ipn/ipnlocal: set default NoStatefulFiltering in ipn.NewPrefs (#12031)

This way the default gets populated on first start, when no existing
state exists to migrate. Also fix `ipn.PrefsFromBytes` to preserve empty
fields, rather than layering `NewPrefs` values on top.

Updates https://github.com/tailscale/corp/issues/19623

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

ipn/ipnlocal/profiles.go

@@ -353,9 +353,13 @@ func (pm *profileManager) loadSavedPrefs(key ipn.StateKey) (ipn.PrefsView, error
 	if err != nil {
 		return ipn.PrefsView{}, err
 	}
-	savedPrefs, err := ipn.PrefsFromBytes(bs)
-	if err != nil {
-		return ipn.PrefsView{}, fmt.Errorf("PrefsFromBytes: %v", err)
+	savedPrefs := ipn.NewPrefs()
+	// NewPrefs sets a default NoStatefulFiltering, but we want to actually see
+	// if the saved state had an empty value. The empty value gets migrated
+	// based on NoSNAT, while a default "false" does not.
+	savedPrefs.NoStatefulFiltering = ""
+	if err := ipn.PrefsFromBytes(bs, savedPrefs); err != nil {
+		return ipn.PrefsView{}, fmt.Errorf("parsing saved prefs: %v", err)
 	}
 	pm.logf("using backend prefs for %q: %v", key, savedPrefs.Pretty())
 
@@ -495,7 +499,6 @@ var defaultPrefs = func() ipn.PrefsView {
 	prefs := ipn.NewPrefs()
 	prefs.LoggedOut = true
 	prefs.WantRunning = false
-	prefs.NoStatefulFiltering = "false"
 
 	return prefs.View()
 }()

ipn/ipnlocal/profiles_test.go

@@ -681,8 +681,21 @@ func TestProfileBackfillStatefulFiltering(t *testing.T) {
 			// StatefulFiltering boolean.
 			pf := pm.CurrentPrefs()
 			if !pf.NoStatefulFiltering().EqualBool(tt.want) {
-				t.Fatalf("got NoStatefulFiltering=%v, want %v", pf.NoStatefulFiltering(), tt.want)
+				t.Fatalf("got NoStatefulFiltering=%q, want %v", pf.NoStatefulFiltering(), tt.want)
 			}
 		})
 	}
 }
+
+// TestDefaultPrefs tests that defaultPrefs is just NewPrefs with
+// LoggedOut=true (the Prefs we use before connecting to control). We shouldn't
+// be putting any defaulting there, and instead put all defaults in NewPrefs.
+func TestDefaultPrefs(t *testing.T) {
+	p1 := ipn.NewPrefs()
+	p1.LoggedOut = true
+	p1.WantRunning = false
+	p2 := defaultPrefs
+	if !p1.View().Equals(p2) {
+		t.Errorf("defaultPrefs is %s, want %s; defaultPrefs should only modify WantRunning and LoggedOut, all other defaults should be in ipn.NewPrefs.", p2.Pretty(), p1.Pretty())
+	}
+}

ipn/ipnlocal/profiles_windows.go

@@ -57,7 +57,7 @@ func (pm *profileManager) loadLegacyPrefs() (string, ipn.PrefsView, error) {
 	}
 
 	prefsPath := filepath.Join(userLegacyPrefsDir, legacyPrefsFile+legacyPrefsExt)
-	prefs, err := ipn.LoadPrefs(prefsPath)
+	prefs, err := ipn.LoadPrefsWindows(prefsPath)
 	pm.dlogf("ipn.LoadPrefs(%q) = %v, %v", prefsPath, prefs, err)
 	if errors.Is(err, fs.ErrNotExist) {
 		return "", ipn.PrefsView{}, errAlreadyMigrated