Use BindsTo dependency for mounting Removable storage devices (i.e USB)

[RushingAlien]

Description

Currently mount units has a Requires dependency against the block device or backing file specified under What. This is not desirable for usecases where backing file or block device can be easily detached, which may cause lingering mounts on some filesystems (i.e BTRFS). Examples are USB storage, NBDs, and loop devices.

I have seen this myself because I use BTRFS for my USB drive, where accidental detaching causes a lingering mount.

Stuff outside of fstab are given ephemeral mount units and as I am aware has no way to set a BindsTo directive other than mounting via systemd-mount. Or by adding mount unit overrides.

For stuff in fstab you can use x-systemd.device-bound in the mount option.

Desired Outcome

For udisks to mount usb devices with a BindsTo relationship against the block device.

Note on Lingering Mounts

Not all filesystems are susceptible to lingering mounts
filesytems susceptible to lingering mounts (mounted, but block device is disconnected)

• btrfs
• any filesystem using nbd

I believe there are more, but those are just the ones I know.

Related

systemd/systemd#36634
https://www.freedesktop.org/software/systemd/man/latest/systemd.mount.html
https://www.freedesktop.org/software/systemd/man/latest/systemd-mount.html

[RushingAlien]

Not all filesystems are susceptible to lingering mounts
filesytems susceptible to lingering mounts (mounted, but block device is disconnected)

• btrfs
• any filesystem using nbd

Now that i think about it, since this is on a filesytem case, I think udisks should just have x-systemd.device-bound as a default mount option for offending filesystems, like btrfs, so we should have a mega list of affected filesystems.

I've set x-systemd.device-bound for btrfs in my /etc/udisks2/mount-options.conf

[RushingAlien]

systemd/systemd#36634 (comment)

Oh perhaps udisks could use systemd-mount --discover probing? It can probe if the backing device is a removable storage, and applies a BindsTo dependency against said backing dev for the mount unit.

[tbzatek]

Cleaning up lingering mounts after its backing block device disappears is done in the UDisksState cleanup thread. If that doesn't work, it should be fixed. UDisks doesn't interact with an init system regarding mounts.