Eliminate BidiFlow.identity in atop when possible #29901

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

@@ -7,9 +7,7 @@ package akka.stream.scaladsl
 import scala.collection.immutable
 import scala.concurrent.Await
 import scala.concurrent.duration._
-
 import scala.annotation.nowarn
-
 import akka.NotUsed
 import akka.stream._
 import akka.stream.testkit.StreamSpec
@@ -121,6 +119,47 @@ class BidiFlowSpec extends StreamSpec {
       b.traversalBuilder.attributes.getFirst[AsyncBoundary.type] shouldEqual Some(AsyncBoundary)
     }
 
+    "short circuit identity in atop" in {
+      val myBidi = BidiFlow.fromFlows(Flow[Long].map(_ + 1L), Flow[ByteString])
+      val identity = BidiFlow.identity[Long, ByteString]
+
+      // simple ones
+      myBidi.atop(identity) should ===(myBidi)
+      identity.atopMat(myBidi)(Keep.right) should ===(myBidi)
+
+      // optimized but not the same instance (because myBidi mat value is dropped)
+      identity.atop(myBidi) should !==(myBidi)
+      myBidi.atopMat(identity)(Keep.right) should !==(myBidi)
+    }
+
+    "semi-shortcuted atop with identity should still work" in {
+      // atop when the NotUsed matval is kept from identity has a smaller optimization, so verify they still work
+      val myBidi =
+        BidiFlow.fromFlows(Flow[Long].map(_ + 1L), Flow[Long].map(_ + 1L)).mapMaterializedValue(_ => "bidi-matval")
+      val identity = BidiFlow.identity[Long, Long]
+
+      def verify[M](atopBidi: BidiFlow[Long, Long, Long, Long, M], expectedMatVal: M): Unit = {
+        val joinedFlow = atopBidi.joinMat(Flow[Long])(Keep.left)
+        val (bidiMatVal, seqSinkMatValF) =
+          Source(1L :: 2L :: Nil).viaMat(joinedFlow)(Keep.right).toMat(Sink.seq)(Keep.both).run()
+        seqSinkMatValF.futureValue should ===(Seq(3L, 4L))
+        bidiMatVal should ===(expectedMatVal)
+      }
+
+      // identity atop myBidi
+      verify(identity.atopMat(myBidi)(Keep.left), NotUsed)
+      verify(identity.atopMat(myBidi)(Keep.none), NotUsed)
+      verify(identity.atopMat(myBidi)(Keep.right), "bidi-matval")
+      // arbitrary matval combine
+      verify(identity.atopMat(myBidi)((_, m) => m), "bidi-matval")
+
+      // myBidi atop identity
+      verify(myBidi.atopMat(identity)(Keep.left), "bidi-matval")
+      verify(myBidi.atopMat(identity)(Keep.none), NotUsed)
+      verify(myBidi.atopMat(identity)(Keep.right), NotUsed)
+      verify(myBidi.atopMat(identity)((m, _) => m), "bidi-matval")
+    }
+
   }
 
 }

akka-stream/src/main/scala/akka/stream/scaladsl/BidiFlow.scala

@@ -5,7 +5,6 @@
 package akka.stream.scaladsl
 
 import scala.concurrent.duration.FiniteDuration
-
 import akka.NotUsed
 import akka.stream.{ BidiShape, _ }
 import akka.stream.impl.{ LinearTraversalBuilder, Timers, TraversalBuilder }
@@ -59,7 +58,31 @@ final class BidiFlow[-I1, +O1, -I2, +O2, +Mat](
    * flow into the materialized value of the resulting BidiFlow.
    */
   def atopMat[OO1, II2, Mat2, M](bidi: Graph[BidiShape[O1, OO1, II2, I2], Mat2])(
-      combine: (Mat, Mat2) => M): BidiFlow[I1, OO1, II2, O2, M] = {
+      combine: (Mat, Mat2) => M): BidiFlow[I1, OO1, II2, O2, M] =
+    if (this eq BidiFlow.identity) {
+      // optimizations possible since we know that identity matval is NotUsed
+      if (combine eq Keep.right)
+        BidiFlow.fromGraph(bidi).asInstanceOf[BidiFlow[I1, OO1, II2, O2, M]]
+      else if ((combine eq Keep.left) || (combine eq Keep.none))
+        BidiFlow.fromGraph(bidi).mapMaterializedValue(_ => NotUsed).asInstanceOf[BidiFlow[I1, OO1, II2, O2, M]]
+      else {
+        BidiFlow
+          .fromGraph(bidi)
+          .mapMaterializedValue(mat2 => combine(NotUsed.asInstanceOf[Mat], mat2))
+          .asInstanceOf[BidiFlow[I1, OO1, II2, O2, M]]
+      }
+    } else if (bidi eq BidiFlow.identity) {
+      // optimizations possible since we know that identity matval is NotUsed
+      if (combine eq Keep.left)
+        this.asInstanceOf[BidiFlow[I1, OO1, II2, O2, M]]
+      else if ((combine eq Keep.right) || (combine eq Keep.none))
+        this.mapMaterializedValue(_ => NotUsed).asInstanceOf[BidiFlow[I1, OO1, II2, O2, M]]
+      else {
+        this
+          .mapMaterializedValue(mat => combine(mat, NotUsed.asInstanceOf[Mat2]))
+          .asInstanceOf[BidiFlow[I1, OO1, II2, O2, M]]
+      }
+    } else {
       val newBidi1Shape = shape.deepCopy()
       val newBidi2Shape = bidi.shape.deepCopy()