http] object HttpServerBluePrint {
     }
   }
 
+  /**
+   * Ensures that the user handler
+   *  - produces exactly one response per request
+   *  - has not more than ``pipeliningLimit`` responses outstanding
+   */
+  def userHandlerGuard(pipeliningLimit: Int): BidiFlow[HttpResponse, HttpResponse, HttpRequest, HttpRequest, Unit] =
+    One2OneBidiFlow[HttpRequest, HttpResponse](pipeliningLimit).reversed
+
   private trait WebsocketSetup {
     def websocketFlow: Flow[ByteString, ByteString, Any]
     def installHandler(handlerFlow: Flow[FrameEvent, FrameEvent, Any])(implicit mat: Materializer): Unit