feat: fail if current namespaces is required but cannot be resolved

Fixes #433

Author: metacosm

operator-framework-core/src/main/java/io/javaoperatorsdk/operator/Operator.java

@@ -137,13 +137,21 @@ public <R extends CustomResource> void register(
       controller.init(eventSourceManager);
       closeables.add(eventSourceManager);
 
+      final var effectiveNamespaces = configuration.getEffectiveNamespaces();
+      if (configuration.isCurrentNamespaceMissing() && configuration.watchCurrentNamespace()) {
+        throw new OperatorException(
+            "Controller '"
+                + controllerName
+                + "' is configured to watch the current namespace but it couldn't be inferred from the current configuration. ");
+      }
+
+      final var watchedNS =
+          configuration.watchAllNamespaces() ? "[all namespaces]" : effectiveNamespaces;
       log.info(
           "Registered Controller: '{}' for CRD: '{}' for namespace(s): {}",
           controllerName,
           resClass,
-          configuration.watchAllNamespaces()
-              ? "[all namespaces]"
-              : configuration.getEffectiveNamespaces());
+          watchedNS);
     }
   }
 }

operator-framework-core/src/main/java/io/javaoperatorsdk/operator/api/config/ControllerConfiguration.java

@@ -3,6 +3,7 @@
 import io.fabric8.kubernetes.client.CustomResource;
 import io.javaoperatorsdk.operator.api.Controller;
 import java.util.Collections;
+import java.util.Objects;
 import java.util.Set;
 
 public interface ControllerConfiguration<R extends CustomResource> {
@@ -41,6 +42,13 @@ static boolean currentNamespaceWatched(Set<String> namespaces) {
         && namespaces.contains(Controller.WATCH_CURRENT_NAMESPACE);
   }
 
+  /**
+   * Computes the effective namespaces based on the set specified by the user, in particular
+   * retrieves the current namespace from the client when the user specified that they wanted to
+   * watch the current namespace only.
+   *
+   * @return a Set of namespace names the associated controller will watch
+   */
   default Set<String> getEffectiveNamespaces() {
     var targetNamespaces = getNamespaces();
     if (watchCurrentNamespace()) {
@@ -54,6 +62,12 @@ default Set<String> getEffectiveNamespaces() {
     return targetNamespaces;
   }
 
+  default boolean isCurrentNamespaceMissing() {
+    final var effectiveNamespaces = getEffectiveNamespaces();
+    return effectiveNamespaces.size() == 1
+        && effectiveNamespaces.stream().allMatch(Objects::isNull);
+  }
+
   default RetryConfiguration getRetryConfiguration() {
     return RetryConfiguration.DEFAULT;
   }