=str Fix maybe throw for MinimalStage.

stream-tests/src/test/java-jdk9-only/org/apache/pekko/stream/javadsl/FlowUnfoldAsyncTest.java

@@ -0,0 +1,57 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.pekko.stream.javadsl;
+
+import org.apache.pekko.japi.Pair;
+import org.apache.pekko.stream.StreamTest;
+import org.apache.pekko.testkit.PekkoJUnitActorSystemResource;
+import org.apache.pekko.testkit.PekkoSpec;
+import org.junit.Assert;
+import org.junit.ClassRule;
+import org.junit.Test;
+
+import java.util.Optional;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.TimeUnit;
+
+public class FlowUnfoldAsyncTest extends StreamTest {
+    @ClassRule
+    public static PekkoJUnitActorSystemResource actorSystemResource =
+        new PekkoJUnitActorSystemResource("SourceTest", PekkoSpec.testConf());
+
+    public FlowUnfoldAsyncTest() {
+        super(actorSystemResource);
+    }
+
+    @Test
+    public void testFoldAsync() throws Exception {
+        final Integer result = Source.unfoldAsync(
+                0,
+                idx -> {
+                    if (idx >= 10) {
+                        return CompletableFuture.completedStage(Optional.empty());
+                    } else {
+                        return CompletableFuture.completedStage(Optional.of(Pair.create(idx + 1, idx)));
+                    }
+                })
+            .runFold(0, Integer::sum, system)
+            .toCompletableFuture()
+            .get(3, TimeUnit.SECONDS);
+        Assert.assertEquals(45, result.intValue());
+    }
+}

stream/src/main/scala/org/apache/pekko/stream/impl/Unfold.scala

@@ -124,7 +124,7 @@ import scala.util.{ Failure, Success, Try }
       }
 
       def onPull(): Unit = {
-        f.apply(state) match {
+        f.apply(state).toCompletableFuture match {
           case cf: CompletableFuture[Optional[Pair[S, E]] @unchecked] if cf.isDone && !cf.isCompletedExceptionally =>
             handle(cf.join())
           case future =>