web: resolve relative album artpath before serving art

[seitzbg]

Description

GET /album/<id>/art returns a 500 when an album's artpath is stored relative to the library directory.

album_art() passes album.artpath straight to flask.send_file(). beets stores artpath relative to the music directory and only resolves it lazily, through the music_dir ContextVar, in the thread that set it. The web server runs with app.run(threaded=True), so worker threads never set that ContextVar; there the path stays relative and send_file() resolves it against the app root (beetsplug/web/), raising FileNotFoundError, so Flask returns a 500. Absolute artpaths are unaffected.

Fix

Resolve a relative artpath against g.lib.directory before serving it, through util.syspath():

if not os.path.isabs(artpath):
    artpath = os.path.join(g.lib.directory, artpath)
return flask.send_file(util.syspath(artpath))

Testing

Added a regression test that reproduces the failure from a separate thread (which, like the threaded=True workers, doesn't inherit the ContextVar) against an on-disk library, and asserts a 200 with the file's bytes. It fails without the fix and passes with it.

To Do

• Changelog.
• Tests.

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.25%. Comparing base (103390f) to head (c9105ee).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
beetsplug/web/__init__.py	60.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7015      +/-   ##
==========================================
- Coverage   77.39%   77.25%   -0.14%     
==========================================
  Files         163      163              
  Lines       21841    21844       +3     
  Branches     3370     3371       +1     
==========================================
- Hits        16904    16876      -28     
- Misses       4112     4146      +34     
+ Partials      825      822       -3     

Files with missing lines	Coverage Δ
beetsplug/web/__init__.py	73.09% <60.00%> (+0.66%)	⬆️

... and 1 file with indirect coverage changes

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[Serene-Arc]

Hi! Thanks for the PR. Overall, it looks good except for one thing. We're currently in the process of moving everything that we can to the more modern pathlib library in Python. If you could change the path stuff to use pathlib, that would be great.

[Serene-Arc]

We are moving to pathlib instead of os functions where possible. These can be done with pathlib functions.

[seitzbg]

Switched to pathlib here in c9105ee. Using the album.art_filepath property now and joining the relative case against the library dir with /. Kept os.fsdecode since that's the bytes to str step art_filepath itself does; pathlib won't take bytes and a plain .decode() would choke on non-utf8 filenames.

[Serene-Arc]

More places where pathlib can be used instead.

[seitzbg]

Same commit. os.makedirs/os.path.dirname are now abspath.parent.mkdir(parents=True, exist_ok=True), and the joins use /.

[seitzbg]

Pushed the pathlib conversion in c9105ee, both the route and the test. The only os call left is os.fsdecode, which is the bytes to str bridge beets already uses in Album.art_filepath (pathlib doesn't accept bytes). Should be good for another look.