chore: Fix leak in FlatMapPrefix operator.

Author: He-Pin

stream/src/main/scala/org/apache/pekko/stream/impl/fusing/FlatMapPrefix.scala

@@ -43,8 +43,10 @@ import pekko.util.OptionVal
         .mandatoryAttribute[Attributes.NestedMaterializationCancellationPolicy]
         .propagateToNestedMaterialization
     val matPromise = Promise[M]()
-    val logic = new GraphStageLogic(shape) with InHandler with OutHandler {
-      val accumulated = collection.mutable.Buffer.empty[In]
+    object logic extends GraphStageLogic(shape) with InHandler with OutHandler {
+      private var left = if (n < 0) 0 else n
+      private var builder = Vector.newBuilder[In]
+      builder.sizeHint(left)
 
       private var subSource = OptionVal.none[SubSourceOutlet[In]]
       private var subSink = OptionVal.none[SubSinkInlet[Out]]
@@ -65,8 +67,9 @@ import pekko.util.OptionVal
         subSource match {
           case OptionVal.Some(s) => s.push(grab(in))
           case _ =>
-            accumulated.append(grab(in))
-            if (accumulated.size == n) {
+            builder += grab(in)
+            left -= 1
+            if (left == 0) {
               materializeFlow()
             } else {
               // gi'me some more!
@@ -98,12 +101,10 @@ import pekko.util.OptionVal
             // delegate to subSink
             s.pull()
           case _ =>
-            if (accumulated.size < n) pull(in)
-            else if (accumulated.size == n) {
+            if (left > 0) pull(in)
+            else if (left == 0) {
               // corner case for n = 0, can be handled in FlowOps
               materializeFlow()
-            } else {
-              throw new IllegalStateException(s"Unexpected accumulated size: ${accumulated.size} (n: $n)")
             }
         }
       }
@@ -114,7 +115,7 @@ import pekko.util.OptionVal
           case _ =>
             if (propagateToNestedMaterialization) {
               downstreamCause = OptionVal.Some(cause)
-              if (accumulated.size == n) {
+              if (left == 0) {
                 // corner case for n = 0, can be handled in FlowOps
                 materializeFlow()
               } else if (!hasBeenPulled(in)) { // if in was already closed, nested flow would have already been materialized
@@ -128,8 +129,8 @@ import pekko.util.OptionVal
 
       def materializeFlow(): Unit =
         try {
-          val prefix = accumulated.toVector
-          accumulated.clear()
+          val prefix = builder.result()
+          builder = null // free for GC
           subSource = OptionVal.Some(new SubSourceOutlet[In]("FlatMapPrefix.subSource"))
           val theSubSource = subSource.get
           subSink = OptionVal.Some(new SubSinkInlet[Out]("FlatMapPrefix.subSink"))