Fix metadata fields list caching for subclasses

Because the _allFieldsCache attribute gets inherited, when it gets set for a class, all subclasses will also see that list rather than their own, potentially different list. To fix this, use a global dict indexing on the metadata class instead.

codearchiver/core.py

@@ -104,7 +104,9 @@ class Metadata(list[tuple[str, str]]):
 	version: int = 0
 	'''Version, incremented on every backward-incompatible change'''
 
-	_allFieldsCache: typing.Optional[tuple[MetadataField]] = None
+	# This cache needs to be different for each subclass.
+	# The easiest way to achieve that is by mapping class objects to the corresponding cache.
+	_allFieldsCache: dict[typing.Type['Metadata'], tuple[MetadataField]] = {}
 
 	def append(self, *args):
 		if len(args) == 1:
@@ -116,12 +118,13 @@ class Metadata(list[tuple[str, str]]):
 	def _allFields(self):
 		'''All fields known by this metadata collection, own ones and all from superclasses'''
 
-		if type(self)._allFieldsCache is None:
+		cls = type(self)
+		if cls not in cls._allFieldsCache:
 			fields = []
-			for cls in reversed(type(self).mro()):
-				fields.extend(getattr(cls, 'fields', []))
-			type(self)._allFieldsCache = tuple(fields)
-		return type(self)._allFieldsCache
+			for cls_ in reversed(cls.mro()):
+				fields.extend(getattr(cls_, 'fields', []))
+			cls._allFieldsCache[cls] = tuple(fields)
+		return cls._allFieldsCache[cls]
 
 	def validate(self):
 		'''Check that all keys and values conform to the specification'''