[frameworks-solid] [Bug 427092] btrfs multiple device handling

[Chris Murphy]

https://bugs.kde.org/show_bug.cgi?id=427092

--- Comment #20 from Chris Murphy <bugzilla at colorremedies.com> ---
>Possibly such functionality belongs in udisksd, if it's the service being requested to do the mount.

A possibly complicating factor is that it's legit to mount a Btrfs file system
more than one time, e.g. 
mount -o subvol=accounting /dev/vdc1 /mnt/accounting
mount -o subvol=photos /dev/vdc1 /mnt/photos

In effect, all btrfs mounts are bind mounts, even without this option because
it's always implied.

A mount without -o subvol means "use the default subvolume" which out of the
box is an unnamed subvolume created at mkfs time, and can't be renamed or
deleted. It's also called the "top-level" of the file system. The subvolume
mounted by default can be changed using 'btrfs subvolume set-default', which
leads to the case of how do you get back to this invisible "top-level" file
system subvolume? These are equivalents:

mount -o subvol=/ /dev/vdc1 /mnt
mount -o subvolid=5 /dev/vdc1 /mnt
mount -o subvolid=0 /dev/vdc1 /mnt

This is why in the mount command at the top of ' solid-hardware5 details,
mounted multiple' attachment, you see 'subvolid=5,subvol=/'

Btrfs is a file system of btrees, they each have a number, and the original
design made the file btree id 5. And then soon after it was aliased to 0 to
make it easier to remember I guess. Anyway, all subsequently created subvolume,
and subvolume snapshots, are additional file btrees. Hence all of them can be
mounted in this fashion, depending on layout preference.
https://btrfs.wiki.kernel.org/index.php/SysadminGuide#Layout

But that doesn't really answer the question how to handle this case. All the
desktops have some variation on this problem, which suggests a need for a
common way to handle it if possible.

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