refactor append metadata exchange filter (istio#48152)

Author: hzxuzhonghu

pilot/pkg/networking/core/v1alpha3/listener_builder.go

@@ -378,21 +378,7 @@ func (lb *ListenerBuilder) buildHTTPConnectionManager(httpOpts *httpListenerOpts
 
 		// Metadata exchange filter needs to be added before any other HTTP filters are added. This is done to
 		// ensure that mx filter comes before HTTP RBAC filter. This is related to https://github.com/istio/istio/issues/41066
-		if features.MetadataExchange && !httpOpts.hbone && !lb.node.IsAmbient() {
-			if features.NativeMetadataExchange && util.IsIstioVersionGE119(lb.node.IstioVersion) {
-				if httpOpts.class == istionetworking.ListenerClassSidecarInbound {
-					filters = append(filters, xdsfilters.SidecarInboundMetadataFilter)
-				} else {
-					if httpOpts.skipIstioMXHeaders {
-						filters = append(filters, xdsfilters.SidecarOutboundMetadataFilterSkipHeaders)
-					} else {
-						filters = append(filters, xdsfilters.SidecarOutboundMetadataFilter)
-					}
-				}
-			} else {
-				filters = append(filters, xdsfilters.HTTPMx)
-			}
-		}
+		filters = appendMxFilter(httpOpts, lb.node, filters)
 		// TODO: how to deal with ext-authz? It will be in the ordering twice
 		filters = append(filters, lb.authzCustomBuilder.BuildHTTP(httpOpts.class)...)
 		filters = extension.PopAppendHTTP(filters, wasm, extensions.PluginPhase_AUTHN)
@@ -449,3 +435,21 @@ func (lb *ListenerBuilder) buildHTTPConnectionManager(httpOpts *httpListenerOpts
 	}
 	return connectionManager
 }
+
+func appendMxFilter(httpOpts *httpListenerOpts, node *model.Proxy, filters []*hcm.HttpFilter) []*hcm.HttpFilter {
+	if !features.MetadataExchange || httpOpts.hbone || node.IsAmbient() {
+		return filters
+	}
+	if !features.NativeMetadataExchange || !util.IsIstioVersionGE119(node.IstioVersion) {
+		return append(filters, xdsfilters.HTTPMx)
+	}
+
+	if httpOpts.class == istionetworking.ListenerClassSidecarInbound {
+		return append(filters, xdsfilters.SidecarInboundMetadataFilter)
+	}
+
+	if httpOpts.skipIstioMXHeaders {
+		return append(filters, xdsfilters.SidecarOutboundMetadataFilterSkipHeaders)
+	}
+	return append(filters, xdsfilters.SidecarOutboundMetadataFilter)
+}