Prevent duplicate Feature Flagging runtimes

products/feature-flagging/feature-flagging-agent/src/main/java/com/datadog/featureflag/FeatureFlaggingSystem.java

@@ -6,6 +6,7 @@
 import datadog.communication.ddagent.SharedCommunicationObjects;
 import datadog.trace.api.Config;
 import datadog.trace.api.featureflag.FeatureFlaggingGateway;
+import datadog.trace.api.featureflag.FeatureFlaggingGateway.RuntimeMode;
 import datadog.trace.api.featureflag.config.FeatureFlaggingConfig;
 import datadog.trace.api.featureflag.flagevaluation.FlagEvaluationWriter;
 import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
@@ -80,6 +81,12 @@ private static void initializeOrRollBack(
       final SharedCommunicationObjects sco,
       final Config config,
       final SystemInitializer systemInitializer) {
+    if (!FeatureFlaggingGateway.claimRuntime(RuntimeMode.AGENT)) {
+      LOGGER.debug(
+          "Feature Flagging agent runtime not started because {} already owns the subsystem",
+          FeatureFlaggingGateway.activeRuntime());
+      return;
+    }
     try {
       systemInitializer.initialize(sco, config);
     } catch (final RuntimeException | Error e) {
@@ -180,6 +187,7 @@ public static synchronized void stop() {
     SPAN_ENRICHMENT_WRITER = null;
     EXPOSURE_WRITER = null;
     CONFIG_SERVICE = null;
+    FeatureFlaggingGateway.releaseRuntime(RuntimeMode.AGENT);
     if (activationListener != null) {
       FeatureFlaggingGateway.removeActivationListener(activationListener);
     }

products/feature-flagging/feature-flagging-agent/src/test/java/com/datadog/featureflag/FeatureFlaggingSystemTest.java

@@ -28,6 +28,7 @@
 import datadog.remoteconfig.Product;
 import datadog.trace.api.Config;
 import datadog.trace.api.featureflag.FeatureFlaggingGateway;
+import datadog.trace.api.featureflag.FeatureFlaggingGateway.RuntimeMode;
 import datadog.trace.api.featureflag.config.FeatureFlaggingConfig;
 import datadog.trace.api.featureflag.flagevaluation.FlagEvaluationWriter;
 import datadog.trace.test.junit.utils.config.WithConfig;
@@ -86,6 +87,25 @@ void agentlessActivationInitializesSystemOnce() {
 
     verify(systemInitializer).initialize(eq(sharedCommunicationObjects), any(Config.class));
     assertFalse(FeatureFlaggingSystem.isAwaitingApplicationActivation());
+    assertSame(RuntimeMode.AGENT, FeatureFlaggingGateway.activeRuntime());
+  }
+
+  @Test
+  @WithConfig(key = FEATURE_FLAGS_CONFIGURATION_SOURCE, value = "agentless")
+  void agentlessActivationDoesNotStartWhenStandaloneRuntimeOwnsTheProcess() {
+    final SharedCommunicationObjects sharedCommunicationObjects = sharedCommunicationObjects();
+    final FeatureFlaggingSystem.SystemInitializer systemInitializer =
+        mock(FeatureFlaggingSystem.SystemInitializer.class);
+    assertTrue(FeatureFlaggingGateway.claimRuntime(RuntimeMode.STANDALONE));
+    try {
+      FeatureFlaggingSystem.start(sharedCommunicationObjects, systemInitializer);
+      FeatureFlaggingGateway.activate();
+
+      verifyNoInteractions(systemInitializer);
+      assertSame(RuntimeMode.STANDALONE, FeatureFlaggingGateway.activeRuntime());
+    } finally {
+      FeatureFlaggingGateway.releaseRuntime(RuntimeMode.STANDALONE);
+    }
   }
 
   @Test
@@ -170,6 +190,7 @@ void testFeatureFlagSystemInitialization() {
     FeatureFlaggingSystem.stop();
     assertFalse(FeatureFlaggingGateway.isFlagEvaluationEnqueueEnabled());
     assertNull(FeatureFlaggingGateway.getFlagEvalWriter());
+    assertNull(FeatureFlaggingGateway.activeRuntime());
     // stop() is idempotent: a second call must be a safe no-op.
     FeatureFlaggingSystem.stop();
 

products/feature-flagging/feature-flagging-bootstrap/src/main/java/datadog/trace/api/featureflag/FeatureFlaggingGateway.java

@@ -4,12 +4,18 @@
 import datadog.trace.api.featureflag.flagevaluation.FlagEvaluationWriter;
 import datadog.trace.api.featureflag.ufc.v1.ServerConfiguration;
 import java.util.List;
+import java.util.Objects;
 import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.Consumer;
 
 public abstract class FeatureFlaggingGateway {
 
+  public enum RuntimeMode {
+    AGENT,
+    STANDALONE
+  }
+
   public interface ConfigListener extends Consumer<ServerConfiguration> {}
 
   public interface ActivationListener {
@@ -29,6 +35,8 @@ public interface SpanEnrichmentListener extends Consumer<SpanEnrichmentEvent> {}
   private static final AtomicReference<ServerConfiguration> CURRENT_CONFIG =
       new AtomicReference<>();
 
+  private static final AtomicReference<RuntimeMode> ACTIVE_RUNTIME = new AtomicReference<>();
+
   /**
    * The active EVP flagevaluation writer. Registered by {@code FlagEvaluationWriterImpl.start()}
    * when the killswitch {@code DD_FLAGGING_EVALUATION_COUNTS_ENABLED} is on (default). Read by
@@ -72,6 +80,28 @@ public static void activate() {
     ACTIVATION_LISTENERS.forEach(ActivationListener::activate);
   }
 
+  /**
+   * Claims process-wide ownership of Feature Flagging configuration and event delivery.
+   *
+   * <p>The claim is idempotent for the current owner. A different runtime must not start while an
+   * owner is active because doing so would create duplicate configuration pollers and duplicate
+   * exposure or evaluation delivery.
+   */
+  public static boolean claimRuntime(final RuntimeMode runtime) {
+    Objects.requireNonNull(runtime, "runtime");
+    return ACTIVE_RUNTIME.compareAndSet(null, runtime) || ACTIVE_RUNTIME.get() == runtime;
+  }
+
+  /** Releases process-wide ownership when {@code runtime} is the current owner. */
+  public static void releaseRuntime(final RuntimeMode runtime) {
+    ACTIVE_RUNTIME.compareAndSet(runtime, null);
+  }
+
+  /** Returns the runtime currently responsible for configuration and event delivery. */
+  public static RuntimeMode activeRuntime() {
+    return ACTIVE_RUNTIME.get();
+  }
+
   public static void addExposureListener(final ExposureListener listener) {
     EXPOSURE_LISTENERS.add(listener);
   }

products/feature-flagging/feature-flagging-bootstrap/src/test/java/datadog/trace/api/featureflag/FeatureFlaggingGatewayTest.java

@@ -1,5 +1,10 @@
 package datadog.trace.api.featureflag;
 
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.verifyNoMoreInteractions;
@@ -41,6 +46,8 @@ void tearDown() {
     FeatureFlaggingGateway.removeSpanEnrichmentListener(spanEnrichmentListener);
     FeatureFlaggingGateway.setFlagEvalWriter(null);
     FeatureFlaggingGateway.setFlagEvaluationEnqueueEnabled(true);
+    FeatureFlaggingGateway.releaseRuntime(FeatureFlaggingGateway.RuntimeMode.AGENT);
+    FeatureFlaggingGateway.releaseRuntime(FeatureFlaggingGateway.RuntimeMode.STANDALONE);
   }
 
   @Test
@@ -53,6 +60,30 @@ void testProviderActivationListener() {
     verifyNoMoreInteractions(activationListener);
   }
 
+  @Test
+  void runtimeOwnershipIsExclusiveAndIdempotent() {
+    assertNull(FeatureFlaggingGateway.activeRuntime());
+
+    assertTrue(FeatureFlaggingGateway.claimRuntime(FeatureFlaggingGateway.RuntimeMode.STANDALONE));
+    assertTrue(FeatureFlaggingGateway.claimRuntime(FeatureFlaggingGateway.RuntimeMode.STANDALONE));
+    assertFalse(FeatureFlaggingGateway.claimRuntime(FeatureFlaggingGateway.RuntimeMode.AGENT));
+    assertEquals(
+        FeatureFlaggingGateway.RuntimeMode.STANDALONE, FeatureFlaggingGateway.activeRuntime());
+
+    FeatureFlaggingGateway.releaseRuntime(FeatureFlaggingGateway.RuntimeMode.AGENT);
+    assertEquals(
+        FeatureFlaggingGateway.RuntimeMode.STANDALONE, FeatureFlaggingGateway.activeRuntime());
+
+    FeatureFlaggingGateway.releaseRuntime(FeatureFlaggingGateway.RuntimeMode.STANDALONE);
+    assertNull(FeatureFlaggingGateway.activeRuntime());
+    assertTrue(FeatureFlaggingGateway.claimRuntime(FeatureFlaggingGateway.RuntimeMode.AGENT));
+  }
+
+  @Test
+  void runtimeOwnershipRejectsNull() {
+    assertThrows(NullPointerException.class, () -> FeatureFlaggingGateway.claimRuntime(null));
+  }
+
   @Test
   void testAttachingAConfigListener() {
     clearCurrentServerConfiguration();