use Flow.lazyInit in Artery TCP (#24590)

akka-remote/src/main/scala/akka/remote/artery/tcp/ArteryTcpTransport.scala

@@ -124,17 +124,19 @@ private[remote] class ArteryTcpTransport(_system: ExtendedActorSystem, _provider
     def connectionFlowWithRestart: Flow[ByteString, ByteString, NotUsed] = {
       val flowFactory = () ⇒ {
 
-        afr.loFreq(
-          TcpOutbound_Connected,
-          s"${outboundContext.remoteAddress.host.get}:${outboundContext.remoteAddress.port.get} " +
-            s"/ ${streamName(streamId)}")
-
-        // FIXME use the Flow.lazyInit from https://github.com/akka/akka/pull/24527
-
         val flow =
           Flow[ByteString]
-            .prepend(Source.single(TcpFraming.encodeConnectionHeader(streamId)))
-            .via(connectionFlow)
+            .via(Flow.lazyInit(_ ⇒ {
+              // only open the actual connection if any new messages are sent
+              afr.loFreq(
+                TcpOutbound_Connected,
+                s"${outboundContext.remoteAddress.host.get}:${outboundContext.remoteAddress.port.get} " +
+                  s"/ ${streamName(streamId)}")
+              Future.successful(
+                Flow[ByteString]
+                  .prepend(Source.single(TcpFraming.encodeConnectionHeader(streamId)))
+                  .via(connectionFlow))
+            }, () ⇒ NotUsed))
             .recoverWithRetries(1, { case ArteryTransport.ShutdownSignal ⇒ Source.empty })
             .log(name = s"outbound connection to [${outboundContext.remoteAddress}], ${streamName(streamId)} stream")
             .addAttributes(Attributes.logLevels(onElement = LogLevels.Off, onFailure = Logging.WarningLevel))