USB disks and SD cards mount for wrong user

[Chris]

I have MATE 20.04 LTS installed on a laptop with 4 users. Plugging in SD cards and USB sticks works perfectly, unless there is someone else logged in on the machine.
eg:
1- user A logs in, then walks away and screen locks
2- user B switches to the login screen and logs in as user B
3- user B plugs in a USB stick, but it doesn't mount for user B
4- (hidden from sight) user A gets an "authentication is required to mount... /dev/sdb1" error and the window is asking for admin password
- details of this are:
polkit.subject-pid: 102318
polkit.caller-pid: 806
Action: org.freedesktop.udisks2.filesystem-mount-other-seat
Vendor: The Udisks Project

user B has no indication on why it didn't mount. If they click on the USB stick in the Caja side bar, they get an "Unable to mount USB Disk - an operation is already pending"

So it seems that automount is trying to show the USB stick to the inactive user A, totally blocking user B. (user B doesn't even get the chance to enter an admin password to override)

the only solution is basically to "switch account", log in as user A and log them out.
(this clearly doesn't work if user B doesn't know their password!)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Gordon N. Squash (thesquash)]

For the record, this is not specific to Ubuntu or Ubuntu MATE at all -- I see this on Gentoo regularly, for example. It seems to be caused by a race condition in Caja and UDisks which didn't appear until UDisks started mounting volumes under user-specific subdirectories of `/media` -- so for example `user1` has volumes mounted under `/media/user1` and `user2` has volumes mounted under `/media/user2`. In ye olden days, `user1`'s UDisks-utilizing process (such as Caja) would mount the volume; `user2`'s instance of Caja would wait for the volume to get mounted by `user1`'s Caja, then open the shared mountpoint at `/media/[volume]`. Nowadays this doesn't work in any case, since `user1` puts in the request to mount the volume first and the volume is mounted under `user1`'s private mount directory -- `/media/user1/[volume]`. I'm not sure how to fix this yet, though. Maybe (but just a WAG) UDisks needs some kind of integration with the display manager or logind?

[Chris (chrisonbuntu)]

Thanks Gordon, I can remember seeing the issue when running Linux Mint back at version 17 - can't remember if it was under MATE (Caja) or Cinnamon (nemo).

Can we as udisk to mount physical media in /media/[volume] and not in the user-specific sub folders? For local users, that would be totally acceptable (and even useful) to be able to share access to USB sticks while plugged in.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.