fix: standalone workflows now fail on top-level cycles (#1997)

Author: yarinkos

operator-framework-core/src/main/java/io/javaoperatorsdk/operator/processing/dependent/workflow/DefaultWorkflow.java

@@ -79,6 +79,10 @@ private Map<String, DependentResourceNode> toMap(Set<DependentResourceNode> node
       }
       map.put(node.getName(), node);
     }
+    if (topLevelResources.size() == 0) {
+      throw new IllegalStateException(
+          "No top-level dependent resources found. This might indicate a cyclic Set of DependentResourceNode has been provided.");
+    }
     return map;
   }
 

operator-framework-core/src/test/java/io/javaoperatorsdk/operator/processing/dependent/workflow/WorkflowTest.java

@@ -14,6 +14,7 @@
 import io.javaoperatorsdk.operator.sample.simple.TestCustomResource;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.junit.Assert.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 import static org.mockito.Mockito.mock;
@@ -24,6 +25,22 @@ class WorkflowTest {
 
   ExecutorService executorService = Executors.newCachedThreadPool();
 
+  @Test
+  void zeroTopLevelDRShouldThrowException() {
+    var dr1 = mock(DependentResource.class);
+    var dr2 = mock(DependentResource.class);
+    var dr3 = mock(DependentResource.class);
+
+    var cyclicWorkflowBuilderSetup = new WorkflowBuilder<TestCustomResource>()
+        .addDependentResource(dr1).dependsOn()
+        .addDependentResource(dr2).dependsOn(dr1)
+        .addDependentResource(dr3).dependsOn(dr2)
+        .addDependentResource(dr1).dependsOn(dr2);
+
+    assertThrows(IllegalStateException.class,
+        cyclicWorkflowBuilderSetup::build);
+  }
+
   @Test
   void calculatesTopLevelResources() {
     var dr1 = mock(DependentResource.class);