[Merge] ~ubuntu-core-dev/ubuntu/+source/systemd:ubuntu-groovy into ubuntu/+source/systemd:ubuntu/devel

[Rafael David Tinoco]

Rafael David Tinoco has proposed merging ~ubuntu-core-dev/ubuntu/+source/systemd:ubuntu-groovy into ubuntu/+source/systemd:ubuntu/devel.

Commit message:
About udev/blkid fix: 

Ryan has explained carefully how to obtain FS UUID from the backing device at:

https://github.com/systemd/systemd/pull/16317#issuecomment-655574875

and about bcache-export-cached wrapper he created for simulating CACHED_UUID and CACHED_LABEL (at 69-bcache.rules) and to create /dev/bcache/{by-uuid,by-label}/{CACHED_UUID,CACHED_LABEL} symlinks.

He explained that having /dev/bcache/{by-uuid,by-label} is mandatory to maintain same names for multiple bcache'd environments over reboots.

Currently, with the bcache-export-cached fix applied, it is *not* mandatory to have systemd-udev fix to correctly create bcache symlinks (or not) at /dev/disk/{by-uuid,by-label}: libblkid could have more information regarding bcache backing devices, but judging by upstream developers disposition they don't seem too worried about having that feature since ID_FS_TYPE=bcache is already provided. (So, having /dev/disk/by-uuid/XXX pointing to bcache backing devices would continue happening...)

Summary:

Having the bcache-tools fix is enough for LP: #1861941 and there could be changes to systemd-udev (I doubt judging by discussions at https://github.com/systemd/systemd/pull/16317), or libblkid, that could provide more information regarding bcache device types. If those changes happen, we would need to revisit this bug.

[WHAT THIS MERGE REQUEST IS ABOUT]

Meanwhile, IMHO,

I like the "skip_bcache_fs_type" approach done by Ryan to systemd-udev. I'll maintain that as a delta to udev until https://github.com/systemd/systemd/pull/16317 is fixed (if ever).

So, for now, PPA contains fix for bcache-tools for Groovy and Focal (and will for Bionic) and that is enough to make /dev/bcache/XXX to work and be stable after reboots. I'll keep the udev fix for Groovy and SRU that fix for Focal and Bionic and upload to the PPA as well.

After all that is done, I'll tell kernel team to revert the bcache patch on Bionic, Focal and Groovy kernels since it is not needed and will reduce their delta and reduce chances of regressions over time.

Requested reviews:
  Dan Streetman (ddstreet)
  Balint Reczey (rbalint)
Related bugs:
  Bug #1861941 in systemd (Ubuntu): "bcache by-uuid links disappear after mounting bcache0"
  https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1861941
  Bug #1885755 in systemd (Ubuntu): "test-cap-list fails with Linux 5.8 due to new capabilities"
  https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1885755

For more details, see:
https://code.launchpad.net/~ubuntu-core-dev/ubuntu/+source/systemd/+git/systemd/+merge/387991
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