+str Add flatmapConcat with parallelism.

akka-bench-jmh/src/main/scala/akka/stream/FlatMapConcatBenchmark.scala

@@ -9,6 +9,7 @@ import java.util.concurrent.TimeUnit
 
 import scala.concurrent.Await
 import scala.concurrent.duration._
+import scala.concurrent.Future
 
 import com.typesafe.config.ConfigFactory
 import org.openjdk.jmh.annotations._
@@ -88,6 +89,18 @@ class FlatMapConcatBenchmark {
     awaitLatch(latch)
   }
 
+  @Benchmark
+  @OperationsPerInvocation(OperationsPerInvocation)
+  def completedFuture(): Unit = {
+    val latch = new CountDownLatch(1)
+
+    testSource
+      .flatMapConcat(n => Source.future(Future.successful(n)))
+      .runWith(new LatchSink(OperationsPerInvocation, latch))
+
+    awaitLatch(latch)
+  }
+
   @Benchmark
   @OperationsPerInvocation(OperationsPerInvocation)
   def mapBaseline(): Unit = {

akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowFlatMapConcatSpec.scala

@@ -0,0 +1,124 @@
+/*
+ * Copyright (C) 2014-2023 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package akka.stream.scaladsl
+
+import akka.stream.OverflowStrategy
+import akka.stream.testkit._
+import akka.stream.testkit.scaladsl.TestSink
+
+import java.util.concurrent.ThreadLocalRandom
+import java.util.concurrent.atomic.AtomicInteger
+import scala.concurrent.Future
+import scala.concurrent.duration.DurationInt
+import scala.util.control.NoStackTrace
+
+class FlowFlatMapConcatSpec extends StreamSpec("""
+    akka.stream.materializer.initial-input-buffer-size = 2
+  """) with ScriptedTest {
+  val toSeq = Flow[Int].grouped(1000).toMat(Sink.head)(Keep.right)
+
+  class BoomException extends RuntimeException("BOOM~~") with NoStackTrace
+  "A flatMapConcat" must {
+
+    "work with value presented sources" in {
+      Source(
+        List(
+          Source.empty[Int],
+          Source.single(1),
+          Source.empty[Int],
+          Source(List(2, 3, 4)),
+          Source.future(Future.successful(5)),
+          Source.lazyFuture(() => Future.successful(6))))
+        .flatMapConcat(ThreadLocalRandom.current().nextInt(1, 129), identity)
+        .runWith(toSeq)
+        .futureValue should ===(1 to 6)
+    }
+
+    "work with value presented failed sources" in {
+      val ex = new BoomException
+      Source(
+        List(
+          Source.empty[Int],
+          Source.single(1),
+          Source.empty[Int],
+          Source(List(2, 3, 4)),
+          Source.future(Future.failed(ex)),
+          Source.lazyFuture(() => Future.successful(5))))
+        .flatMapConcat(ThreadLocalRandom.current().nextInt(1, 129), identity)
+        .onErrorComplete[BoomException]()
+        .runWith(toSeq)
+        .futureValue should ===(1 to 4)
+    }
+
+    "work with value presented sources when demands slow" in {
+      val prob = Source(
+        List(Source.empty[Int], Source.single(1), Source(List(2, 3, 4)), Source.lazyFuture(() => Future.successful(5))))
+        .flatMapConcat(ThreadLocalRandom.current().nextInt(1, 129), identity)
+        .runWith(TestSink())
+
+      prob.request(1)
+      prob.expectNext(1)
+      prob.expectNoMessage(1.seconds)
+      prob.request(2)
+      prob.expectNext(2, 3)
+      prob.expectNoMessage(1.seconds)
+      prob.request(2)
+      prob.expectNext(4, 5)
+      prob.expectComplete()
+    }
+
+    "can do pre materialization when parallelism > 1" in {
+      val materializationCounter = new AtomicInteger(0)
+      val randomParallelism = ThreadLocalRandom.current().nextInt(4, 65)
+      val prob = Source(1 to (randomParallelism * 3))
+        .flatMapConcat(
+          randomParallelism,
+          value => {
+            Source
+              .lazySingle(() => {
+                materializationCounter.incrementAndGet()
+                value
+              })
+              .buffer(1, overflowStrategy = OverflowStrategy.backpressure)
+          })
+        .runWith(TestSink())
+
+      expectNoMessage(1.seconds)
+      materializationCounter.get() shouldBe 0
+
+      prob.request(1)
+      prob.expectNext(1.seconds, 1)
+      expectNoMessage(1.seconds)
+      materializationCounter.get() shouldBe (randomParallelism + 1)
+      materializationCounter.set(0)
+
+      prob.request(2)
+      prob.expectNextN(List(2, 3))
+      expectNoMessage(1.seconds)
+      materializationCounter.get() shouldBe 2
+      materializationCounter.set(0)
+
+      prob.request(randomParallelism - 3)
+      prob.expectNextN(4 to randomParallelism)
+      expectNoMessage(1.seconds)
+      materializationCounter.get() shouldBe (randomParallelism - 3)
+      materializationCounter.set(0)
+
+      prob.request(randomParallelism)
+      prob.expectNextN(randomParallelism + 1 to randomParallelism * 2)
+      expectNoMessage(1.seconds)
+      materializationCounter.get() shouldBe randomParallelism
+      materializationCounter.set(0)
+
+      prob.request(randomParallelism)
+      prob.expectNextN(randomParallelism * 2 + 1 to randomParallelism * 3)
+      expectNoMessage(1.seconds)
+      materializationCounter.get() shouldBe 0
+      prob.expectComplete()
+    }
+
+  }
+
+}

akka-stream/src/main/scala/akka/stream/impl/Stages.scala

@@ -79,6 +79,7 @@ import akka.stream.Attributes._
     val mergePreferred = name("mergePreferred")
     val mergePrioritized = name("mergePrioritized")
     val flattenMerge = name("flattenMerge")
+    val flattenConcat = name("flattenConcat")
     val recoverWith = name("recoverWith")
     val onErrorComplete = name("onErrorComplete")
     val broadcast = name("broadcast")

akka-stream/src/main/scala/akka/stream/impl/TraversalBuilder.scala

@@ -6,12 +6,12 @@ package akka.stream.impl
 
 import scala.collection.immutable.Map.Map1
 import scala.language.existentials
-
 import akka.annotation.{ DoNotInherit, InternalApi }
 import akka.stream._
 import akka.stream.impl.StreamLayout.AtomicModule
 import akka.stream.impl.TraversalBuilder.{ AnyFunction1, AnyFunction2 }
 import akka.stream.impl.fusing.GraphStageModule
+import akka.stream.impl.fusing.GraphStages.IterableSource
 import akka.stream.impl.fusing.GraphStages.SingleSource
 import akka.stream.scaladsl.Keep
 import akka.util.OptionVal
@@ -371,6 +371,37 @@ import akka.util.unused
     }
   }
 
+  def getValuePresentedSource[A >: Null](graph: Graph[SourceShape[A], _]): OptionVal[Graph[SourceShape[A], _]] = {
+    def isValuePresentedSource(graph: Graph[SourceShape[_ <: A], _]): Boolean = graph match {
+      case _: SingleSource[_] | _: IterableSource[_] | EmptySource => true
+      case _                                                       => false
+    }
+    graph match {
+      case _ if isValuePresentedSource(graph) => OptionVal.Some(graph)
+      case _ =>
+        graph.traversalBuilder match {
+          case l: LinearTraversalBuilder =>
+            l.pendingBuilder match {
+              case OptionVal.Some(a: AtomicTraversalBuilder) =>
+                a.module match {
+                  case m: GraphStageModule[_, _] =>
+                    m.stage match {
+                      case _ if isValuePresentedSource(m.stage.asInstanceOf[Graph[SourceShape[A], _]]) =>
+                        // It would be != EmptyTraversal if mapMaterializedValue was used and then we can't optimize.
+                        if ((l.traversalSoFar eq EmptyTraversal) && !l.attributes.isAsync)
+                          OptionVal.Some(m.stage.asInstanceOf[Graph[SourceShape[A], _]])
+                        else OptionVal.None
+                      case _ => OptionVal.None
+                    }
+                  case _ => OptionVal.None
+                }
+              case _ => OptionVal.None
+            }
+          case _ => OptionVal.None
+        }
+    }
+  }
+
   /**
    * Test if a Graph is an empty Source.
    * */

akka-stream/src/main/scala/akka/stream/impl/fusing/Ops.scala

@@ -1272,6 +1272,7 @@ private[stream] object Collect {
  */
 @InternalApi private[akka] final case class MapAsync[In, Out](parallelism: Int, f: In => Future[Out])
     extends GraphStage[FlowShape[In, Out]] {
+  require(parallelism >= 1, "parallelism should >= 1")
 
   import MapAsync._