[Bug 1987679] Re: os-prober leaves filesystems (lvm-thin, lvm snap) mounted

[Heitor Alves de Siqueira]

Hi mikeaa,

no worries, your contribution is quite valuable! It's great to have a
view from a different perspective or use-case, as that helps us uncover
different scenarios of the bug to tackle.

For the other bugs you found, would you be able to open up a new report
for them? It makes tracking across different releases easier, as we'd
have a unique identifier per issue. Let's keep this one focused on the
leftover partition mounts; I think we have good enough evidence to point
out it's being caused by grub-probe terminating early, we just need to
figure out the best approach to fixing it :)

I'm uploading a v3 of my debdiff, as I missed one of the original parts
of the code that writes "fuseblk" to type if it's empty. This can
presumably happen when grub-probe does not error out, but returns an
empty string.

I still think we should follow existing code under os-probes for the
fix, and let LVM partitions be probed/mounted by later grub scripts (I'd
argue that booting from LVM snapshots is a net-positive, and probably
intended given the current code). I'm happy to defer to a more
experienced grub/os-prober developer on this matter and go with a
different fix if it makes more sense, though.

** Patch added: "lp1987679-plucky-v3.debdiff"
   https://bugs.launchpad.net/ubuntu/+source/os-prober/+bug/1987679/+attachment/5855399/+files/lp1987679-plucky-v3.debdiff

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Title:
  os-prober leaves filesystems (lvm-thin, lvm snap) mounted

Status in os-prober package in Ubuntu:
  In Progress
Status in os-prober source package in Focal:
  In Progress
Status in os-prober source package in Jammy:
  In Progress
Status in os-prober source package in Kinetic:
  Won't Fix
Status in os-prober source package in Noble:
  In Progress
Status in os-prober source package in Oracular:
  In Progress
Status in os-prober source package in Plucky:
  In Progress

Bug description:
  If one happens to have by accident (or on purpose) a thick LVM snapshot, or a thin-LV with a copy of a system on it, update-grub reports
  /sbin/grub-probe: error: unknown filesystem.
  Found Ubuntu 22.04.1 LTS on /dev/mapper/vgxubuntu-s22.04
  and
  /sbin/grub-probe: error: disk `lvmid/X7waXv-rMDA-5fQA-aw6l-ei1I-8Gz3-pQl3lr/k3Ze1u-Mfwc-pxUt-7fXa-FjSZ-7IkX-HAxf3b' not found.
  Found Ubuntu 22.04.1 LTS (22.04) on /dev/mapper/vgxubuntu-XR22.04.1U

  and leaves the systems mounted rw on /var/lib/os-prober/mount.
  On subsequent invocations update-grub spews out multiple
  mdir: failed to remove '/var/lib/os-prober/mount': Device or resource busy

  I'm pretty sure this also causes a new install to hang (though a
  power-reset seems to load the new system - not sure if it is complete
  or not.)

  The missing umounts should be relatively easy to fix, but the thick
  snapshot error is particularly vague as most users (I think) would
  take the "unknown filesystem" to refer to the previous entry.

  I have been playing with grub2-common-2.06-2ubuntu7 on xubuntu
  22.04.1, though I am sure this has been around a while

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