beet update re-reports same metadata changes on subsequent runs because item mtime is not persisted

[arsaboo]

Running beet update once should make a second run mostly quiet for unchanged files.
Instead, the same noisy diffs (especially arrangers, composers, lyricists) are shown again on the next run.

I have a large library, and I see a ton of these scrolling by

arsaboo@arsmusic:~$ beet -v update
user configuration: /home/arsaboo/.config/beets/config.yaml
data directory: /home/arsaboo/.config/beets
plugin paths: []
Loading plugins: badfiles, convert, deezer, discogs, duplicates, edit, embedart, fetchart, gaana, importreplace, info, inline, lastgenre, lastimport, limit, listenbrainz, lyrics, mbsubmit, mbsync, missing, musicbrainz, plexsync, rewrite, scrub, spotify, subsonic, types, web, xtractor, youtube
fetchart: google: Disabling art source due to missing key
lastgenre: Loading whitelist ~/.config/beets/genres/genres.txt
lastgenre: Loading canonicalization tree ~/.config/beets/genres/genres-trees.yaml
rewrite: adding template field artist Rahul Dev Burman
inline: adding item field initial_char
inline: adding item field folder
inline: adding item field clean_comments
inline: adding album field format
inline: adding album field alb_data_source
inline: adding album field alb_spotify_album_id
inline: adding album field alb_mtime
inline: adding album field plex_matched
Initializing cache at: /home/arsaboo/.config/beets/plexsync_cache.db
Cache database initialized successfully
Spotify cache tables verified
Sending event: pluginload
library database: /home/arsaboo/.config/beets/musiclibrary.blb
library directory: /data/music
Sending event: library_opened
Parsed query: AndQuery([TrueQuery()])
Parsed sort: NullSort()
Manmohini (1994) - Manmohini Manmohini
  arrangers:
    +
  lyricists:
    +
Sending event: database_change
Mtv Hustle 03 Represent Episode 3 (2023) - Amravati Potta
  arrangers:
    +
Sending event: database_change
Street Ghazals (2018) - Desi Hip-Hop
  arrangers:
    +
Sending event: database_change
Aah Chak 2016 (2015) - 32 Bor
  arrangers:
    +
  composers:
    +
  lyricists:
    +
Sending event: database_change
Koshur Drill (2025) - Koshur Drill
  arrangers:
    +
Sending event: database_change
Achanak Bhayanak (2021) - Achanak Bhayanak
  arrangers:
    +
Sending event: database_change
Aakhir Tak (2021) - Aakhir Tak
  arrangers:
    +
Sending event: database_change
Tu Yaa Main (2026) - Chandni
  arrangers:
    +
  composers:
    +
  lyricists:
    +
Sending event: database_change
Tu Yaa Main (2026) - Fame Us
  arrangers:
    +
Sending event: database_change
MTV Hustle 4, Episode. 8 (2024) - Du Buckle
  arrangers:
    +
Sending event: database_change
Bhram (2008) - Jaane Kaisa Hai Tu
  arrangers:
    +
Sending event: database_change
Bhram (2008) - I Am a Bad Boy
  arrangers:
    +
Sending event: database_change
Bhram (2008) - Meri Akhiyaan
  arrangers:
    +
Sending event: database_change
Bhram (2008) - Teri Aankhon Ki Narmi
  arrangers:
    +
Sending event: database_change
Gaane Bhi Do Yaaro (2005) - Nain Katari
  arrangers:
    +
Sending event: database_change
Kiss... Kis Ko (2004) - Teri Neend
  arrangers:
    +
Sending event: database_change
Ye Bhi Woh Bhi (2002) - Gori
  arrangers:
    +
Sending event: database_change
A Band of Boys Reignite (2024) - Manwa Re
  arrangers:
    +
Sending event: database_change
A Band of Boys Reignite (2024) - Le Chaloon
  arrangers:
    +
Sending event: database_change
Bheegi Bheegi (2026) - Bheegi Bheegi
  arrangers:
    +
  composers:
    +
  lyricists:
    +
Sending event: database_change
Ladies, Thank You (2024) - Piya
  arrangers:
    +
Sending event: database_change
Sunny Sanskari Ki Tulsi Kumari (2025) - Ishq Manzoor
  arrangers:
    +
  composers:
    +
  lyricists:
    +
Sending event: database_change
Bombay (1994) - Bombay Theme
  arrangers:
    +
  lyricists:
    +
Sending event: database_change
...

Setup

• OS: Ubuntu
• Python version: 3.11
• beets version: on the latest github build
• Turning off plugins made problem go away (yes/no):

Notes from investigation

During update, item metadata is read from disk and in-memory mtime is refreshed. But the subsequent store operation may only persist selected media fields, and mtime is not guaranteed to be included.
If mtime is not written back to DB, next run treats files as needing reprocessing again.
This matches the repeated noisy diffs seen across runs.

[snejus]

I don't think this has to do with mtime. Can you run

echo first run
beet update | head -50
echo second run
beet update | head -50

And copy the output?

[snejus]

Note that after arrangers, composers and other multi-valued tags migration you are expected to run beet write in order to write these updated tags to files.

I remember seeing changes like

  arrangers:
    +
  composers:
    +
  lyricists:
    +

when I did this. Could you try writing and then try running update after?

[arsaboo]

I don't think this has to do with mtime. Can you run

echo first run
beet update | head -50
echo second run
beet update | head -50
And copy the output?

arsaboo@arsmusic:~$ echo first run
beet update | head -50
echo second run
beet update | head -50
first run
Meri Biwi Ka Jawab Nahin (2004) - Hum Sang Kitna Pyaar Hai
  arrangers:
    +
  composers:
    +
  lyricists:
    +
Mohra (1994) - Tip Tip Barsa Paani
  arrangers:
    +
Mujhe Meri Biwi Se Bachaao (2000) - Thehro
  arrangers:
    +
  composers:
    +
  lyricists:
    +
Muskurahat (1943) - Phool Ki Patti Sa Tan Iska
  arrangers:
    +
  lyricists:
    +
One Two Ka Four (2001) - Sona Nahi Na Sahi
  arrangers:
    +
Pehchaan (1993) - Nazuk Nazuk Hoon
  arrangers:
    +
  lyricists:
    +
Prem Deewane (1992) - Aise Lagi Aag Barabar
  arrangers:
    +
  composers:
    +
  lyricists:
    +
Pyaar Ishq Aur Mohabbat (2001) - Pyar Ishq Aur Mohabbat
  arrangers:
    +
  composers:
    +
  lyricists:
    +
Pyaar Ishq Aur Mohabbat (2001) - Pyar Ishq Aur Mohabbat - Remix
  arrangers:
    +
Pyaar Koi Khel Nahin (1999) - Kuchh Hamare Hai
  arrangers:
    +
second run
Meri Biwi Ka Jawab Nahin (2004) - Hum Sang Kitna Pyaar Hai
  arrangers:
    +
  composers:
    +
  lyricists:
    +
Mohra (1994) - Tip Tip Barsa Paani
  arrangers:
    +
Mujhe Meri Biwi Se Bachaao (2000) - Thehro
  arrangers:
    +
  composers:
    +
  lyricists:
    +
Muskurahat (1943) - Phool Ki Patti Sa Tan Iska
  arrangers:
    +
  lyricists:
    +
One Two Ka Four (2001) - Sona Nahi Na Sahi
  arrangers:
    +
Pehchaan (1993) - Nazuk Nazuk Hoon
  arrangers:
    +
  lyricists:
    +
Prem Deewane (1992) - Aise Lagi Aag Barabar
  arrangers:
    +
  composers:
    +
  lyricists:
    +
Pyaar Ishq Aur Mohabbat (2001) - Pyar Ishq Aur Mohabbat
  arrangers:
    +
  composers:
    +
  lyricists:
    +
Pyaar Ishq Aur Mohabbat (2001) - Pyar Ishq Aur Mohabbat - Remix
  arrangers:
    +
Pyaar Koi Khel Nahin (1999) - Kuchh Hamare Hai
  arrangers:
    +

The above is without the fix. I don't recall ever using beet write command.

[arsaboo]

@snejus I did some more investigation, and there are actually two distinct bugs causing the repeated output.

1. Bug 1 — DelimitedString roundtrip ([''] ↔ [])

The arrangers: + with nothing after the + means an empty string is being added. Files with a blank arranger tag cause MediaFile to return [''], but DelimitedString.to_sql and parse don't round-trip that correctly:

MULTI_VALUE_DSV.to_sql([''])  # → ''
MULTI_VALUE_DSV.parse('')     # → []
# [''] != [] → diff appears every time the file is re-read

You're right that running beet write might clear these tags from the files (if MediaFile removes the tag instead of writing it back empty), which would eliminate the source of the diff. But the roundtrip inconsistency itself is still a bug.

2. Bug 2 — mtime not persisted after item.store(fields=item_fields)

mtime is not in Item._media_fields, so item.store(fields=item_fields) never writes the updated mtime to the database. Worse, store() unconditionally calls clear_dirty() at the end, silently discarding the dirty flag without persisting the value. The DB retains the old (stale) mtime, so every subsequent beet update run sees current_mtime() > db_mtime and re-reads the file — triggering Bug 1 again.

Overall, the arrangers: + diff fires on every single run because Bug 2 forces a file re-read, which exposes Bug 1 on each pass.

The PR fixes Bug 2. Bug 1 is a separate DelimitedString issue worth addressing independently.

[snejus]

Could you try writing and then try running update after?

Could you do this? As I said you're expected to run beet write to update your music files after the multi-valued fields update.

[arsaboo]

How long is beet write expected to take? I have been stuck on the following for over 7 hours.

Mumbaikar (2023) - Maahi Mere - Duet
  arrangers:
    -
Nimmo Lucknow Wali (2023) - Rukna Tu Chalte Jana
  arrangers:
    -
Nimmo Lucknow Wali (2023) - Guddu Pappu Chintu
  arrangers:
    -
Mismatched: Season 3 (Soundtrack from the Netflix Series) (2024) - Rang Laago
  arrangers:
    -
Mismatched: Season 2 (Music from the Netflix Series) (2022) - Musafir x Wanderer - Mismatched Version
  arrangers:
    -
The Biggest Desi Bhangra Hits, Vol. 2 (2009) - Chan Warga Chamkara
  arrangers:
    -
  composers:
    -
  lyricists:
    -
The Biggest Desi Bhangra Hits, Vol. 2 (2009) - Kala Tikka
  arrangers:
    -
  composers:
    -
  lyricists:
    -
RADE RAPATE (2026) - RADE RAPATE
  arrangers:
    -

[arsaboo]

I terminated the beet write run with CTRL+C and ran beet -vv update and now I see

skipping /data/music/Hindi Music/T/The Biggest UK Bhangra Hits, Vol. 3 (2010)/The Biggest UK Bhangra Hits, Vol. 3 (2010) - Nachdi De.flac because mtime is up to date (1777857735.0)
skipping /data/music/Hindi Music/T/The Biggest UK Bhangra Hits, Vol. 3 (2010)/The Biggest UK Bhangra Hits, Vol. 3 (2010) - Do Naina Di Daru.flac because mtime is up to date (1777857735.0)
skipping /data/music/Hindi Music/T/The Biggest UK Bhangra Hits, Vol. 4 (2012)/The Biggest UK Bhangra Hits, Vol. 4 (2012) - Ajj Nachna.flac because mtime is up to date (1777857735.0)

I guess, I will close this issue for now.