cmd/tailscale/cli: treat nil and non-nil zero length slices as equiv prefs

Updates #1725

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/cli_test.go

@@ -8,6 +8,7 @@ import (
 	"flag"
 	"testing"
 
+	"inet.af/netaddr"
 	"tailscale.com/ipn"
 )
 
@@ -102,6 +103,19 @@ func TestCheckForAccidentalSettingReverts(t *testing.T) {
 			},
 			want: "",
 		},
+		{
+			name:    "empty_slice_equals_nil_slice",
+			flagSet: f("hostname"),
+			curPrefs: &ipn.Prefs{
+				AdvertiseRoutes: []netaddr.IPPrefix{},
+			},
+			mp: &ipn.MaskedPrefs{
+				Prefs: ipn.Prefs{
+					AdvertiseRoutes: nil,
+				},
+			},
+			want: "",
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {

cmd/tailscale/cli/up.go

@@ -501,8 +501,16 @@ func checkForAccidentalSettingReverts(flagSet map[string]bool, curPrefs *ipn.Pre
 			continue
 		}
 		// Get explicit value and implicit value
-		evi, ivi := ev.Field(i).Interface(), iv.Field(i).Interface()
-		if reflect.DeepEqual(evi, ivi) {
+		ex, im := ev.Field(i), iv.Field(i)
+		switch ex.Kind() {
+		case reflect.String, reflect.Slice:
+			if ex.Kind() == reflect.Slice && ex.Len() == 0 && im.Len() == 0 {
+				// Treat nil and non-nil empty slices as equivalent.
+				continue
+			}
+		}
+		exi, imi := ex.Interface(), im.Interface()
+		if reflect.DeepEqual(exi, imi) {
 			continue
 		}
 		switch flagName {
@@ -515,7 +523,7 @@ func checkForAccidentalSettingReverts(flagSet map[string]bool, curPrefs *ipn.Pre
 			}
 		default:
 			errs = append(errs, fmt.Errorf("'tailscale up' without --reset requires all preferences with changing values to be explicitly mentioned; --%s is not specified but its default value of %v differs from current value %v",
-				flagName, fmtSettingVal(ivi), fmtSettingVal(evi)))
+				flagName, fmtSettingVal(imi), fmtSettingVal(exi)))
 		}
 	}
 	return multierror.New(errs)