Make partition busy detection method selectable per file system (#723842)

GParted's primary inbuilt busy detection method is "is the partition
mounted?".  A custom method is used for LVM2 PV because its not a
mounted file system.

Make busy detection selectable per file system type.

    .fs.busy = FS::NONE  (default)
        No busy detection.

    .fs.busy = FS::GPARTED
        Use internal GParted method which checks if the partition is
        mounted.

    .fs.busy = FS:EXTERNAL
        Call the file system type's member function is_busy().

LVM2 PV busy detection changes from a special case to just electing to
call the lvm2_pv::is_busy() method.  Linux Software RAID remains a
special case because it's only recognised, but not otherwise supported.

Bug #723842 - GParted resizes the wrong filesystem (does not pass the
              devid to btrfs filesystem resize)
Restore busy detection of unknown mounted file systems (#723842)

Previous commit:
    Make partition busy detection method selectable per file system
    (#723842)
causes GParted to no longer detect an unknown mounted file system as
busy.

This is needed because there are many less popular file systems which
Linux can mount but GParted doesn't recognise, such as: Acorn ADFS,
AFFS, BeFS, BFS, etc.  Encountering an unrecognised mounted device can
also occur with btrfs on Linux 3.4 and earlier when the mounting device
is deleted from a multi-device btrfs file system.  Happens because
/proc/mounts continues to show the original mounting device even after
that device was deleted from the file system.

    # mkfs.btrfs /dev/sdb1
    # mount /dev/sdb1 /mnt/1
    # btrfs device add /dev/sdb2 /mnt/1
    # btrfs device delete /dev/sdb1 /mnt/1
    # btrfs filesystem sync /mnt/1
    # grep btrfs /proc/mounts
    /dev/sdb1 /mnt/1 btrfs rw,seclabel,relatime,ssd,space_cache 0 0
    # blkid /dev/sdb1
    # btrfs filesystem show /dev/sdb1
    # blkid /dev/sdb2
    /dev/sdb2: UUID="9c75647c-217a-4718-bcc7-f3ccd8cc7dc6" UUID_SUB="b5d43630-80d4-42ac-b406-185e97cd5bbe" TYPE="btrfs"
    # btrfs filesystem show /dev/sdb2
    Label: none  uuid: 9c75647c-217a-4718-bcc7-f3ccd8cc7dc6
            Total devices 1 FS bytes used 28.00KB
            devid    2 size 2.00GB used 1.02GB path /dev/sdb2

Reinstate the search for mounted partitions for busy detection even for
unrecognised file system types.

Bug #723842 - GParted resizes the wrong filesystem (does not pass the
              devid to btrfs filesystem resize)
Add is_dev_mounted() to expose core partition is mounted test (#723842)

Add static member function GParted_Core::is_dev_mounted() so that other
modules can determine if a particular partition contains a mounted file
system or not.

Make it a static member function so that it can be called without
needing the gparted_core object.  Extend to make the group of
manipulated variables (mount_info, fstab_info) and manipulating
functions (init_maps(), read_mountpoints_from_file(),
read_mountpoints_from_file_swaps(), get_all_mountpoints()) static too.

Bug #723842 - GParted resizes the wrong filesystem (does not pass the
              devid to btrfs filesystem resize)
Detect busy status of multi-device btrfs file systems (#723842)

Busy detection of file systems works by checking if the device is
mounted (appears in the mount_info map).  For a multi-device btrfs file
system this will only report one of the devices as busy, not all of
them.

    # btrfs filesystem show /dev/sdb1
    Label: none  uuid: 36eb51a2-2927-4c92-820f-b2f0b5cdae50
            Total devices 2 FS bytes used 156.00KB
            devid    2 size 2.00GB used 512.00MB path /dev/sdb2
            devid    1 size 2.00GB used 240.75MB path /dev/sdb1
    # mount /dev/sdb1 /mnt/1
    # grep btrfs /proc/mounts
    /dev/sdb1 /mnt/1 btrfs rw,seclabel,relatime,ssd,space_cache 0 0

GParted will only report /dev/sdb1 as busy, but not /dev/sdb2.

Add btrfs specific is_busy() method which reports the device as busy if
any of the devices in the btrfs file system are mounted.  This uses a
cache which maps device membership in all btrfs file systems.  The cache
is cleared on GParted refresh and incrementally populated as each btrfs
partition is checked for busy status.

WARNING:
Removal of the mounting device from a btrfs file system makes it
impossible to determine whether the file system is mounted or not for
linux <= 3.4.  This is because /proc/mounts continues to show the old
device which is no longer a member of the file system.

    # btrfs device delete /dev/sdb1 /mnt/1
    # sync
    # grep btrfs /proc/mounts
    /dev/sdb1 /mnt/1 btrfs rw,seclabel,relatime,ssd,space_cache 0 0
    # btrfs filesystem show /dev/sdb1
    # btrfs filesystem show /dev/sdb2
    Label: none  uuid: 36eb51a2-2927-4c92-820f-b2f0b5cdae50
            Total devices 1 FS bytes used 28.00KB
            devid    2 size 2.00GB used 1.02GB path /dev/sdb2

Fixed in linux 3.5 by commit:
    Btrfs: implement ->show_devname
    https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=9c5085c147989d48dfe74194b48affc23f376650

Bug #723842 - GParted resizes the wrong filesystem (does not pass the
              devid to btrfs filesystem resize)
Display mount points for multi-device btrfs file systems (#723842)

Linux can only show a single device name in /proc/mounts and /etc/mtab
for each mounted btrfs, even if it is a multi-device file system.  So
GParted only shows a mount point for one of the devices in the btrfs, no
matter how many devices are part of the file system.

    # mkfs.btrfs /dev/sdb1 /dev/sdb2
    # btrfs filesystem show /dev/sdb1
    Label: none  uuid: 36eb51a2-2927-4c92-820f-b2f0b5cdae50
            Total devices 2 FS bytes used 156.00KB
            devid    2 size 2.00GB used 512.00MB path /dev/sdb2
            devid    1 size 2.00GB used 240.75MB path /dev/sdb1
    # mount /dev/sdb1 /mnt/1
    # grep btrfs /proc/mounts
    /dev/sdb1 /mnt/1 btrfs rw,seclabel,relatime,ssd,space_cache 0 0

GParted only shows the mount point for /dev/sdb1 as /mnt/1, but nothing
for /dev/sdb2.

Make GParted report the same mount point for all devices included in a
multi-device btrfs file system.

Add btrfs specific get_mount_device() method to report the mounting
device, if any, for the btrfs file system in the occupying the device in
question.  Uses the existing cache of btrfs file system device
membership.  Also extract common code from GParted_Core::
set_mountpoints() into set_mountpoints_helper().

Bug #723842 - GParted resizes the wrong filesystem (does not pass the
              devid to btrfs filesystem resize)
Display usage for multi-device btrfs file systems (#723842)

Currently GParted fails to report the usage of a multi-device btrfs file
system if it is mounted or if the used space is larger than the size of
an individual member device.  When GParted does display usage figures it
also incorrectly reports the file system wide used figure against every
member device.

Mounted case:
    statvfs() provides an FS size which is larger than any individual
    device so is rejected.  See:
        GParted_Core::mounted_set_used_sectors()
            Utils::get_mounted_filesystem_usage()
            partition .set_sector_usage()

Unmounted case, FS used > device size:
    FS used figure is larger than any individual device so free space is
    calculated as a negative number and rejected.  See:
        btrfs::set_used_sectors()

Btrfs has a volume manager layer within the file system which allows it
to provide multiple levels of data redundancy, RAID levels, and use
multiple devices both of which can be changed while the file system is
mounted.  To achieve this btrfs has to allocate space at two different
level: (1) chunks of 256 MiB or more at the volume manager level; and
(2) extents at the file data level.
References:
*   Btrfs: Working with multiple devices
    https://lwn.net/Articles/577961/
*   Btrfs wiki: Glossary
    https://btrfs.wiki.kernel.org/index.php/Glossary

This makes the question of how much disk space is being used in an
individual device a complicated question to answer.  Further, the
current btrfs tools don't provide the required information.

Btrfs filesystem show only provides space usage information at the chunk
level per device.  At the file extent level only a single figure for the
whole file system is provided.  It also reports size of the data and
metadata being stored, not the larger figure of the amount of space
taken after redundancy is applied.  So it is impossible to answer the
question of how much disk space is being used in an individual device.
Example output:

    Label: none  uuid: 36eb51a2-2927-4c92-820f-b2f0b5cdae50
            Total devices 2 FS bytes used 156.00KB
            devid    2 size 2.00GB used 512.00MB path /dev/sdb2
            devid    1 size 2.00GB used 240.75MB path /dev/sdb1

Fix by guesstimating the per device used figure as the fraction of the
file system wide extent usage based on chunk usage per device.
Calculation:
    ptn fs used = total fs used * devid used / sum devid used

Positives:
1) Per device used figure will correctly be between zero and allocated
   chunk size.

Known inaccuracies:
[for single and multi-device btrfs file systems]
1) Btrfs filesystem show reports file system wide file extent usage
   without considering redundancy applied to that data.  (By default
   btrfs stores two copies of metadata and one copy of data).
2) At minimum size when all data has been consolidated there will be a
   few partly filled chunks of 256 MiB or more for data and metadata of
   each storage profile (RAID level).
[for multi-device btrfs file systems only]
3) Data may be far from evenly distributed between the chunks on
   multiple devices.
4) Extents can be and are relocated to other devices within the file
   system when shrinking a device.

Bug #723842 - GParted resizes the wrong filesystem (does not pass the
              devid to btrfs filesystem resize)
Add devid to the cache of btrfs device information (#723842)

Bug #723842 - GParted resizes the wrong filesystem (does not pass the
              devid to btrfs filesystem resize)
Pass devid when resizing btrfs file systems (#723842)

GParted doesn't specify the devid when resizing a btrfs file system, so
the kernel defaults to resizing devid 1.  On a multi-device btrfs this
may not be the same partition which GParted is resizing.  This will
result in file system truncation and corruption.  Shrinking the wrong
partition example:

    1)  Create a btrfs file system spanning 2 partitions:
            # mkfs.btrfs /dev/sdb1 /dev/sdb2
            # btrfs filesystem show /dev/sdb1
            Label: none  uuid: 41654265-9840-45c4-aca1-55989da358d6
                    Total devices 2 FS bytes used 112.00KiB
                    devid    1 size 2.00GiB used 437.50MiB path /dev/sdb1
                    devid    2 size 2.00GiB used 417.50MiB path /dev/sdb2

    2)  Resize /dev/sdb2 down to 1 GiB using GParted.  This command was
        run:
            btrfs filesystem resize 1048576K /tmp/gparted-ddyGRh
        which resized devid 1 (/dev/sdb1) to 1 GiB:
            # btrfs filesystem show /dev/sdb1
            Label: none  uuid: 41654265-9840-45c4-aca1-55989da358d6
                    Total devices 2 FS bytes used 256.00KiB
                    devid    1 size 1.00GiB used 437.50MiB path /dev/sdb1
                    devid    2 size 2.00GiB used 417.50MiB path /dev/sdb2
        but GParted instead resized /dev/sdb2 to 1 GiB:
            # sfdisk -s /dev/sdb1
            2097152
            # sfdisk -s /dev/sdb2
            1048576

Even on a single device btrfs devid 1 may no longer exist if the file
system has had the initial device removed from it.  Example:

    1)  Create a single btrfs file system, add a second device and
        remove the first:
            # mkfs.btrfs /dev/sdb1
            # mount /dev/sdb1 /mnt/1
            # btrfs device add /dev/sdb2 /mnt/1
            # btrfs device remove /dev/sdb1 /mnt/1
            # umount /mnt/1
            # btrfs filesystem show /dev/sdb2
            Label: none  uuid: 2cbf3ac3-1344-472a-a0c7-1476d23bdc9f
                    Total devices 1 FS bytes used 256.00KiB
                    devid    2 size 2.00GiB used 480.00MiB path /dev/sdb2

    2)  Again resize /dev/sdb2 down to 1 GiB using GParted.  This
        command was run:
            btrfs filesystem resize 1048576K /tmp/gparted-ddyGRh
        but it failed with:
            ERROR: unable to resize 'tmp/gparted-lEyGaY' - No such device
        A more informative error message was written to syslog:
            # tail -1 /var/log/messages
            Mar 12 14:15:01 localhost kernel: btrfs: resizer unable to find device 1

This is with Linux kernel 3.13.5 on Fedora 20, circa March 2014.

Fix by specifying the devid when resizing (part of) a btrfs file system.
Example command specifying devid 2:

    btrfs filesystem resize 2:1048576K /tmp/1

This will always work because it is the kernel which interprets the
devid colon size parameter and has always done so since btrfs was first
added to the kernel in version 2.6.32 [1].

Reference:
[1] linux v2.6.32 fs/btrfs/ioctl.c btrfs_ioctl_resize()
    https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/fs/btrfs/ioctl.c?id=v2.6.32#n578

Bug #723842 - GParted resizes the wrong filesystem (does not pass the
              devid to btrfs filesystem resize)
Add fallback busy detection for btrfs file systems (#723842)

In a number of cases there may be no btrfs device cache entry.  Some of
the reasons why this can occur:
1)  Mounting device removed from btrfs on linux <= 3.4 so old mount
    point in /proc/mounts;
2)  btrfs and btrfs-show commands don't exist;
3)  btrfs or btrfs-show command returned non-zero exit status;
4)  get_cache_entry() failed to parse output from btrfs filesystem show
    or btrfs-show.

Without a valid btrfs device cache entry, busy detection for all member
devices fails.  Search the GParted internal mounted partitions map as
the fallback busy detection method.  This can only determine if the
mounting device is mounted or not, not any of the other members of a
multi-device btrfs file system.

Bug #723842 - GParted resizes the wrong filesystem (does not pass the
              devid to btrfs filesystem resize)
Fallback to reading mount command output instead of /etc/mtab (#723842)

With linux 3.5 and later, the device used to mount a btrfs file system
is updated in /proc/mounts when the previous mounting device is removed
from the file system.  Most recent distributions make /etc/mtab a
symbolic link to /proc/mounts.  However some still have /etc/mtab as a
plain file only updated by mount and umount, thus showing the old device
name which is no longer part of the file system.

On Ubuntu 13.10, which has /etc/mtab as a plain file managed by mount
and umount:

    # mkfs.btrfs /dev/sdb1
    # mount /dev/sdb1 /mnt/1
    # btrfs device add /dev/sdb2 /mnt/1
    # btrfs device delete /dev/sdb1 /mnt/1
    # sync
    # btrfs filesystem show /dev/sdb1
    # btrfs filesystem show /dev/sdb2
    Label: none  uuid: e47775a6-e5ad-4fb4-9ea4-1570aa5b4009
            Total devices 2 FS bytes used 28.00KB
            devid    2 size 2.00GB used 272.00MB path /dev/sdb2

    # fgrep btrfs /proc/mounts
    /dev/sdb2 /mnt/1 btrfs rw,relatime,space_cache 0 0
    # ls -l /etc/mtab
    -rw-r--r-- 1 root root 842 Apr 15 19:41 /etc/mtab
    # fgrep btrfs /etc/mtab
    /dev/sdb1 /mnt/1 btrfs rw 0 0

This causes GParted to report /dev/sdb1 as busy and mounted at /mnt/1
when it is no longer mounted.  This effects recent releases of Ubuntu,
13.04, 13.10 and 14.04.

Either /etc/mtab is a symlink and is identical to /proc/mounts or
/etc/mtab is a plain file with wrong information.  Fix by not reading
mounted file systems from /etc/mtab.

However old distributions only contain 'rootfs' and '/dev/root' device
names for the / (root) file system with '/dev/root' being a block device
rather than a symlink to the true device.  For example from CentOS 5.x:

    # fgrep ' / ' /proc/mounts
    rootfs / rootfs rw 0 0
    /dev/root / ext3 rw,data=ordered 0 0
    # ls -l /dev/root
    brw------- 1 root root 8, 3 Jun  4  2013 /dev/root

This prevents identification, and therefore busy detection, of the
device containing the / (root) file system.  Used to read /etc/mtab to
get the root file system device name.

    # fgrep ' / ' /etc/mtab
    /dev/sda3 / ext3 rw 0 0
    # ls -l /dev/sda3
    brw-r----- 1 root disk 8, 3 Jun  4  2013 /dev/sda3

As per commit:

    409096f739118af95e3bff4484fdedb7885c97a2
    improved scanning for root mountpoint (/) ...

but, as discussed above, this contains an out of date device name after
the mounting device has been dynamically removed from a multi-device
btrfs, thus identifying the wrong device as busy.  Instead fall back to
reading mounted file systems from the output of the mount command, but
only when required.

    # mount | fgrep ' / '
    /dev/sda3 on / type ext3 (rw)

Bug #723842 - GParted resizes the wrong filesystem (does not pass the
              devid to btrfs filesystem resize)
Display btrfs members in the Partition Information dialog (#723842)

Bug #723842 - GParted resizes the wrong filesystem (does not pass the
              devid to btrfs filesystem resize)
Flush devices when scanning to prevent reading stale signatures (#723842)

While one partition is busy, reformat another partition from the command
line.  Afterwards parted/libparted still detects the original file
system and GParted shows errors from the file system specific tools
reporting the new file system doesn't exist.  Only limitation is that
the new new file system must be recognised by libparted (or by GParted's
fallback file system signature detection).

Case #1, File system reformatting:

    # parted /dev/sdb print
    Model: ATA SAMSUNG SSD UM41 (scsi)
    Disk /dev/sdb: 8012MB
    Sector size (logical/physical): 512B/512B
    Partition Table: msdos

    Number  Start   End     Size    Type     File system  Flags
     1      1049kB  2149MB  2147MB  primary  ext2
     2      2149MB  4296MB  2147MB  primary  ext2
    # mount | fgrep sdb
    /dev/sdb1 on /mnt/1 type ext2 (rw)

    # mkfs.xfs -f /dev/sdb2
    # blkid /dev/sdb2
    /dev/sdb2: UUID="c31823a2-b81b-46fa-8246-0a59695e4834" TYPE="xfs"
    # parted /dev/sdb print
    Model: ATA SAMSUNG SSD UM41 (scsi)
    Disk /dev/sdb: 8012MB
    Sector size (logical/physical): 512B/512B
    Partition Table: msdos

    Number  Start   End     Size    Type     File system  Flags
     1      1049kB  2149MB  2147MB  primary  ext2
     2      2149MB  4296MB  2147MB  primary  ext2
    # e2label /dev/sdb2
    e2label: Bad magic number in super-block while trying to open /dev/sdb2
    Couldn't find valid filesystem superblock.
    # dumpe2fs /dev/sdb2
    dumpe2fs 1.41.12 (17-May-2010)
    dumpe2fs: Bad magic number in super-block while trying to open /dev/sdb2
    Couldn't find valid filesystem superblock.

Case #2, Removing device from multi-device btrfs:

    # btrfs filesystem show /dev/sdb1
    Label: none  uuid: a05db434-efd5-4e8c-902f-05f89a88b610
            Total devices 2 FS bytes used 156.00KB
            devid    2 size 2.00GB used 512.00MB path /dev/sdb2
            devid    1 size 2.00GB used 240.75MB path /dev/sdb1

    # mount /dev/sdb1 /mnt/1
    # btrfs device delete /dev/sdb2
    # btrfs filesystem show /dev/sdb1
    Label: none  uuid: a05db434-efd5-4e8c-902f-05f89a88b610
            Total devices 1 FS bytes used 92.00KB
            devid    1 size 2.00GB used 714.25MB path /dev/sdb1
    # btrfs filesystem show /dev/sdb2

and GParted reports this error for partition /dev/sdb2:
    Unable to read the contents of this file system!
    Because of this some operations may be unavailable.
    The cause might be a missing software package.
    The following list of software packages is required for btrfs
    file system support:  btrfs-tools.

This is another case of libparted reading from the whole disk device
(/dev/sdb) yet the file system tools use the partition specific block
device (/dev/sdb2), and the Linux buffer cache not providing cache
coherency.  Previous scenario was fixed with:

    797f0b8eeb61f81da1791c3cf048b5c0abf9a550
    Flush device after wiping a file system (#688882)

This affects libparted 2.0 to 3.1 inclusive and is fixed by:

    http://git.savannah.gnu.org/cgit/parted.git/commit/?id=fb99ba5ebd0dc34204fc9f1014131d5d494805bc
    Revert "linux-commit: do not unnecessarily open partition device nodes"

Fix by calling ped_device_sync() to guarantee cache coherency for each
device during scanning.

Bug #723842 - GParted resizes the wrong filesystem (does not pass the
              devid to btrfs filesystem resize)