Multiple mounts over a single filesystem (on a single block object)

[tbzatek]

(Note: created as a result of my attempt to call bd_fs_unmount() over a block device path instead of a mount point path)

The org.freedesktop.UDisks2.Filesystem interface contains a MountPoints property - an array of paths/mount points this block device is mounted on. This properly reflects the state where filesystem can be mounted multiple times, either natively (for filesystems that supports multiple mounts) or by bind mounting.
(TODO: check rbind and --make-slave, --make-private, --make-unbindable mount options)

Question: let's have a filesystem that supports mounting subvolumes/snapshots via mount options - would that create a new block device or would we stack it all on a single block object? Hints: btrfs, zfs, stratis?! ...

The org.freedesktop.UDisks2.Filesystem.Unmount() method takes no specific mount point argument and as long as it's attached to a particular block device, it makes sense to unmount all mounts listed in the MountPoints property. For unmounting a specifc mount point we should either think about extending the options argument of the Unmount() method or adding a new method call.

For multiple mounts support I guess we also need to extend UDisksState matching as well.

edit: Don't make me start on supporting mount namespaces...