CoverArt: SHA-256 digest and (background) color

[uklotzde]

Includes a database schema migration

The next evolution of cover art display:

• New SHA-256 digest with 64-bit in-memory cache keys to reliably prevent wrong or swapping cover art display: https://bugs.launchpad.net/mixxx/+bug/1607097
• Optional background color that is calculated from scaled-down images (1x1 pixel) and displayed immediately as a preview while the actual cover art is still loading
• Completely backwards compatible and therefore maybe a valid candidate for 2.3:
  • you can seamlessly switch between Mixxx versions without any data inconsistencies
  • the old hash is still calculated and stored in the database for backwards compatibility with previous Mixxx versions
  • the new table columns digest and color are optional, initialized with default values (NULL), and populated lazily on demand when first needed
• Fixes https://bugs.launchpad.net/mixxx/+bug/1879160 by logging image load failures only once fixed in lp1879160: Log warning about cover load failure only once #2815 and merged into 2.3

The only drawback is that displaying tracks in the track table view for the first time will take more time until all tracks have been updated with the additional data. This is mainly triggered by CoverArtDelegate, because usually tracks a first displayed in the track table view before they are actually loaded by TrackDAO.

This PR finally enables cover art display for the track table view in #2282.

The following image shows a side-by-side comparison of two subsequent screenshots. On the left side the covers have only partially been loaded while on the right side loading of all covers has finished:

[uklotzde]

I consider this ready for 2.3.0. Remove the label if you disagree.

[Be-ing]

Thank you for working on this, but this looks like a pretty substantial change and I am not comfortable rushing it for 2.3. We already rushing the changes for Serato metadata import.

[uklotzde]

After the prerequisite PRs have been merged into master this feature is considered done.

I will rebase on master from time to time for the time being until review and merge are considered.

[uklotzde]

Back on track :)

[Holzhaus]

Can you add a change log entry? Going through hundreds of old PRs before release is cumbersome. Let's just do it as we go.

[Holzhaus]

Ready to merge?

[uklotzde]

Ready to merge?

I decided to first prepare a backport for 2.3 fixing https://bugs.launchpad.net/mixxx/+bug/1879160. PR will follow. This requires another rebase of this PR.

[ronso0]

Unfortunately this doesn't solve the high CPU either.
(Same testing procedure as with #2815 )

• pick a track, make sure it doesn't have coverart embedded
• assign any cover image from file system
• delete/move cover image in file system
  = cover error is logged once
  = idle CPU stays very high (~105% here)
• restore cover image
  = idle CPU load is back to normal (~18% here)

[Holzhaus]

LGTM, thank you. I'll hold back merge until @ronso0 confirms that this works fine for him, too.

[uklotzde]

Anything else to do? I would like to get rid of this long pending PR.

[ronso0]

I won't be at my pc until sunday. So if this can't wait until then basically anyone else could do the simple test I described above #2524 (comment)

[ronso0]

(sry for closing, but weirder stuff happened when my phone slipped out of my hand... ;)

[Be-ing]

ping @ronso0

[ronso0]

Yes, CPU issue is solved. Thanks!

[Holzhaus]

Alright, let's merge then.

[uklotzde]

@Holzhaus Thank you :)