Fix #6302: musicbrainz: crash when releases lack the "track" key. (#6364)

## Description

Fixes #6302

Todo: Test. I could not figure out how to add a test for this bug.

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Author: henry-oberholtzer

beetsplug/musicbrainz.py

@@ -477,7 +477,7 @@ def album_info(self, release: JSONDict) -> beets.autotag.hooks.AlbumInfo:
             release["artist-credit"], include_join_phrase=False
         )
 
-        ntracks = sum(len(m["tracks"]) for m in release["media"])
+        ntracks = sum(len(m.get("tracks", [])) for m in release["media"])
 
         # The MusicBrainz API omits 'relations'
         # when the release has more than 500 tracks. So we use browse_recordings
@@ -512,7 +512,7 @@ def album_info(self, release: JSONDict) -> beets.autotag.hooks.AlbumInfo:
             if format in config["match"]["ignored_media"].as_str_seq():
                 continue
 
-            all_tracks = medium["tracks"]
+            all_tracks = medium.get("tracks", [])
             if (
                 "data-tracks" in medium
                 and not config["match"]["ignore_data_tracks"]

docs/changelog.rst

@@ -16,6 +16,8 @@ Unreleased
 Bug fixes
 ~~~~~~~~~
 
+- :doc:`plugins/musicbrainz`: Fix crash when release mediums lack the ``tracks``
+  key. :bug:`6302`
 - :doc:`plugins/musicbrainz`: Fix search terms escaping. :bug:`6347`
 - :doc:`plugins/musicbrainz`: Fix support for ``alias`` and ``tracks``
   :conf:`plugins.musicbrainz:extra_tags`.

test/plugins/test_musicbrainz.py

@@ -691,6 +691,23 @@ def test_track_disambiguation(self):
         assert t[0].trackdisambig is None
         assert t[1].trackdisambig == "SECOND TRACK"
 
+    def test_missing_tracks(self):
+        tracks = [
+            self._make_track("TITLE ONE", "ID ONE", 100.0 * 1000.0),
+            self._make_track(
+                "TITLE TWO",
+                "ID TWO",
+                200.0 * 1000.0,
+                disambiguation="SECOND TRACK",
+            ),
+        ]
+        release = self._make_release(tracks=tracks)
+        release["media"].append(release["media"][0])
+        del release["media"][0]["tracks"]
+        del release["media"][0]["data-tracks"]
+        d = self.mb.album_info(release)
+        assert d.mediums == 2
+
 
 class ArtistFlatteningTest(unittest.TestCase):
     def _credit_dict(self, suffix=""):