Do not delete the genres tag when musicbrainz.genres is False.

[djl]

Description

Fixes #6403

This commit also introduces a way to explicitly exclude fields from being updated when mbsync runs.

To Do

• Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• Tests. (Very much encouraged but not strictly required.)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.42%. Comparing base (fd586ef) to head (d451c62).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6513      +/-   ##
==========================================
+ Coverage   70.40%   70.42%   +0.02%     
==========================================
  Files         148      148              
  Lines       18806    18819      +13     
  Branches     3067     3071       +4     
==========================================
+ Hits        13240    13253      +13     
  Misses       4916     4916              
  Partials      650      650              

Files with missing lines	Coverage Δ
beetsplug/mbsync.py	84.94% <100.00%> (+2.44%)	⬆️

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[snejus]

Given that the bug seems to stem from the musicbrainz plugin, I expected the fix to be there. Using excluded fields in this seems to be a workaround, rather than a solution. For example, if I do not use mbsync but rather reimport music with beet import -LI, presumably we will still have the same issue?

[djl]

Ah I didn't think about re-importing. That will still show the same issue. I'll move the fix over to the musicbrainz plugin.

[snejus]

I had a look at the source code: it doesn't seem that genres field even gets set when musicbrainz.genres is False. In this case pre-existing genres should not be overwritten.

Are you able to reproduce this issue when reimporting?

[djl]

So I think this happens because the genres field on the data returned by musicbrainz plugin is None, which results in the existing genres being also set to None. This would mean it's not an issue with the musicbrainz plugin directly but would happen with any field that is different from existing data.

Here I updated the musicbrainz plugin to set info.country = None:

$ bq mbsync -p Këkht Aräkh pale
Këkht Aräkh - Pale Swordsman - Intro
  country: XW ->
  genres:
    - black metal
Këkht Aräkh - Pale Swordsman - Thorns
  country: XW ->
  genres:
    - black metal
Këkht Aräkh - Pale Swordsman - Night Descends
  country: XW ->
  genres:
    - black metal
Këkht Aräkh - Pale Swordsman - In the Garden
  country: XW ->
  genres:
    - black metal
Këkht Aräkh - Pale Swordsman - Amor
  country: XW ->
  genres:
    - black metal
Këkht Aräkh - Pale Swordsman - Nocturne
  country: XW ->
  genres:
    - black metal
Këkht Aräkh - Pale Swordsman - Amid the Stars
  country: XW ->
  genres:
    - black metal
Këkht Aräkh - Pale Swordsman - Lily
  country: XW ->
  genres:
    - black metal
Këkht Aräkh - Pale Swordsman - Crystal
  country: XW ->
  genres:
    - black metal
Këkht Aräkh - Pale Swordsman - Swordsman
  country: XW ->
  genres:
    - black metal

If this is an issue beyond just the musicbrainz plugin, my immediate thought for a solution would be to have a way for a metadata plugin to say "I have nothing for this field, use any existing data". Something other than None, which could mean that, but also mean "this should be set to null".

And yes, this does happen on re-importing.