feature: log warning when passing a running event source

Author: csviri

Diff for: operator-framework-core/src/main/java/io/javaoperatorsdk/operator/processing/event/internal/InformerEventSource.java

@@ -5,6 +5,9 @@
 import java.util.Set;
 import java.util.function.Function;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 import io.fabric8.kubernetes.api.model.HasMetadata;
 import io.fabric8.kubernetes.client.KubernetesClient;
 import io.fabric8.kubernetes.client.informers.ResourceEventHandler;
@@ -15,6 +18,8 @@
 
 public class InformerEventSource<T extends HasMetadata> extends AbstractEventSource {
 
+  private static final Logger log = LoggerFactory.getLogger(InformerEventSource.class);
+
   private final SharedInformer<T> sharedInformer;
   private final Function<T, Set<String>> resourceToUIDs;
   private final Function<HasMetadata, T> associatedWith;
@@ -41,6 +46,11 @@ public InformerEventSource(SharedInformer<T> sharedInformer,
       Function<T, Set<String>> resourceToUIDs,
       Function<HasMetadata, T> associatedWith,
       boolean skipUpdateEventPropagationIfNoChange) {
+    if (sharedInformer.isRunning()) {
+      log.warn(
+          "Informer is already running on event source creation, this is not desirable and may " +
+              "lead to non deterministic behavior.");
+    }
     this.sharedInformer = sharedInformer;
     this.resourceToUIDs = resourceToUIDs;
     this.skipUpdateEventPropagationIfNoChange = skipUpdateEventPropagationIfNoChange;
@@ -81,6 +91,7 @@ private void propagateEvent(InformerEvent.Action action, T object, T oldObject)
     }
     uids.forEach(uid -> {
       InformerEvent event = new InformerEvent(uid, this, action, object, oldObject);
+      // In case user passes an already running informer this would fail.
       if (this.eventHandler != null) {
         this.eventHandler.handleEvent(event);
       }

Diff for: sample-operators/tomcat-operator/src/main/java/io/javaoperatorsdk/operator/sample/TomcatController.java

@@ -45,7 +45,7 @@ public void init(EventSourceManager eventSourceManager) {
     SharedIndexInformer<Deployment> deploymentInformer =
         kubernetesClient.apps().deployments().inAnyNamespace()
             .withLabel("app.kubernetes.io/managed-by", "tomcat-operator")
-            .inform();
+            .runnableInformer(0);
 
     this.informerEventSource = new InformerEventSource<>(deploymentInformer, d -> {
       var ownerReferences = d.getMetadata().getOwnerReferences();