[Bug 1861941] Re: bcache by-uuid links disappear after mounting bcache0

[Rafael David Tinoco]

About 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 this bug (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.

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.

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https://bugs.launchpad.net/bugs/1861941

Title:
  bcache by-uuid links disappear after mounting bcache0

Status in bcache-tools package in Ubuntu:
  In Progress
Status in systemd package in Ubuntu:
  In Progress
Status in bcache-tools source package in Bionic:
  In Progress
Status in systemd source package in Bionic:
  In Progress
Status in bcache-tools source package in Focal:
  In Progress
Status in systemd source package in Focal:
  In Progress

Bug description:
  1.
  root at ubuntu:~# lsb_release -rd
  Description:	Ubuntu Focal Fossa (development branch)
  Release:	20.04

  2. 
  root at ubuntu:~# lsb_release -rd
  Description:	Ubuntu Focal Fossa (development branch)
  Release:	20.04
  root at ubuntu:~# apt-cache policy linux-image-virtual 
  linux-image-virtual:
    Installed: 5.4.0.12.15
    Candidate: 5.4.0.12.15
    Version table:
   *** 5.4.0.12.15 500
          500 http://archive.ubuntu.com/ubuntu focal/main amd64 Packages
          100 /var/lib/dpkg/status
  root at ubuntu:~# apt-cache policy linux-image-5.4.0-12-generic 
  linux-image-5.4.0-12-generic:
    Installed: 5.4.0-12.15
    Candidate: 5.4.0-12.15
    Version table:
   *** 5.4.0-12.15 500
          500 http://archive.ubuntu.com/ubuntu focal/main amd64 Packages
          100 /var/lib/dpkg/status

  3. mount /dev/bcache0 && ls -al /dev/bcache/by-uuid/
  + ls -al /dev/bcache/by-uuid/
  total 0
  drwxr-xr-x 2 root root 60 Feb  4 23:31 .
  drwxr-xr-x 3 root root 60 Feb  4 23:31 ..
  lrwxrwxrwx 1 root root 13 Feb  4 23:31 abdfd1f6-44ce-4266-91db-24667b9ae51a -> ../../bcache0

  4.
  root at ubuntu:~# ls -al /dev/bcache/by-uuid
  ls: cannot access '/dev/bcache/by-uuid': No such file or directory

  ProblemType: Bug
  DistroRelease: Ubuntu 20.04
  Package: linux-image-5.4.0-12-generic 5.4.0-12.15
  ProcVersionSignature: Ubuntu 5.4.0-12.15-generic 5.4.8
  Uname: Linux 5.4.0-12-generic x86_64
  ApportVersion: 2.20.11-0ubuntu16
  Architecture: amd64
  Date: Tue Feb  4 23:31:52 2020
  ProcEnviron:
   TERM=xterm-256color
   PATH=(custom, no user)
   LANG=C.UTF-8
   SHELL=/bin/bash
  SourcePackage: linux-signed-5.4
  UpgradeStatus: No upgrade log present (probably fresh install)

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