cmd/tailscale/cli: Add a test for addServiceToPrefs

Signed-off-by: KevinLiang10 <37811973+KevinLiang10@users.noreply.github.com>

cmd/tailscale/cli/serve_v2_test.go

@@ -12,6 +12,7 @@ import (
 	"os"
 	"path/filepath"
 	"reflect"
+	"slices"
 	"strconv"
 	"strings"
 	"testing"
@@ -1180,6 +1181,54 @@ func TestCleanURLPath(t *testing.T) {
 	}
 }
 
+func TestAddServiceToPrefs(t *testing.T) {
+	tests := []struct {
+		name          string
+		dnsName       string
+		startServices []string
+		expected      []string
+	}{
+		{
+			name:     "add service to empty prefs",
+			dnsName:  "svc:foo",
+			expected: []string{"svc:foo"},
+		},
+		{
+			name:          "add service to existing prefs",
+			dnsName:       "svc:bar",
+			startServices: []string{"svc:foo"},
+			expected:      []string{"svc:foo", "svc:bar"},
+		},
+		{
+			name:          "add existing service to prefs",
+			dnsName:       "svc:foo",
+			startServices: []string{"svc:foo"},
+			expected:      []string{"svc:foo"},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			lc := &fakeLocalServeClient{}
+			ctx := t.Context()
+			lc.EditPrefs(ctx, &ipn.MaskedPrefs{
+				AdvertiseServicesSet: true,
+				Prefs: ipn.Prefs{
+					AdvertiseServices: tt.startServices,
+				},
+			})
+			e := &serveEnv{lc: lc, bg: bgBoolFlag{true, false}}
+			err := e.addServiceToPrefs(ctx, tt.dnsName)
+			if err != nil {
+				t.Fatalf("addServiceToPrefs(%q) returned unexpected error: %v", tt.dnsName, err)
+			}
+			if !slices.Equal(lc.prefs.AdvertiseServices, tt.expected) {
+				t.Errorf("addServiceToPrefs(%q) = %v, want %v", tt.dnsName, lc.prefs.AdvertiseServices, tt.expected)
+			}
+		})
+	}
+
+}
+
 func TestMessageForPort(t *testing.T) {
 	svcIPMap := tailcfg.ServiceIPMappings{
 		"svc:foo": []netip.Addr{