Fix accidental concurrent preference writes (#2450)

By syncing the File write we can be sure a second write puts the updated file after.

Fixes #2449

app/preferences.go

@@ -56,11 +56,17 @@ func (p *preferences) saveToFile(path string) error {
 			return err
 		}
 	}
+	defer file.Close()
 	encode := json.NewEncoder(file)
 
 	p.InMemoryPreferences.ReadValues(func(values map[string]interface{}) {
 		err = encode.Encode(&values)
 	})
+
+	err2 := file.Sync()
+	if err == nil {
+		err = err2
+	}
 	return err
 }
 

app/preferences_test.go

@@ -40,6 +40,26 @@ func TestPreferences_Save(t *testing.T) {
 	assert.JSONEq(t, string(expected), string(content))
 }
 
+func TestPreferences_Save_OverwriteFast(t *testing.T) {
+	p := loadPreferences("dummy2")
+	p.WriteValues(func(val map[string]interface{}) {
+		val["key"] = "value"
+	})
+
+	path := filepath.Join(os.TempDir(), "fynePrefs2.json")
+	defer os.Remove(path)
+	p.saveToFile(path)
+
+	p.WriteValues(func(val map[string]interface{}) {
+		val["key2"] = "value2"
+	})
+	p.saveToFile(path)
+
+	p2 := loadPreferences("dummy")
+	p2.loadFromFile(path)
+	assert.Equal(t, "value2", p2.String("key2"))
+}
+
 func TestPreferences_Load(t *testing.T) {
 	p := loadPreferences("dummy")
 	p.loadFromFile(filepath.Join("testdata", "preferences.json"))