Add cache_version flag to saved state

Can be used to determine whether the cache file format has been broken after an update.
If so, the cache will be discarded and recreated.

src/config.rs

@@ -13,6 +13,7 @@ use crate::queue;
 use crate::serialization::{Serializer, CBOR, TOML};
 
 pub const CLIENT_ID: &str = "d420a117a32841c2b3474932e49fb54b";
+pub const CACHE_VERSION: u16 = 1;
 
 #[derive(Clone, Serialize, Deserialize, Debug, Default)]
 pub struct ConfigValues {
@@ -81,6 +82,7 @@ pub struct UserState {
     pub repeat: queue::RepeatSetting,
     pub queuestate: QueueState,
     pub playlist_orders: HashMap<String, SortingOrder>,
+    pub cache_version: u16,
 }
 
 impl Default for UserState {
@@ -91,6 +93,7 @@ impl Default for UserState {
             repeat: queue::RepeatSetting::None,
             queuestate: QueueState::default(),
             playlist_orders: HashMap::new(),
+            cache_version: 0,
         }
     }
 }
@@ -150,6 +153,9 @@ impl Config {
     }
 
     pub fn save_state(&self) {
+        // update cache version number
+        self.with_state_mut(|mut state| state.cache_version = CACHE_VERSION);
+
         let path = config_path("userstate.cbor");
         debug!("saving user state to {}", path.display());
         if let Err(e) = CBOR.write(path, self.state().clone()) {