lastgenre: Fix multi-artist-albums (squashed)

Often last.fm does not have artist genres for delimiter-separated artist
names (eg. "Artist One, Artist Two"). This splits out by (the
configured) delimiter and gathers each artist's last.fm genre.

FIXME: It would be cool if a configured list of artist name separators
would be used instead of a hard-coded one.

Author: JOJ0

beetsplug/lastgenre/__init__.py

@@ -24,6 +24,7 @@
 
 import codecs
 import os
+import re
 import traceback
 from typing import Union
 
@@ -290,6 +291,15 @@ def fetch_album_artist_genre(self, obj):
         """Return raw album artist genres from Last.fm for this Item or Album."""
         return self._last_lookup("artist", LASTFM.get_artist, obj.albumartist)
 
+    def fetch_split_album_artist_genre(self, split_artist):
+        """Return the artist genre for any passed artist name.
+
+        Used for multi-artist albums where the artist name may not match
+        the album artist exactly and a split by separator is needed to get a last.fm
+        result.
+        """
+        return  self._last_lookup("artist", LASTFM.get_artist, split_artist)
+
     def fetch_artist_genre(self, item):
         """Returns raw track artist genres from Last.fm for this Item."""
         return self._last_lookup("artist", LASTFM.get_artist, item.artist)
@@ -408,6 +418,47 @@ def _try_resolve_stage(stage_label: str, keep_genres, new_genres):
             elif obj.albumartist != config["va_name"].as_str():
                 new_genres = self.fetch_album_artist_genre(obj)
                 stage_label = "album artist"
+                if not new_genres:
+                    if self.config["extended_debug"]:
+                        self._log.debug(
+                            'No album artist genre found for "{0.albumartist}"',
+                            obj,
+                        )
+                    separators = [
+                        re.escape(self.config["separator"].get()),
+                        " feat\\. ",
+                        " featuring ",
+                        " & ",
+                        " vs\\. ",
+                        " x ",
+                        " / ",
+                        " + ",
+                        " and ",
+                        " \\| ",
+                    ]
+                    if any(
+                        re.sub(r"\\", "", sep) in obj.albumartist
+                        for sep in separators
+                    ):
+                        if self.config["extended_debug"]:
+                            self._log.debug(
+                                "Found separators in album artist - splitting..."
+                            )
+                        # Split on all separators using regex
+                        pattern = "|".join(separators)
+                        albumartists = re.split(pattern, obj.albumartist)
+                        for albumartist in albumartists:
+                            albumartist = albumartist.strip()
+                            if self.config["extended_debug"]:
+                                self._log.debug(
+                                    'Fetching multi-artist album genre for "{0}"',
+                                    albumartist,
+                                )
+                            new_genres += self.fetch_split_album_artist_genre(
+                                albumartist
+                            )
+                            if new_genres:
+                                label = "album artist (split)"
             else:
                 # For "Various Artists", pick the most popular track genre.
                 item_genres = []