fix(analytics): surface polling-loop errors so schema drift can't loop silently (SDK-78)

lib/mixpanel-ruby/flags/local_flags_provider.rb

@@ -49,13 +49,13 @@ def start_polling_for_definitions!
               begin
                 fetch_flag_definitions
               rescue StandardError => e
-                @error_handler.handle(e) if @error_handler
+                log_polling_error(e)
               end
             end
           end
         end
       rescue StandardError => e
-        @error_handler.handle(e) if @error_handler
+        log_polling_error(e)
       end
 
       def stop_polling_for_definitions!
@@ -141,6 +141,19 @@ def get_all_variants(context)
 
       private
 
+      # Surface polling-loop failures unconditionally. The default
+      # Mixpanel::ErrorHandler is a no-op, so dispatching only via
+      # @error_handler swallows schema drift (NoMethodError,
+      # JSON::ParserError, etc.) — the loop runs forever undetected.
+      # Always warn so the failure is visible without an error_handler
+      # being configured. Matches the convention in mixpanel-python /
+      # mixpanel-java / mixpanel-go / mixpanel-node, all of which log
+      # unconditionally and keep polling.
+      def log_polling_error(error)
+        warn "[Mixpanel] Failed to fetch flag definitions: #{error.class}: #{error.message}"
+        @error_handler.handle(error) if @error_handler
+      end
+
       def fetch_flag_definitions
         response = call_flags_endpoint
 

spec/mixpanel-ruby/flags/local_flags_spec.rb

@@ -755,5 +755,39 @@ def user_context_with_properties(properties)
         polling_provider.stop_polling_for_definitions!
       end
     end
+
+    it 'warns to stderr when fetch raises and no error_handler is configured' do
+      stub_request(:get, endpoint_url_regex).to_return(status: 500, body: 'server down')
+
+      polling_provider = Mixpanel::Flags::LocalFlagsProvider.new(
+        test_token,
+        { enable_polling: false },
+        mock_tracker,
+        nil
+      )
+
+      expect { polling_provider.start_polling_for_definitions! }
+        .to output(/\[Mixpanel\] Failed to fetch flag definitions: Mixpanel::ServerError/).to_stderr
+    end
+
+    it 'surfaces unexpected errors (schema drift) instead of swallowing them silently' do
+      stub_flag_definitions([create_test_flag])
+
+      polling_provider = Mixpanel::Flags::LocalFlagsProvider.new(
+        test_token,
+        { enable_polling: false },
+        mock_tracker,
+        mock_error_handler
+      )
+
+      # Simulate schema drift: server returns a payload the parser doesn't expect.
+      # Without this fix the error would be dispatched only to @error_handler (whose
+      # default ErrorHandler#handle is a no-op) — operators would never notice.
+      allow(polling_provider).to receive(:fetch_flag_definitions).and_raise(NoMethodError, "undefined method `[]' for nil:NilClass")
+
+      expect(mock_error_handler).to receive(:handle).with(an_instance_of(NoMethodError))
+      expect { polling_provider.start_polling_for_definitions! }
+        .to output(/\[Mixpanel\] Failed to fetch flag definitions: NoMethodError/).to_stderr
+    end
   end
 end