This is really strange. I have to I agree. A fresh installation of 18.04.1 does not have the problem, but one that is updated does. Updating
evince is all it takes to make it not work.
Oddly, this doesn’t seem to make any significant difference (it only adds to the supported MIME types) to the
/usr/share/thumbnailers/evince.thumbnailer file. In fact, copying the original content back over doesn’t make for any difference.
Additionally, one should be able to
evince-thumbnailer /path/to/pdf /path/to/png and end up with a PNG thumbnail. This works fine in the live 18.04.1, but in the updated installed system, it fails silently with an odd exit code of 254 which I can’t find referenced in the code but I think the negative
return values might actually be what that refers to. Still doesn’t help that much. There’s also supposed to be
~/.cache/thumbnails which includes a
fail folder if there are issues. Neither it nor its parent are there.
There are plenty of bugs related to thumbnails but most of them seem to be crashes and not particularly relevant, either. And there doesn’t seem to be anything good upstream, either. Weird.
Looking at the changelog there appears to be a security bug that was fixed, making the thumbnailer more restricted. I think this is behind a different but related issue in Cosmic. The workaround there doesn’t help. It makes things worse, actually. Still, it smells like the problem would be around there somewhere.
Curiously, using the live Ubuntu system and upgrading
evince causes a fail, too, though with a different error. Exit code 255 and
Error opening directory "/usr/lib/x86_64-linux-gnu/evince/4/backends": Permission denied, which still reeks of that aforementioned issue, but the workaround again doesn’t help.
I need to investigate some more to figure out what is amiss.