Preserve feature flag configuration when Remote Config replaces a target

products/feature-flagging/feature-flagging-lib/src/main/java/com/datadog/featureflag/RemoteConfigServiceImpl.java

@@ -25,6 +25,9 @@ public class RemoteConfigServiceImpl implements ConfigurationChangesTypedListene
   private final List<Consumer<byte[]>> subscribers = new CopyOnWriteArrayList<>();
   private volatile byte[] current;
 
+  /** Key of the current document, guarded by the subscribers lock. */
+  @Nullable private String currentConfigKey;
+
   /**
    * Whether the current document was received before any subscriber, so the poller acknowledged it
    * without validation. Guarded by the subscribers lock.
@@ -114,8 +117,11 @@ public synchronized void close() {
     this.configurationPoller.removeCapabilities(CAPABILITY_FFE_FLAG_CONFIGURATION_RULES);
     this.configurationPoller.removeListeners(FFE_FLAGS);
     this.configurationPoller.stop();
-    this.subscribers.clear();
-    this.current = null;
+    synchronized (this.subscribers) {
+      this.subscribers.clear();
+      this.current = null;
+      this.currentConfigKey = null;
+    }
   }
 
   /**
@@ -134,6 +140,11 @@ public void accept(
       final PollingRateHinter pollingRateHinter) {
     final Iterator<Consumer<byte[]>> snapshot;
     synchronized (this.subscribers) {
+      // RC applies a replacement before removing the old target.
+      if (content == null && !configKey.equals(this.currentConfigKey)) {
+        return;
+      }
+      this.currentConfigKey = content == null ? null : configKey;
       this.current = content;
       this.unvalidated = content != null && this.subscribers.isEmpty();
       snapshot = this.subscribers.iterator();

products/feature-flagging/feature-flagging-lib/src/test/java/com/datadog/featureflag/RemoteConfigServiceImplTest.java

@@ -41,6 +41,7 @@
 import org.junit.jupiter.api.Test;
 import org.mockito.ArgumentCaptor;
 import org.slf4j.LoggerFactory;
+import org.tabletest.junit.TableTest;
 
 class RemoteConfigServiceImplTest {
   private static final byte[] DOCUMENT = "{\"flags\":{}}".getBytes(UTF_8);
@@ -80,6 +81,39 @@ void forwardsDocumentsAndRemovalsToAllSubscribers() {
     assertEquals(2, second.size());
   }
 
+  @TableTest({
+    "Scenario           | previousKey | replacementKey",
+    "enable dependency  | base        | dependent     ",
+    "disable dependency | dependent   | base          ",
+    "semver upgrade     | base        | semver        "
+  })
+  void keepsReplacementWhenPreviousConfigurationIsRemoved(
+      final String previousKey, final String replacementKey) {
+    final RemoteConfigServiceImpl service =
+        new RemoteConfigServiceImpl(mock(ConfigurationPoller.class));
+    final List<byte[]> received = new ArrayList<>();
+    service.subscribe(received::add);
+    final byte[] replacement = "{\"flags\":{\"replacement\":{}}}".getBytes(UTF_8);
+
+    // The RC poller applies the new target before removing the obsolete target.
+    service.accept(previousKey, DOCUMENT, null);
+    service.accept(replacementKey, replacement, null);
+    service.accept(previousKey, null, null);
+
+    assertEquals(2, received.size(), "Removing the old target must not clear its replacement");
+    assertArrayEquals(replacement, received.get(1));
+    final List<byte[]> replayed = new ArrayList<>();
+    service.subscribe(replayed::add);
+    assertEquals(1, replayed.size());
+    assertArrayEquals(replacement, replayed.get(0));
+
+    service.accept(replacementKey, null, null);
+    assertEquals(3, received.size());
+    assertNull(received.get(2));
+    assertEquals(2, replayed.size());
+    assertNull(replayed.get(1));
+  }
+
   @Test
   void reportsRejectedDocumentsOnceEverySubscriberIsNotified() {
     final RemoteConfigServiceImpl service =
@@ -339,6 +373,7 @@ void reportsReceivedConfiguration() {
 
     @Test
     void reportsRemovedConfiguration() {
+      service.accept("config", DOCUMENT, mock(PollingRateHinter.class));
       service.accept("config", null, mock(PollingRateHinter.class));
 
       assertLogged("configuration removed: key=config");