Thanks for the reminder re: “candidate” channel Simos - I set it to that to pick up a bugfix a while ago when snap “helpfully” upgraded lxd behind my back and broke all my containers.
I had already tried the squashfuse package, in case that provided what was needed. All I got was a slightly different error message I’m afraid.
root@debian-outer:~# dpkg-query --list 'squash*' | grep ii
ii squashfs-tools 1:4.3-12 amd64 Tool to create and append to squashfs filesystems
ii squashfuse 0.1.103-1 amd64 FUSE filesystem to mount squashfs archives
root@debian-outer:~# snap install hello-world
error: system does not fully support snapd: cannot mount squashfs image using "fuse.squashfuse":
mount: /tmp/sanity-mountpoint-939393695: wrong fs type, bad option, bad superblock on
/tmp/sanity-squashfs-352129600, missing codepage or helper program, or other error.
I managed to capture a copy of the temporary squashfile it tried to open and manually mounted it in /tmp just to check
root@debian-outer:/tmp# squashfuse xsquash mountpoint
root@debian-outer:/tmp# ls mountpoint/
Of course, there doesn’t seem to be a debug/verbose option to snap.
However, I can get the same error message as it gives with this:
root@debian-outer:/tmp# mount -t fuse.squashfuse xsquash mountpoint/
mount: /tmp/mountpoint: wrong fs type, bad option, bad superblock on /tmp/xsquash, missing codepage or helper program, or other error.
Where it is getting that filesystem type from is a mystery to me though. I don’t see any reason to think it is supplied by the squash/fuse packages in debian.