Custom thumbnailers don’t work on Ubuntu 18.10 and 18.04

I'm using 18.04, but after 2018.11.28 update I had same issue. It is caused by sandboxing thumbnailers with bubblewrap (it cost me 2 days of research!!!).

So I downgraded gnome-desktop3-data, gir1.2-gnomedesktop-3.0, libgnome-desktop-3-17 (not sure if all 3 needed) and then removed bubblewrap.

It's a dirty hack and it weakens the security, but I don't care...

Maybe it will be not so easy on 18.10.

I hope somebody can write better solution...

I've gone thru the same trouble with some of my external thumbnailers.

After some researches, I realised that bubblewarp call fails under Ubuntu and Debian based distros because of --symlink option on /bin and /sbin. In fact under these distros, /bin and /usr/bin are not merged. So, both should be declared with --ro-bind instead of --symlink.

To make bwrap call from Nautilus work under Ubuntu, you need to replace --symlink calls with --ro-bind calls for /bin and /sbin

To solve speed issue with thumbnailers using imagemagick tools, you also need to add a --ro-bind for /etc/alternatives and /var/cache/fontconfig

So, simplest solution is to place a wrapper script under /usr/local/bin/bwrap which will do the job and then call original /usr/bin/bwrap. As /usr/local/bin is before /usr/bin in the PATH, it is transparent for Nautilus.

Wrapper script is available on my Github repo

I've written a small article to explain all into details and to provide wrapper script installation instruction. It has been tested under 18.04 and 18.10

http://bernaerts.dyndns.org/linux/360-ubuntu-nautilus-external-thumbnailer-failure

Hope it helps