Create lock before launching the coroutine,

return always a copy of the list and not the real one.

ultrasonic/src/main/kotlin/org/moire/ultrasonic/playback/LegacyPlaylistManager.kt

@@ -77,7 +77,7 @@ class LegacyPlaylistManager : KoinComponent {
 
     // Public facing playlist (immutable)
     val playlist: List<DownloadFile>
-        get() = _playlist
+        get() = _playlist.toList()
 
     @get:Synchronized
     val playlistDuration: Long

ultrasonic/src/main/kotlin/org/moire/ultrasonic/service/PlaybackStateSerializer.kt

@@ -43,8 +43,8 @@ class PlaybackStateSerializer : KoinComponent {
     ) {
         if (!setup.get()) return
 
-        ioScope.launch {
         if (lock.tryLock()) {
+            ioScope.launch {
                 try {
                     serializeNow(songs, currentPlayingIndex, currentPlayingPosition)
                 } finally {