Fix ForwardBackend filters initialization in routegroup (#4122)

Align filter cleanup behavior between ingress and routegroup processing
in kubernetes dataclient.
Filters are cleaned up during the processing of ingress and left as is
during the processing of routegroups.

IIUC the logic, default filters will not be cleared anyway, because they
are added later in polling.go.
This is the risk we should accept.

Signed-off-by: Aleksandr Ponimaskin <aleksandr.ponimaskin@zalando.de>

Author: a4180p

dataclients/kubernetes/routegroup.go

@@ -298,6 +298,7 @@ func applyBackend(ctx *routeGroupContext, backend *definitions.SkipperBackend, r
 	case eskip.ForwardBackend:
 		r.Backend = ctx.forwardBackendURL
 		r.BackendType = eskip.NetworkBackend
+		r.Filters = []*eskip.Filter{}
 	}
 
 	if ctx.backendNameTracingTag {

dataclients/kubernetes/testdata/routegroups/backends/forward-with-filters.eskip

@@ -0,0 +1,8 @@
+kube_rg____example_org__catchall__0_0:
+	Host("^(example[.]org[.]?(:[0-9]+)?)$")
+	-> <shunt>;
+
+kube_rg__default__myapp__all__0_0:
+	Host("^(example[.]org[.]?(:[0-9]+)?)$")
+	&& PathSubtree("/")
+	-> "http://forward.example";

dataclients/kubernetes/testdata/routegroups/backends/forward-with-filters.kube

@@ -0,0 +1 @@
+forward-backend-url: http://forward.example

dataclients/kubernetes/testdata/routegroups/backends/forward-with-filters.yaml

@@ -0,0 +1,16 @@
+apiVersion: zalando.org/v1
+kind: RouteGroup
+metadata:
+  name: myapp
+spec:
+  hosts:
+    - example.org
+  backends:
+    - name: fwd
+      type: forward
+  routes:
+    - pathSubtree: /
+      filters:
+        - status(200)
+      backends:
+        - backendName: fwd

routesrv/routesrv_test.go

@@ -651,6 +651,37 @@ func TestRoutesWithForwardBackend(t *testing.T) {
 	wantHTTPCode(t, w, http.StatusOK)
 }
 
+func TestRoutesWithForwardBackendFiltersDropped(t *testing.T) {
+	defer tl.Reset()
+	ks, _ := newKubeServer(t, loadKubeYAML(t, "testdata/forward-backend-with-filter.yaml"))
+	ks.Start()
+	defer ks.Close()
+	rs := newRouteServerWithOptions(t, skipper.Options{
+		SourcePollTimeout: pollInterval,
+		Kubernetes:        true,
+		KubernetesURL:     ks.URL,
+		ForwardBackendURL: "http://forward.example",
+	})
+
+	rs.StartUpdates()
+	defer rs.StopUpdates()
+
+	if err := tl.WaitFor(routesrv.LogRoutesInitialized, waitTimeout); err != nil {
+		t.Fatalf("routes not initialized: %v", err)
+	}
+	w := getRoutes(rs)
+
+	want := parseEskipFixture(t, "testdata/forward-backend-with-filter.eskip")
+	got, err := eskip.Parse(w.Body.String())
+	if err != nil {
+		t.Fatalf("served routes are not valid eskip: %s", w.Body)
+	}
+	if !eskip.EqLists(got, want) {
+		t.Errorf("expected to receive route without filters: %s", cmp.Diff(got, want))
+	}
+	wantHTTPCode(t, w, http.StatusOK)
+}
+
 func TestESkipBytesHandlerWithCorrectEtag(t *testing.T) {
 	defer tl.Reset()
 	ks, _ := newKubeServer(t, loadKubeYAML(t, "testdata/lb-target-multi.yaml"))

routesrv/testdata/forward-backend-with-filter.eskip

@@ -0,0 +1 @@
+kube_rg__default__my_test__all__0_0: PathSubtree("/") -> "http://forward.example";

routesrv/testdata/forward-backend-with-filter.yaml

@@ -0,0 +1,14 @@
+apiVersion: zalando.org/v1
+kind: RouteGroup
+metadata:
+  name: my-test
+spec:
+  backends:
+    - name: fwd
+      type: forward
+  routes:
+    - pathSubtree: /
+      filters:
+        - status(200)
+      backends:
+        - backendName: fwd