apt purge snapd fails in LXD (on amd64)

[Balint Reczey]

I have started an LXD container and tried removing snapd:
$ lxc launch ubuntu-daily:groovy snapd-postrm
Creating snapd-postrm
Starting snapd-postrm
$ lxc shell snapd-postrm
root@snapd-postrm:~# apt purge snapd
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages were automatically installed and are no longer required:
  libfreetype6 squashfs-tools
Use 'apt autoremove' to remove them.
The following packages will be REMOVED:
  snapd*
0 upgraded, 0 newly installed, 1 to remove and 0 not upgraded.
After this operation, 105 MB disk space will be freed.
Do you want to continue? [Y/n]
(Reading database ... 31149 files and directories currently installed.)
Removing snapd (2.44.3+20.04) ...
Stopping snap.lxd.activate.service
Stopping unit snap.lxd.activate.service
Waiting until unit snap.lxd.activate.service is stopped [attempt 1]
snap.lxd.activate.service is stopped.
Stopping snap.lxd.daemon.service
Stopping unit snap.lxd.daemon.service
Waiting until unit snap.lxd.daemon.service is stopped [attempt 1]
snap.lxd.daemon.service is stopped.
Stopping snap.lxd.daemon.unix.socket
Stopping unit snap.lxd.daemon.unix.socket
Waiting until unit snap.lxd.daemon.unix.socket is stopped [attempt 1]
snap.lxd.daemon.unix.socket is stopped.
dpkg: warning: while removing snapd, unable to remove directory '/snap': Device or resource busy - directory may be a mount point?
Processing triggers for man-db (2.9.1-1) ...
Processing triggers for mime-support (3.64ubuntu1) ...
(Reading database ... 31079 files and directories currently installed.)
Purging configuration files for snapd (2.44.3+20.04) ...
Stopping snap.lxd.activate.service
Stopping unit snap.lxd.activate.service
Waiting until unit snap.lxd.activate.service is stopped [attempt 1]
snap.lxd.activate.service is stopped.
Removing snap.lxd.activate.service
Stopping snap.lxd.daemon.service
Stopping unit snap.lxd.daemon.service
Waiting until unit snap.lxd.daemon.service is stopped [attempt 1]
snap.lxd.daemon.service is stopped.
Removing snap.lxd.daemon.service
Stopping snap-core18-1754.mount
Stopping unit snap-core18-1754.mount
Waiting until unit snap-core18-1754.mount is stopped [attempt 1]
snap-core18-1754.mount is stopped.
Removing snap core18 and revision 1754
Removing snap-core18-1754.mount
Stopping snap-lxd-14954.mount
Stopping unit snap-lxd-14954.mount
Waiting until unit snap-lxd-14954.mount is stopped [attempt 1]
snap-lxd-14954.mount is stopped.
Removing snap lxd and revision 14954
Stopping unit snap.lxd.daemon.unix.socket
Waiting until unit snap.lxd.daemon.unix.socket is stopped [attempt 1]
snap.lxd.daemon.unix.socket is stopped.
Stopping unit snap.lxd.daemon.unix.socket
Waiting until unit snap.lxd.daemon.unix.socket is stopped [attempt 1]
snap.lxd.daemon.unix.socket is stopped.
Removing snap-lxd-14954.mount
Stopping snap-snapd-7264.mount
Stopping unit snap-snapd-7264.mount
Waiting until unit snap-snapd-7264.mount is stopped [attempt 1]
snap-snapd-7264.mount is stopped.
Removing snap snapd and revision 7264
Removing snap-snapd-7264.mount
Final directory cleanup
rm: cannot remove '/snap': Device or resource busy
dpkg: error processing package snapd (--purge):
 installed snapd package post-removal script subprocess returned error exit status 1
Errors were encountered while processing:
 snapd
E: Sub-process /usr/bin/dpkg returned an error code (1)
root@snapd-postrm:~# echo $?
100

I noticed the failure first when building LXD autopkgtest backend images:

autopkgtest-build-lxd ubuntu-daily:groovy
...
Removing snap snapd and revision 7264
Removing snap-snapd-7264.mount
Final directory cleanup
rm: cannot remove '/snap': Device or resource busy
dpkg: error processing package snapd (--purge):
 installed snapd package post-removal script subprocess returned error exit status 1
...

[Launchpad Janitor (janitor)]

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

[Johan Ehnberg (johan-ehnberg)]

I am seeing this as well. A workaround is to remove all snaps but core, then unmount the snap core before purging with apt.

[Thiago Martins (martinx)]

I can also confirm this too! Here is a list of problems that I'm seeing with snap inside LXD containers:

https://askubuntu.com/questions/1288779/the-snapd-package-fails-to-upgrade-inside-of-ubuntu-20-04-lxd-container

At first, "apt update ; apt full-upgrade" fails... Then, I thought, let's remove this `snapd` thing. Nop, doesn't work either!

Here is the error when I try `apt remove --purge snapd` inside of an LXD Container:

https://pastebin.com/TTj4Z4Jp

Johan, do you mind sharing how are you removing snapd out from your containers?

Thanks!

[Johan Ehnberg (johan-ehnberg)]

In our case we decided to spend the same effort on orchestrating for images:ubuntu/focal/cloud (which comes without snapd) rather than working around issues with snap on the thicker default image.

But the workaround above can be used, and I remember it was documented elsewhere on launchpad or maybe askubuntu. Going back on our git I can see the following (YMMV):

# Remove snapd
snap remove lxd
snap remove core18
snap remove snapd
umount /snap/
rm -rf ~/snap /snap /var/snap /var/lib/snapd
apt-get purge snapd

[Thiago Martins (martinx)]

Thanks Johan! But this procedure failed too.

I'll try the images without snapd!

But, currently, I'm using an image called just "focal", which comes with this creepy snapd. I can't find an image called "ubuntu/focal/cloud"...

Which source are you using for your images? I'm using "https://cloud-images.ubuntu.com/releases" simplestreams.

Thanks again!

[Thiago Martins (martinx)]

Never mind! Found it at https://images.linuxcontainers.org! No more snapd!! Wheee!!