Delete the SYLT frame by its HashKey

[stefanvanburen]

synced_lyrics can be written but not deleted. Mutagen stores the SYLT frame under the HashKey SYLT::XXX, while MP3SYLTStorageStyle inherits a delete() that looks for a bare SYLT key, so it never matches and the deletion is a silent no-op.

Repro

from mediafile import MediaFile
import mutagen.id3

mf = MediaFile("x.mp3")
mf.synced_lyrics = [("hello", 1000)]
mf.save()

mf = MediaFile("x.mp3")
del mf.synced_lyrics                      # or mf.update({"synced_lyrics": None})
mf.save()

mutagen.id3.ID3("x.mp3").getall("SYLT")   # still there

MediaFile.update() is affected too, since it calls delattr for any key whose value is None — so its documented "if a key has the value None, the corresponding property is deleted" doesn't hold for this field.

Why it was missed

The class already gets this right in store(), which clears with delall("SYLT"); only the delete path was missing. Assignment-clearing (mf.synced_lyrics = None) routes through store(), which is why the existing test_clear passes and this went unnoticed.

Sibling styles whose frames carry a non-bare HashKey all override delete for the same reason — MP3DescStorageStyle (USLT) deletes via frame.HashKey, and MP3ImageStorageStyle (APIC) uses delall.

Fix

Three lines, mirroring MP3ImageStorageStyle:

def delete(self, mutagen_file):
    mutagen_file.tags.delall("SYLT")

Tests

Four added to TestSyncedLyrics, all of which fail without the fix and pass with it:

• test_delete[mp3], test_delete[aiff] — del removes the frame
• test_update_with_none_clears — update({"synced_lyrics": None}) removes it
• test_delete_leaves_plain_lyrics — deleting SYLT doesn't disturb USLT

Full suite is 491 passed, 1 failed: OpusTest::test_read_audio_properties, which fails identically on unmodified master (#109). ruff check and ruff format --check are clean.

How I hit it

Turning on beets' lyrics.synced and then turning it back off. USLT reverts, but every MP3 keeps a stale SYLT frame that beets has no way to remove.