cmd/k8s-operator/proxygroup.go

@@ -1104,9 +1104,35 @@ func (r *ProxyGroupReconciler) findStaticEndpoints(ctx context.Context, existing
 		return nil, &FindStaticEndpointErr{msg: fmt.Sprintf("failed to find any `status.addresses` of type %q on nodes using configured Selectors on `spec.staticEndpoints.nodePort.selectors` for ProxyClass %q", corev1.NodeExternalIP, proxyClass.Name)}
 	}
 
+	// If we ended up selecting the same set of addresses already in use, keep
+	// the existing order. nodes.Items from r.List is not guaranteed to be in
+	// a stable order across calls, so without this the slice can permute on
+	// each reconcile, making the marshalled config Secret differ byte-for-byte
+	// even though nothing has effectively changed. That trips the DeepEqual
+	// check on the config Secret, which writes the Secret, which fires a
+	// watch event, which re-enqueues the ProxyGroup, and so on.
+	if len(currAddrs) > 0 && sameAddrPortSet(endpoints, currAddrs) {
+		return currAddrs, nil
+	}
+
 	return endpoints, nil
 }
 
+// sameAddrPortSet reports whether a and b contain the same AddrPorts,
+// ignoring order. Both slices are assumed to be free of duplicates, which
+// holds for callers in this package.
+func sameAddrPortSet(a, b []netip.AddrPort) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for _, x := range a {
+		if !slices.Contains(b, x) {
+			return false
+		}
+	}
+	return true
+}
+
 func getStaticEndpointAddress(a *corev1.NodeAddress, port uint16) *netip.AddrPort {
 	addr, err := netip.ParseAddr(a.Address)
 	if err != nil {

cmd/k8s-operator/proxygroup_test.go

@@ -811,6 +811,90 @@ func TestProxyGroupWithStaticEndpoints(t *testing.T) {
 	}
 }
 
+// TestFindStaticEndpointsStableOrder verifies that findStaticEndpoints returns
+// the existing endpoint order from the config Secret when the resulting set of
+// addresses is unchanged. nodes.Items from r.List is not order-stable across
+// calls, so without this guarantee the slice can permute on each reconcile,
+// triggering a spurious config Secret rewrite which fires a watch event that
+// re-enqueues the ProxyGroup, looping forever (issue #19700).
+func TestFindStaticEndpointsStableOrder(t *testing.T) {
+	const (
+		addrA = "10.0.0.1"
+		addrB = "10.0.0.2"
+		port  = uint16(30001)
+	)
+
+	pc := &tsapi.ProxyClass{
+		ObjectMeta: metav1.ObjectMeta{Name: "test-pc"},
+		Spec: tsapi.ProxyClassSpec{
+			StaticEndpoints: &tsapi.StaticEndpointsConfig{
+				NodePort: &tsapi.NodePortConfig{
+					Ports:    []tsapi.PortRange{{Port: port}},
+					Selector: map[string]string{"foo/bar": "baz"},
+				},
+			},
+		},
+	}
+
+	// Existing config Secret already pins the order [B, A]. The fake client
+	// lists nodes in name order ([node-a, node-b]) so without the stable-order
+	// guard findStaticEndpoints would return [A, B], differing from currAddrs
+	// and causing a spurious Secret rewrite.
+	currAddrs := []netip.AddrPort{
+		netip.MustParseAddrPort(addrB + ":30001"),
+		netip.MustParseAddrPort(addrA + ":30001"),
+	}
+	cfg := ipn.ConfigVAlpha{StaticEndpoints: currAddrs}
+	cfgJSON, err := json.Marshal(cfg)
+	if err != nil {
+		t.Fatalf("marshal config: %v", err)
+	}
+	existingSecret := &corev1.Secret{
+		ObjectMeta: metav1.ObjectMeta{Name: "test-0-config", Namespace: tsNamespace},
+		Data:       map[string][]byte{tsoperator.TailscaledConfigFileName(106): cfgJSON},
+	}
+
+	nodes := []*corev1.Node{
+		{
+			ObjectMeta: metav1.ObjectMeta{Name: "node-a", Labels: map[string]string{"foo/bar": "baz"}},
+			Status: corev1.NodeStatus{Addresses: []corev1.NodeAddress{
+				{Type: corev1.NodeExternalIP, Address: addrA},
+			}},
+		},
+		{
+			ObjectMeta: metav1.ObjectMeta{Name: "node-b", Labels: map[string]string{"foo/bar": "baz"}},
+			Status: corev1.NodeStatus{Addresses: []corev1.NodeAddress{
+				{Type: corev1.NodeExternalIP, Address: addrB},
+			}},
+		},
+	}
+
+	fc := fake.NewClientBuilder().
+		WithScheme(tsapi.GlobalScheme).
+		WithObjects(pc, nodes[0], nodes[1], existingSecret).
+		Build()
+
+	zl, _ := zap.NewDevelopment()
+	r := &ProxyGroupReconciler{Client: fc}
+
+	got, err := r.findStaticEndpoints(t.Context(), existingSecret, pc, port, zl.Sugar())
+	if err != nil {
+		t.Fatalf("findStaticEndpoints: %v", err)
+	}
+	if !slices.Equal(got, currAddrs) {
+		t.Errorf("findStaticEndpoints returned %v, want %v (order must match currAddrs to avoid reconcile churn)", got, currAddrs)
+	}
+
+	// Repeat to confirm the result is stable across calls.
+	got2, err := r.findStaticEndpoints(t.Context(), existingSecret, pc, port, zl.Sugar())
+	if err != nil {
+		t.Fatalf("findStaticEndpoints (2nd call): %v", err)
+	}
+	if !slices.Equal(got, got2) {
+		t.Errorf("findStaticEndpoints not stable across calls: first=%v second=%v", got, got2)
+	}
+}
+
 func TestProxyGroup(t *testing.T) {
 	pc := &tsapi.ProxyClass{
 		ObjectMeta: metav1.ObjectMeta{