Fix NPE in ServerGrpcChannelBackoffResetHandler (#18139)

Author: timothy-e

pinot-server/src/main/java/org/apache/pinot/server/starter/helix/ServerGrpcChannelBackoffResetHandler.java

@@ -77,13 +77,16 @@ public ServerGrpcChannelBackoffResetHandler(HelixAdmin helixAdmin, String cluste
   @Override
   public synchronized void onInstanceConfigChange(List<InstanceConfig> instanceConfigs,
       NotificationContext context) {
+    // Only process INIT (listener registration) and CALLBACK (ZK data/child change).
+    // Ignore FINALIZE (listener unregistration) and other types.
+
     // INIT: first callback when the listener is registered (full cluster snapshot).
     // isChildChange: an instance ZK node was added or removed under /CONFIGS/PARTICIPANT.
     // Both require a full scan to rebuild _shuttingDownServers from the current cluster state.
     NotificationContext.Type type = context.getType();
     if (type == NotificationContext.Type.INIT || context.getIsChildChange()) {
       handleFullScan();
-    } else {
+    } else if (type == NotificationContext.Type.CALLBACK) {
       // An existing instance's config changed (e.g. IS_SHUTDOWN_IN_PROGRESS toggled).
       // pathChanged is the ZK path of the specific instance that changed.
       String pathChanged = context.getPathChanged();

pinot-server/src/test/java/org/apache/pinot/server/starter/helix/ServerGrpcChannelBackoffResetHandlerTest.java

@@ -229,6 +229,22 @@ private NotificationContext createCallbackContextForInstance(String instanceId)
     return context;
   }
 
+  @Test
+  public void testFinalizeNotificationIsNoOp() {
+    // Simulate a FINALIZE notification where pathChanged is null. The listener should ignore this.
+    _handler.onInstanceConfigChange(Collections.emptyList(), createFinalizeContextWithNullPath());
+
+    verify(_mailboxService, never()).resetConnectBackoff(any(), anyInt());
+    verify(_helixAdmin, never()).getInstancesInCluster(any());
+  }
+
+  private NotificationContext createFinalizeContextWithNullPath() {
+    NotificationContext context = new NotificationContext(_helixManager);
+    context.setType(NotificationContext.Type.FINALIZE);
+    // pathChanged is not set, so getPathChanged() returns null
+    return context;
+  }
+
   private NotificationContext createInitContext() {
     NotificationContext context = new NotificationContext(_helixManager);
     context.setType(NotificationContext.Type.INIT);