Embedart: fails on some jpegs because imghdr doesn't recognize the mimetype

[nathdwek]

This errors pops up

embedart: Embedding album art into Queen - Live at Wembley ’86

embedart: not embedding image of unsupported type: image/x-None

with this cover (found by beets-fetchart).

This is due to this issue. Fortunately, imghdr provides the list imghdr.tests to add extra mimetype tests. I added a test which identifies jpeg files based on their magic bytes, which is the method used by file.

This is my first time contributing to beets, so I think some code style guidance is probably needed (mainly regarding the modification of imghdr.tests, which should only happen once). I also absolutely need some help with adding the right tests. There is some testing code in this PR, which is very naively based on the rest of test_mediafile.py, but the test doesn't fail without the patch, and I think it actually never runs. I should probably also find/build a smaller cover which can only be recognized based on the magic bytes.

[nathdwek]

I added a commit to fix the code style issues.

[sampsyo]

Awesome! Thanks for sorting this out. Looks great so far.

• I think the test belongs in a standalone test function, probably in the simpler test_mediafile_edge.py. The test can just call _image_mime_type() directly rather than using the automated test-battery machinery. Does that make sense?
• I would love to avoid modifying imghdr.tests is possible. Could we just run your new magic-number check ourselves if imghdr gives us None? (Avoiding changes to global state help make things more predictable down the road.)
• And finally, just to check: do you have reason to believe this check is fairly precise, i.e., there aren't a bunch of other formats that happen to use the same magic number? A link to the file policy would totally suffice.

[nathdwek]

• I'll have a look at test_mediafile_edge.py and see if I can get really started with tests without any external indication.
• I also felt kinda uncomfortable modifying imghdr.tests like that but it had the advantage of really being a "pluggable" solution. I'll look into alternatives.
• https://github.com/file/file/blob/master/magic/Magdir/jpeg#L12

[nathdwek]

I added a commit re:testing. The test now passes and fails adequately.

[sampsyo]

Nice! The test looks great.

Would you mind adding a comment to the code with that link? Could help us stay less confused later on. 😃

[nathdwek]

I added two commits with a more elegant implementation which doesn't require to modify imghdr.tests and also comments for the test.

Should I try to build a smaller image which fits the edge case or is the 1000x1000 image acceptable in the test ressources?

[sampsyo]

This is looking great. Thanks for the extra comments.

A smaller image would be wonderful if it's not too difficult -- it seems a little suboptimal to add a 0.5 MB file to the beets distribution for just one test. (And avoiding copyrighted images is also a plus.)

[nathdwek]

Yeah I don't know I never tried but I guess it shouldn't be too difficult :)

[nathdwek]

I added a commit with a smaller test image.

Anything else to be done?

[nathdwek]

Do I need to do anything else before this can be merged?

[sampsyo]

Nope, I was just being slow. Thanks for your patience.

[sampsyo]

All merged up! Thank you again! ✨

[nathdwek]

Thank you very much for your responsiveness 😃