stream: register newshells with no active interpreters (#29731)

Fixes #29730

akka-actor-tests/src/test/scala/akka/util/ByteStringInitializationSpec.scala

@@ -39,7 +39,12 @@ class ByteStringInitializationSpec extends AnyWordSpec with Matchers {
 
       import scala.language.reflectiveCalls
       type WithRun = { def run(): Unit }
-      cleanCl.loadClass("akka.util.ByteStringInitTest").newInstance().asInstanceOf[WithRun].run()
+      cleanCl
+        .loadClass("akka.util.ByteStringInitTest")
+        .getDeclaredConstructor()
+        .newInstance()
+        .asInstanceOf[WithRun]
+        .run()
     }
   }
 }

akka-remote/src/test/scala/akka/remote/artery/tcp/ssl/PeerSubjectVerifierSpec.scala

@@ -52,7 +52,7 @@ class PeerSubjectVerifierSpec extends AnyWordSpec with Matchers {
       override def removeValue(name: String): Unit = throw new UnsupportedOperationException()
       override def getValueNames: Array[String] = throw new UnsupportedOperationException()
       override def getLocalCertificates: Array[Certificate] = throw new UnsupportedOperationException()
-      override def getPeerCertificateChain: Array[javax.security.cert.X509Certificate] =
+      override def getPeerCertificateChain /*: Array[javax.security.cert.X509Certificate]*/ =
         throw new UnsupportedOperationException()
       override def getPeerPrincipal: Principal = throw new UnsupportedOperationException()
       override def getLocalPrincipal: Principal = throw new UnsupportedOperationException()

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

@@ -584,6 +584,34 @@ class FlowFlatMapPrefixSpec extends StreamSpec {
         subscriber.expectNext("a", "b", "c")
         subscriber.expectComplete()
       }
+
+      "complete newShells registration when all active interpreters are done" in assertAllStagesStopped {
+        @volatile var closeSink: () => Unit = null
+
+        val (fNotUsed, qOut) = Source
+          .empty[Int]
+          .flatMapPrefixMat(1) { seq =>
+            log.debug("waiting for closer to be set")
+            while (null == closeSink) Thread.sleep(50)
+            log.debug("closing sink")
+            closeSink()
+            log.debug("sink closed")
+            //closing the sink before returning means that it's higly probably
+            //for the flatMapPrefix stage to receive the downstream cancellation before the actor graph interpreter
+            //gets a chance to complete the new interpreter shell's registration.
+            //this in turn exposes a bug in the actor graph interpreter when all active flows complete
+            //but there are pending new interpreter shells to be registered.
+            Flow[Int].prepend(Source(seq))
+          }(Keep.right)
+          .toMat(Sink.queue(10))(Keep.both)
+          .run()
+
+        log.debug("assigning closer")
+        closeSink = () => qOut.cancel()
+
+        log.debug("closer assigned, waiting for completion")
+        fNotUsed.futureValue should be(NotUsed)
+      }
     }
   }
 }

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

@@ -753,13 +753,17 @@ import akka.util.OptionVal
     else if (shortCircuitBuffer != null) shortCircuitBatch()
   }
 
-  private def shortCircuitBatch(): Unit = {
-    while (!shortCircuitBuffer.isEmpty && currentLimit > 0 && activeInterpreters.nonEmpty) shortCircuitBuffer
-      .poll() match {
+  @tailrec private def shortCircuitBatch(): Unit = {
+    if (shortCircuitBuffer.isEmpty) ()
+    else if (currentLimit == 0) {
+      self ! Resume
+    } else {
+      shortCircuitBuffer.poll() match {
         case b: BoundaryEvent => processEvent(b)
         case Resume           => finishShellRegistration()
       }
-    if (!shortCircuitBuffer.isEmpty && currentLimit == 0) self ! Resume
+      shortCircuitBatch()
+    }
   }
 
   private def processEvent(b: BoundaryEvent): Unit = {