fix: Removing items during validation breaks validation

Fix #266.

media/cache.py

@@ -14,6 +14,10 @@
 import util
 
 
+class ItemNotCachedError(Exception):
+    pass
+
+
 class MusicCache(dict):
     def __init__(self, db: MusicDatabase):
         super().__init__()
@@ -142,7 +146,7 @@ def item(self):
         if self.id in self.lib:
             return self.lib[self.id]
         else:
-            raise ValueError(f"Uncached item of id {self.id}, type {self.type}.")
+            raise ItemNotCachedError(f"Uncached item of id {self.id}, type {self.type}.")
 
     def to_dict(self):
         dict = self.item().to_dict()

media/playlist.py

@@ -5,7 +5,8 @@
 import time
 
 import variables as var
-from media.cache import CachedItemWrapper, get_cached_wrapper_from_dict, get_cached_wrapper_by_id
+from media.cache import (CachedItemWrapper, ItemNotCachedError,
+                         get_cached_wrapper_from_dict, get_cached_wrapper_by_id)
 from database import Condition
 from media.item import ValidationFailedError, PreparationFailedError
 
@@ -224,6 +225,15 @@ def _check_valid(self):
         self.validating_thread_lock.acquire()
         while len(self.pending_items) > 0:
             item = self.pending_items.pop()
+            try:
+                item.item()
+            except ItemNotCachedError:
+                # In some very subtle case, items are removed and freed from
+                # the playlist and the cache, before validation even starts,
+                # causes, freed items remain in pending_items.
+                # Simply ignore these items here.
+                continue
+
             self.log.debug("playlist: validating %s" % item.format_debug_string())
             ver = item.version