feat: warn that payload filtering has no effect with FDv2

lib/ldclient-rb/config.rb

@@ -362,6 +362,9 @@ def offline?
     # This payload filter key only applies to the default streaming and polling data sources. It will not affect TestData or FileData
     # data sources, nor will it be applied to any data source provided through the {#data_source} config property.
     #
+    # Payload filtering is not supported with the FDv2 data system, so this key has no effect on requests made by FDv2
+    # data sources.
+    #
     attr_reader :payload_filter_key
 
     #

lib/ldclient-rb/impl/data_system/fdv2.rb

@@ -54,6 +54,11 @@ def initialize(sdk_key, config, data_system_config)
           @config = config
           @data_system_config = data_system_config
           @logger = config.logger
+
+          unless config.payload_filter_key.nil?
+            @logger.warn { "[LDClient] Payload filtering is not supported with the FDv2 data system; the configured payload filter has no effect on FDv2 requests" }
+          end
+
           @synchronizer_builders = data_system_config.synchronizers || []
           @fdv1_fallback_synchronizer_builder = data_system_config.fdv1_fallback_synchronizer
           @disabled = @config.offline?

spec/impl/data_system/fdv2_datasystem_spec.rb

@@ -674,6 +674,29 @@ def build(_sdk_key, _config)
             fdv2.stop
           end
         end
+
+        describe "payload filter key" do
+          let(:data_system_config) do
+            LaunchDarkly::DataSystem::ConfigBuilder.new
+              .initializers([])
+              .synchronizers([])
+              .build
+          end
+
+          it "warns when a payload filter key is configured" do
+            logger = double.as_null_object
+            expect(logger).to receive(:warn)
+
+            FDv2.new(sdk_key, LaunchDarkly::Config.new(logger: logger, payload_filter_key: "microservice-1"), data_system_config)
+          end
+
+          it "does not warn when no payload filter key is configured" do
+            logger = double.as_null_object
+            expect(logger).not_to receive(:warn)
+
+            FDv2.new(sdk_key, LaunchDarkly::Config.new(logger: logger), data_system_config)
+          end
+        end
       end
     end
   end