Fix Gateway API HttpRoute cannot strip path prefix.

According to Kubernetes guidlines empty prefix should be allowed.

Fixes: #27994

Signed-off-by: chaunceyjiang <chaunceyjiang@gmail.com>

operator/pkg/model/translation/envoy_virtual_host.go

@@ -270,10 +270,16 @@ func hostRewriteMutation(rewrite *model.HTTPURLRewriteFilter) routeActionMutatio
 
 func pathPrefixMutation(rewrite *model.HTTPURLRewriteFilter) routeActionMutation {
 	return func(route *envoy_config_route_v3.Route_Route) *envoy_config_route_v3.Route_Route {
-		if rewrite == nil || rewrite.Path == nil || len(rewrite.Path.Prefix) == 0 {
+		if rewrite == nil || rewrite.Path == nil || len(rewrite.Path.Exact) != 0 || len(rewrite.Path.Regex) != 0 {
 			return route
 		}
-		route.Route.PrefixRewrite = rewrite.Path.Prefix
+		if len(rewrite.Path.Prefix) == 0 {
+			// Refer to: https://gateway-api.sigs.k8s.io/references/spec/#gateway.networking.k8s.io%2fv1beta1.HTTPPathModifier
+			// ReplacePrefix is allowed to be empty.
+			route.Route.PrefixRewrite = "/"
+		} else {
+			route.Route.PrefixRewrite = rewrite.Path.Prefix
+		}
 		return route
 	}
 }

operator/pkg/model/translation/envoy_virtual_host_test.go

@@ -170,6 +170,19 @@ func Test_pathPrefixMutation(t *testing.T) {
 		res := pathPrefixMutation(rewrite)(route)
 		require.Equal(t, res.Route.PrefixRewrite, "/prefix")
 	})
+	t.Run("with empty prefix rewrite", func(t *testing.T) {
+		route := &envoy_config_route_v3.Route_Route{
+			Route: &envoy_config_route_v3.RouteAction{},
+		}
+		rewrite := &model.HTTPURLRewriteFilter{
+			Path: &model.StringMatch{
+				Prefix: "",
+			},
+		}
+
+		res := pathPrefixMutation(rewrite)(route)
+		require.Equal(t, res.Route.PrefixRewrite, "/")
+	})
 }
 
 func Test_requestMirrorMutation(t *testing.T) {