fix(lastgenre): Canonicalize keep_existing fallback (#6303)

beetsplug/lastgenre/__init__.py

@@ -482,6 +482,13 @@ class LastGenrePlugin(plugins.BeetsPlugin):
         if obj.genre and self.config["keep_existing"]:
             if not self.whitelist or self._is_valid(obj.genre.lower()):
                 return obj.genre, "original fallback"
+            else:
+                # If the original genre doesn't match a whitelisted genre, check
+                # if we can canonicalize it to find a matching, whitelisted genre!
+                if result := _try_resolve_stage(
+                    "original fallback", keep_genres, []
+                ):
+                    return result
 
         # Return fallback string.
         if fallback := self.config["fallback"].get():

docs/changelog.rst

@@ -57,6 +57,9 @@ New features:
 
 Bug fixes:
 
+- :doc:`/plugins/lastgenre`: Canonicalize genres when ``force`` and
+  ``keep_existing`` are ``on``, yet no genre info on lastfm could be found.
+  :bug:`6303`
 - Handle potential OSError when unlinking temporary files in ArtResizer.
   :bug:`5615`
 - :doc:`/plugins/spotify`: Updated Spotify API credentials. :bug:`6270`

test/plugins/test_lastgenre.py

@@ -541,6 +541,31 @@ class LastGenrePluginTest(PluginTestCase):
                 "keep + album, whitelist",
             ),
         ),
+        # 16 - canonicalization transforms non-whitelisted original genres to canonical
+        # forms and deduplication works, **even** when no new genres are found online.
+        #
+        # "Cosmic Disco" is not in the default whitelist, thus gets resolved "up" in the
+        # tree to "Disco" and "Electronic".
+        (
+            {
+                "force": True,
+                "keep_existing": True,
+                "source": "album",
+                "whitelist": True,
+                "canonical": True,
+                "prefer_specific": False,
+                "count": 10,
+            },
+            "Cosmic Disco",
+            {
+                "album": [],
+                "artist": [],
+            },
+            (
+                "Disco, Electronic",
+                "keep + original fallback, whitelist",
+            ),
+        ),
     ],
 )
 def test_get_genre(config_values, item_genre, mock_genres, expected_result):