Fix race condition in StreamConverters.asOutputStream() (#26136)

Fixes #25983

akka-bench-jmh/src/main/scala/akka/stream/impl/OutputStreamSourceStageBenchmark.scala

@@ -0,0 +1,60 @@
+/*
+ * Copyright (C) 2019 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package akka.stream.impl
+
+import java.io.OutputStream
+import java.util.concurrent.TimeUnit
+
+import akka.Done
+import akka.actor.ActorSystem
+import akka.stream.ActorMaterializer
+import akka.stream.scaladsl.{ Keep, Sink, StreamConverters }
+import org.openjdk.jmh.annotations.TearDown
+
+import scala.concurrent.{ Await, Future }
+import scala.concurrent.duration._
+import org.openjdk.jmh.annotations._
+
+object OutputStreamSourceStageBenchmark {
+  final val WritesPerBench = 10000
+}
+@State(Scope.Benchmark)
+@OutputTimeUnit(TimeUnit.MILLISECONDS)
+@BenchmarkMode(Array(Mode.Throughput))
+class OutputStreamSourceStageBenchmark {
+  import OutputStreamSourceStageBenchmark.WritesPerBench
+  implicit val system = ActorSystem("OutputStreamSourceStageBenchmark")
+  implicit val materializer = ActorMaterializer()
+
+  private val bytes: Array[Byte] = Array.emptyByteArray
+
+  private var os: OutputStream = _
+  private var done: Future[Done] = _
+
+  @Benchmark
+  @OperationsPerInvocation(WritesPerBench)
+  def consumeWrites(): Unit = {
+    val (os, done) = StreamConverters.asOutputStream()
+      .toMat(Sink.ignore)(Keep.both)
+      .run()
+    new Thread(new Runnable {
+      def run(): Unit = {
+        var counter = 0
+        while (counter > WritesPerBench) {
+          os.write(bytes)
+          counter += 1
+        }
+        os.close()
+      }
+    }).start()
+    Await.result(done, 30.seconds)
+  }
+
+  @TearDown
+  def shutdown(): Unit = {
+    Await.result(system.terminate(), 5.seconds)
+  }
+
+}