Fix .head causing JsonSupport sink to terminate

http-json/src/main/scala/org/mdedetrich/pekko/http/JsonSupport.scala

@@ -22,21 +22,80 @@ import pekko.http.scaladsl.model.HttpEntity
 import pekko.http.scaladsl.model.MediaTypes.`application/json`
 import pekko.http.scaladsl.unmarshalling.{FromEntityUnmarshaller, Unmarshaller}
 import pekko.http.scaladsl.util.FastFuture
+import pekko.stream._
+import pekko.stream.scaladsl.Sink
+import pekko.stream.stage._
 import org.mdedetrich.pekko.json.stream.JsonStreamParser
 import org.typelevel.jawn.Facade
 
+import scala.concurrent.{Future, Promise}
+
 trait JsonSupport {
 
-  implicit def jsonUnmarshaller[J: Facade]: FromEntityUnmarshaller[J] =
+  implicit def jsonUnmarshaller[J <: AnyRef: Facade]: FromEntityUnmarshaller[J] =
     Unmarshaller
       .withMaterializer[HttpEntity, J](_ =>
         implicit mat => {
           case HttpEntity.Strict(_, data) =>
             FastFuture(JsonStreamParser.parse[J](data))
-          case entity => entity.dataBytes.runWith(JsonStreamParser.head[J])
+          case entity => entity.dataBytes.via(JsonStreamParser[J]).runWith(JsonSupport.firstElementSink[J])
         }
       )
       .forContentTypes(`application/json`)
 }
 
-object JsonSupport extends JsonSupport
+object JsonSupport extends JsonSupport {
+  private def firstElementSink[J <: AnyRef]: Sink[J, Future[J]] =
+    Sink.fromGraph(new FirstElementSinkStage[J])
+
+  private final class FirstElementSinkStage[J <: AnyRef]
+      extends GraphStageWithMaterializedValue[SinkShape[J], Future[J]] {
+    private[this] val in: Inlet[J] = Inlet("firstElement.in")
+
+    override val shape: SinkShape[J]                     = SinkShape.of(in)
+    override protected def initialAttributes: Attributes = Attributes.name("firstElement")
+
+    override def createLogicAndMaterializedValue(inheritedAttributes: Attributes): (GraphStageLogic, Future[J]) = {
+      val p: Promise[J] = Promise()
+      (new GraphStageLogic(shape) with InHandler {
+         private[this] var element: J = null.asInstanceOf[J]
+
+         override def preStart(): Unit = pull(in)
+
+         def onPush(): Unit = {
+           if (element eq null)
+             element = grab(in)
+           pull(in)
+         }
+
+         override def onUpstreamFinish(): Unit = {
+           val el = element
+           element = null.asInstanceOf[J]
+           if (el ne null)
+             p.trySuccess(el)
+           else
+             p.tryFailure(new NoSuchElementException("No complete json entity consumed"))
+           completeStage()
+         }
+
+         override def onUpstreamFailure(ex: Throwable): Unit = {
+           element = null.asInstanceOf[J]
+           p.tryFailure(ex)
+           failStage(ex)
+         }
+
+         override def postStop(): Unit =
+           if (!p.isCompleted) {
+             p.failure(new AbruptStageTerminationException(this))
+             ()
+           }
+
+         setHandler(in, this)
+       },
+       p.future
+      )
+    }
+
+    override def toString: String = "FirstElementSinkStage"
+  }
+}