Mountpoint "auto" can fail for read-only mounts

[unterkomplex]

I encountered one issue while using the option to automatically process all mountpoints
BTRFS_SCRUB_MOUNTPOINTS="auto"

The script returns a single mountpoint for each filesystem
MNT=$(findmnt --types btrfs --first-only --noheadings --output "TARGET" --source "$DEVICE")
https://github.com/kdave/btrfsmaintenance/blob/b1e70f6c99797def13c1d302a603e18cce701a8d/btrfsmaintenance-functions#L25C10-L25C84

This fails if there are both read-only and read-write mounts and findmnt returns a ro mountpoint

In order to fix it I think it would be sufficient to add --options rw
MNT=$(findmnt --types btrfs --first-only --options rw --noheadings --output "TARGET" --source "$DEVICE")

Example output from Fedora IoT on BTRFS filesystem with /sysroot mounted as ro

[root@pi ~]# findmnt --types btrfs --noheadings --output "TARGET" --source "/dev/sda1"
/sysroot
/
/etc
/usr
/sysroot/ostree/deploy/fedora-iot/var
/var
/var/mnt/ssd
[root@pi ~]# findmnt --types btrfs --first-only --noheadings --output "TARGET" --source "/dev/sda1"
/sysroot
[root@pi ~]# findmnt --types btrfs --first-only --options rw --noheadings --output "TARGET" --source "/dev/sda1"
/

[root@pi ~]# journalctl -b0 -u btrfs-scrub.service 
Jul 08 03:09:39 pi btrfs-scrub.sh[1283]: Running scrub on /sysroot
Jul 08 03:09:39 pi btrfs-scrub.sh[1283]: RAID level is not 5 or 6, parallel device scrubbing
Jul 08 03:09:39 pi btrfs-scrub.sh[1283]: ERROR: scrubbing /sysroot failed for device id 1: ret=-1, errno=30 (Read-only>
Jul 08 03:09:39 pi btrfs-scrub.sh[1283]: Starting scrub on devid 1
Jul 08 03:09:39 pi btrfs-scrub.sh[1283]: Scrub device /dev/sda1 (id 1) canceled
Jul 08 03:09:39 pi btrfs-scrub.sh[1283]: Scrub started:    Mon Jul  8 03:09:39 2024
Jul 08 03:09:39 pi btrfs-scrub.sh[1283]: Status:           aborted
Jul 08 03:09:39 pi btrfs-scrub.sh[1283]: Duration:         0:00:00
Jul 08 03:09:39 pi btrfs-scrub.sh[1283]: Total to scrub:   0.00B
Jul 08 03:09:39 pi btrfs-scrub.sh[1283]: Rate:             0.00B/s
Jul 08 03:09:39 pi btrfs-scrub.sh[1283]: Error summary:    no errors found

[Framsfex]

The keyword "auto" also adds btrfs filesystems on loop devices, which is not a good idea.

As a workaround I have added an extra variable BTRFS_ALL to /etc/default/btrfsmaintenance :

# all rw-mounted btrfs filesystems (but skip loop devices)
# see also: lsblk -n -o NAME,FSTYPE,MOUNTPOINT
BTRFS_ALL=$(perl -wne '
  next if m:^/dev/loop:;
  $b{$1} ||= $2 if /^(\S+) (\S+) btrfs rw,/;
  END { print join(":",sort values %b) if %b }
' /proc/mounts)

So I can use:

BTRFS_SCRUB_MOUNTPOINTS="$BTRFS_ALL"