backendRef filters shouldn't affect weighted routing (#3604)

* backendRef filters shouldn't affect weighted routing

This conformance test ensures that adding a backendRef filter
(in this case a request header modifier) should not have any
side effects on the routing. Especially when the xRoute has
multiple weighted backends

* fix copyright year

* fix casing of 'Backend'

* add newline to the end of the file

conformance/tests/httproute-request-header-modifier-backend-weights.go

@@ -0,0 +1,88 @@
+/*
+Copyright 2025 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package tests
+
+import (
+	"strings"
+	"testing"
+
+	"k8s.io/apimachinery/pkg/types"
+
+	"sigs.k8s.io/gateway-api/conformance/utils/http"
+	"sigs.k8s.io/gateway-api/conformance/utils/kubernetes"
+	"sigs.k8s.io/gateway-api/conformance/utils/suite"
+	"sigs.k8s.io/gateway-api/pkg/features"
+)
+
+func init() {
+	ConformanceTests = append(ConformanceTests,
+		HTTPRouteRequestHeaderModifierBackendWeights,
+	)
+}
+
+var HTTPRouteRequestHeaderModifierBackendWeights = suite.ConformanceTest{
+	ShortName:   "HTTPRouteRequestHeaderModifierBackendWeights",
+	Description: "An HTTPRoute with backend request header modifier filter sends traffic to the correct backends",
+	Features: []features.FeatureName{
+		features.SupportGateway,
+		features.SupportHTTPRouteBackendRequestHeaderModification,
+		features.SupportHTTPRoute,
+	},
+	Manifests: []string{"tests/httproute-request-header-modifier-backend-weights.yaml"},
+	Test: func(t *testing.T, suite *suite.ConformanceTestSuite) {
+		ns := "gateway-conformance-infra"
+
+		routeNN := types.NamespacedName{Name: "request-header-modifier-backend-weights", Namespace: ns}
+		gwNN := types.NamespacedName{Name: "same-namespace", Namespace: ns}
+		gwAddr := kubernetes.GatewayAndHTTPRoutesMustBeAccepted(t, suite.Client, suite.TimeoutConfig, suite.ControllerName, kubernetes.NewGatewayRef(gwNN), routeNN)
+		kubernetes.HTTPRouteMustHaveResolvedRefsConditionsTrue(t, suite.Client, suite.TimeoutConfig, routeNN, gwNN)
+
+		roundTripper := suite.RoundTripper
+
+		expected := http.ExpectedResponse{
+			Request:   http.Request{Path: "/"},
+			Response:  http.Response{StatusCode: 200},
+			Namespace: "gateway-conformance-infra",
+		}
+
+		req := http.MakeRequest(t, &expected, gwAddr, "HTTP", "http")
+
+		// Assert request succeeds before checking traffic
+		http.MakeRequestAndExpectEventuallyConsistentResponse(t, suite.RoundTripper, suite.TimeoutConfig, gwAddr, expected)
+
+		for range 100 {
+			cReq, _, err := roundTripper.CaptureRoundTrip(req)
+			if err != nil {
+				t.Fatalf("failed to roundtrip request: %v", err)
+			}
+
+			expectedBackends := cReq.Headers["Backend"]
+
+			if len(expectedBackends) != 1 {
+				t.Fatalf("expected a single 'Backend' header to have been set, got %d", len(expectedBackends))
+			}
+
+			if !strings.HasPrefix(cReq.Pod, expectedBackends[0]) {
+				t.Fatalf(
+					"expected the backendRef to have set the correct headers and sent the request to the correct pod, got %q, want %q",
+					cReq.Pod,
+					expectedBackends[0],
+				)
+			}
+		}
+	},
+}

conformance/tests/httproute-request-header-modifier-backend-weights.yaml

@@ -0,0 +1,28 @@
+apiVersion: gateway.networking.k8s.io/v1
+kind: HTTPRoute
+metadata:
+  name: request-header-modifier-backend-weights
+  namespace: gateway-conformance-infra
+spec:
+  parentRefs:
+  - name: same-namespace
+  rules:
+  - backendRefs:
+    - name: infra-backend-v1
+      port: 8080
+      weight: 10
+      filters:
+      - type: RequestHeaderModifier
+        requestHeaderModifier:
+          set:
+          - name: Backend
+            value: infra-backend-v1
+    - name: infra-backend-v2
+      port: 8080
+      weight: 10
+      filters:
+      - type: RequestHeaderModifier
+        requestHeaderModifier:
+          set:
+          - name: Backend
+            value: infra-backend-v2