Mike Fleetwood authored Jun 25, 2016 and Curtis Gedak committed Aug 06, 2016

On some distributions having btrfs on top of LUKS encrypted partitions,
adding a second device and removing the first device used to mount the
file system causes GParted to no longer be able to report the file
system as busy or the mount points themselves.

For example, on CentOS 7, create a single btrfs file system and mount
it.  The provided /dev/mapper/sdb1_crypt name is reported, via
/proc/mounts, as the mounting device:
    # cryptsetup luksFormat --force-password /dev/sdb1
    # cryptsetup luksOpen /dev/sdb1 sdb1_crypt
    # mkfs.btrfs -L encrypted-btrfs /dev/mapper/sdb1_crypt
    # mount /dev/mapper/sdb1_crypt /mnt/1

    # ls -l /dev/mapper
    total 0
    lrwxrwxrwx. 1 root root       7 Jul  2 14:15 centos-root -> ../dm-1
    lrwxrwxrwx. 1 root root       7 Jul  2 14:15 centos-swap -> ../dm-0
    crw-------. 1 root root 10, 236 Jul  2 14:15 control
    lrwxrwxrwx. 1 root root       7 Jul  2 15:14 sdb1_crypt -> ../dm-2
    # fgrep btrfs /proc/mounts
    /dev/mapper/sdb1_crypt /mnt/1 btrfs rw,seclabel,relatime,space_cache 0 0

Add a second device to the btrfs file system:
    # cryptsetup luksFormat --force-password /dev/sdb2
    # cryptsetup luksOpen /dev/sdb2 sdb2_crypt
    # btrfs device add /dev/mapper/sdb2_crypt /mnt/1

    # ls -l /dev/mapper
    ...
    lrwxrwxrwx. 1 root root       7 Jul  2 15:12 sdb2_crypt -> ../dm-3
    # btrfs filesystem show /dev/mapper/sdb1_crypt
    Label: 'encrypted-btrfs'  uuid: 45d7b1ef-820c-4ef8-8abd-c70d928afb49
            Total devices 2 FS bytes used 32.00KiB
            devid    1 size 1022.00MiB used 12.00MiB path /dev/mapper/sdb1_crypt
            devid    2 size 1022.00MiB used 0.00B path /dev/mapper/sdb2_crypt

Remove the first mounting device from the btrfs file system.  Now the
non-canonical name /dev/dm-3 is reported, via /proc/mounts, as the
mounting device:
    # btrfs device delete /dev/mapper/sdb1_crypt /mnt/1

    # btrfs filesystem show /dev/mapper/sdb2_crypt
    Label: 'encrypted-btrfs'  uuid: 45d7b1ef-820c-4ef8-8abd-c70d928afb49
            Total devices 1 FS bytes used 96.00KiB
            devid    2 size 1022.00MiB used 144.00MiB path /dev/mapper/sdb2_crypt
    # fgrep btrfs /proc/mounts
    /dev/dm-3 /mnt/1 btrfs rw,seclabel,relatime,space_cache 0 0
    # ls -l /dev/dm-3
    brw-rw----. 1 root disk 253, 3 Jul  2 15:12 /dev/dm-3

GParted loads the mount_info mapping from /proc/mounts and with it the
/dev/dm-3 name.  When GParted is determining if the encrypted btrfs file
system is mounted or getting the mount points it is using the
/dev/mapper/sdb2_crypt name.  Therefore no information is found and the
file system is incorrectly reported as unmounted.

Fix by changing mount_info and fstab_info to use BlockSpecial objects
instead of strings so that matching is performed by major, minor device
numbers rather than by string compare.  Note that as BlockSpecial
objects are used as the key of std::map [1] mappings operator<() [2]
needs to be provided to order the key values.

[1] std::map
    http://www.cplusplus.com/reference/map/map/
[2] std::map::key_comp
    http://www.cplusplus.com/reference/map/map/key_comp/

Bug 767842 - File system usage missing when tools report alternate block
             device names