mount: Handle multi-device with 1 device node

[tasleson]

Instead of requiring the user to supply all the device nodes for a multi-device FS, allow them to specifiy 1 of them. We then fetch the UUID for the FS and then find all the disks on the system that match this UUID.

This allows me to bring up a bcachefs FS in /etc/fstab by using a UUID. This works because it appears the mount command looks up the UUID, finds an entry in /dev/disk/by-uuid and then passes that found device node to mount.bcachefs which fails with insufficient_devices_to_start as bcachefs is expecting a list of devices with a ":" between them. This behavior is preserved if a user specifies a list of all the needed device nodes to bring up the FS.

Notes:

• Maybe this isn't the correct approach, but I think the logic should be here instead of util-linux even if this isn't implemented well.
• I'm using a release candidate util-linux, ref. https://github.com/util-linux/util-linux/archive/refs/tags/v2.40-rc1.tar.gz running on Fedora 39 with 6.7.5-200 Fedora kernel.
• I'm not sure why the existing bcachefs user-space doesn't leverage the udev db more, but that might be due to the fact that we maynot have full udev db information populated with current systems with existing util-linux/blkid functionality.
• I'm not sure why the current code dumps the following out the terminal when mounting via UUID= eg.

  # bcachefs mount UUID=11f32f1f-016d-41c2-afd4-cd341c26f133 /mnt/bcachefs_2_dev
  bcachefs (/dev/sda): error reading default superblock: Not a bcachefs superblock (got magic 00000000-0000-0000-0000-000000000000)
  bcachefs (/dev/sda): error reading superblock: Not a bcachefs superblock layout
  bcachefs (/dev/sda1): error reading default superblock: Not a bcachefs superblock (got magic 00000000-0000-0000-0000-000000000000)
  bcachefs (/dev/sda1): error reading superblock: Not a bcachefs superblock layout
  bcachefs (/dev/sda2): error reading default superblock: Not a bcachefs superblock (got magic cd69a632-d51f-4ecc-0000-000000000000)
  bcachefs (/dev/sda2): error reading superblock: Not a bcachefs superblock layout
  bcachefs (/dev/sda3): error reading default superblock: Not a bcachefs superblock (got magic f676ecf2-df1f-6474-0000-000000000000)
  bcachefs (/dev/sda3): error reading superblock: Not a bcachefs superblock layout
  bcachefs (/dev/sdb): error reading default superblock: Not a bcachefs superblock (got magic 00100000-0000-0000-00f0-0f0000000000)
  bcachefs (/dev/sdb): error reading superblock: Not a bcachefs superblock layout
  bcachefs (/dev/sdc): error reading default superblock: Not a bcachefs superblock (got magic 00100000-0000-0000-00f0-0f0000000000)
  bcachefs (/dev/sdc): error reading superblock: Not a bcachefs superblock layout
  bcachefs (/dev/zram0): error reading default superblock: Not a bcachefs superblock (got magic 00000000-0000-0000-0000-000000000000)
  bcachefs (/dev/zram0): error reading superblock: IO error: -5
  bcachefs (/dev/dm-0): error reading default superblock: Not a bcachefs superblock (got magic 18000000-0000-0000-0100-000000000000)
  bcachefs (/dev/dm-0): error reading superblock: Not a bcachefs superblock layout
  bcachefs (/dev/dm-1): error reading default superblock: Not a bcachefs superblock (got magic 00000000-0000-0000-0000-000000000000)
  bcachefs (/dev/dm-1): error reading superblock: Not a bcachefs superblock layout
  bcachefs (/dev/dm-2): error reading default superblock: Not a bcachefs superblock (got magic 00000000-0000-0000-0000-000000000000)
  bcachefs (/dev/dm-2): error reading superblock: Not a bcachefs superblock layout
  bcachefs (/dev/dm-3): error reading default superblock: Not a bcachefs superblock (got magic 00000000-0000-0000-0000-000000000000)
  bcachefs (/dev/dm-3): error reading superblock: Not a bcachefs superblock layout
  bcachefs (/dev/dm-4): error reading default superblock: Not a bcachefs superblock (got magic 00000000-0000-0000-0000-000000000000)
  bcachefs (/dev/dm-4): error reading superblock: Not a bcachefs superblock layout
  bcachefs (/dev/dm-5): error reading default superblock: Not a bcachefs superblock (got magic 00000000-0000-0000-0000-000000000000)
  bcachefs (/dev/dm-5): error reading superblock: Not a bcachefs superblock layout

[tasleson]

Looks like this is similar to PR https://github.com/koverstreet/bcachefs-tools/pull/142 , but different.

[koverstreet]

I had to think about this, and - I think this actually a pretty slick solution. This should help with systemd integration as well; we're trying to avoid systemd having to know which devices it has to wait for, and this helps with that.

[JohnRTitor]

And since systemd initrd still can not properly mount bcachefs on boot (systemd-udev can), I think it will fix that too.

[tasleson]

The first commit in this PR was merged already. The last commit in this PR has been re-worked in https://github.com/koverstreet/bcachefs-tools/pull/254