chore: Fix flaky test in unsafeOptionalDataVia (#1611)

Author: He-Pin

stream/src/main/scala/org/apache/pekko/stream/scaladsl/SourceWithContext.scala

@@ -57,36 +57,44 @@ object SourceWithContext {
     SourceWithContext.fromTuples(Source.fromGraph(GraphDSL.createGraph(source, viaFlow)(combine) {
       implicit b => (s, viaF) =>
         import GraphDSL.Implicits._
-        val broadcast = b.add(Broadcast[(Option[SOut], Ctx)](2))
-        val merge = b.add(Merge[(Option[FOut], Ctx)](2))
 
-        val unzip = b.add(Unzip[SOut, Ctx]())
-        val zipper = b.add(Zip[FOut, Ctx]())
-
-        val filterAvailable = Flow[(Option[SOut], Ctx)].collect {
-          case (Some(f), ctx) => (f, ctx)
-        }
-
-        val filterUnavailable = Flow[(Option[SOut], Ctx)].collect {
-          case (None, ctx) => (Option.empty[FOut], ctx)
+        case class IndexedCtx(idx: Long, ctx: Ctx)
+        val partition = b.add(Partition[(Option[SOut], IndexedCtx)](2,
+          {
+            case (None, _)    => 0
+            case (Some(_), _) => 1
+          }))
+
+        val sequence = Flow[(Option[SOut], Ctx)].zipWithIndex
+          .map {
+            case ((opt, ctx), idx) => (opt, IndexedCtx(idx, ctx))
+          }
+
+        val unzip = b.add(Unzip[Option[SOut], IndexedCtx]())
+        val zipper = b.add(Zip[FOut, IndexedCtx]())
+        val mergeSequence = b.add(MergeSequence[(Option[FOut], IndexedCtx)](2)(_._2.idx))
+        val unwrapSome = b.add(Flow[Option[SOut]].map {
+          case Some(elem) => elem
+          case _          => throw new IllegalStateException("Only expects Some")
+        })
+        val unwrap = Flow[(Option[FOut], IndexedCtx)].map {
+          case (opt, indexedCtx) => (opt, indexedCtx.ctx)
         }
 
-        val mapIntoOption = Flow[(FOut, Ctx)].map {
-          case (f, ctx) => (Some(f), ctx)
+        val mapIntoOption = Flow[(FOut, IndexedCtx)].map {
+          case (elem, indexedCtx) => (Some(elem), indexedCtx)
         }
 
-        s ~> broadcast.in
-
-        broadcast.out(0) ~> filterAvailable ~> unzip.in
-
-        unzip.out0 ~> viaF ~> zipper.in0
-        unzip.out1 ~> zipper.in1
-
-        zipper.out ~> mapIntoOption ~> merge.in(0)
-
-        broadcast.out(1) ~> filterUnavailable ~> merge.in(1)
+        //format: off
+        s ~> sequence ~> partition.in
+        partition.out(0).asInstanceOf[Outlet[(Option[FOut], IndexedCtx)]] ~> mergeSequence.in(0)
+        partition.out(1) ~> unzip.in
+                            unzip.out0 ~> unwrapSome ~> viaF ~> zipper.in0
+                            unzip.out1                       ~> zipper.in1
+                                                                zipper.out ~> mapIntoOption ~> mergeSequence.in(1)
 
-        SourceShape(merge.out)
+        //format: on
+        SourceShape((mergeSequence.out ~> unwrap).outlet)
     }))
 }