[Bug 1849347] Re: /etc/grub.d/10_linux_zfs gives up if any checked /etc/fstab entry is invalid

[Jean-Baptiste Lallement]

** Changed in: grub2 (Ubuntu Focal)
       Status: Triaged => In Progress

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https://bugs.launchpad.net/bugs/1849347

Title:
  /etc/grub.d/10_linux_zfs gives up if any checked /etc/fstab entry is
  invalid

Status in grub2 package in Ubuntu:
  In Progress
Status in grub2 source package in Focal:
  In Progress

Bug description:
  [Impact]
  Ubuntu's ZFS boot support (`/etc/grub.d/10_linux_zfs`) will fail to generate any GRUB entries for ZFS roots if any of the checked `/etc/fstab` entries fails to mount.  This includes all past ZFS snapshots.

  [Test Case]
  1. On a system that has a ZFS root (e.g. one installed with zsys), run `sudo update-grub`.

  2. Verify that there are GRUB boot entries for ZFS:

      sed -n '/^### BEGIN \/etc\/grub\.d\/10_linux_zfs ###$/,/^### END
  \/etc\/grub\.d\/10_linux_zfs ###$/p;/^### END
  \/etc\/grub\.d\/10_linux_zfs ###$/q' /boot/grub/grub.cfg

  3. To reproduce the problem, create or edit a `/boot` entry that will
  fail mounting.  (Back up the existing line if you have one and are
  working on a real system.)  Example:

      echo 'UUID=deaddead-dead-4ead-dead-deaddeaddead /boot
  ext2    discard,nofail        0       2' | sudo tee -a /etc/fstab

  4. (Optional) Take a ZFS snapshot of the dataset that provides `/etc`:

      sudo zfs snap "$(df /etc -t zfs | sed '/^Filesystem/d' | awk
  '{print $1}')"@repro

  5. If you performed step 4, restore the `/etc/fstab` back to its
  original working condition:

      sudo sed -i '/^UUID=deaddead-dead-4ead-dead-deaddeaddead/d'
  /etc/fstab

  6. Run `sudo update-grub` again.

  7. See that the GRUB boot entries for ZFS roots have been wiped out:

      sed -n '/^### BEGIN \/etc\/grub\.d\/10_linux_zfs ###$/,/^### END
  \/etc\/grub\.d\/10_linux_zfs ###$/p;/^### END
  \/etc\/grub\.d\/10_linux_zfs ###$/q' /boot/grub/grub.cfg

  [Regression Potential]
  The patch adds a sanity check for a mount that could fail.  The sanity check prevents the entire `/etc/grub.d/10_linux_zfs` from being wiped out from that failed mount.

  The new behavior causes the `get_system_directory()` function in
  `/etc/grub.d/10_linux_zfs` to act as if the bad mount didn't exist.
  The function would then continue to the next case, which is currently
  documented as "Handle zfs case, which can be a snapshots."

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