Having just installed gnome-desktop-3.26.2 I noticed that thumbnails weren't being generated. A file was being generated in .cache/thumbnails/fail/ but never under normal or large.
As best I could find, this is a gnome-desktop issue  which is resolved by installing a tool called bubblewrap (it's not clear why this isn't part of the configure script)?
Either way, installing the attached (to follow) ebuild (originally for an old EAPI from foo-overlay) resolves the issue and thumbnails are correctly generated and displayed in nautilus.
Created attachment 528228 [details]
Ebuild for bubblewrap, as required by gnome-desktop in order to generate thumbnails.
Sorry,  is the address in the URL field:
yeah, bubblewrap is something on top of libseccomp iirc; there was some upstream churn about it being hard requiring in some package for all linux, which was wrong as libseccomp isn't available for all linux supported architectures, and I believe that patch got accepted. So it's a bit confusing - I understood that without bwrap thumbnailing is just not sandboxed (but the build system would enforce sandboxing by default for linux on architectures that support seccomp) - but here you are seeing silent failures to thumbnail :(
Maybe this was in newer gnome-desktop versions than 3.26, or gdk-pixbuf bits of it.
The bug has been closed via the following commit(s):
Author: Mart Raudsepp <firstname.lastname@example.org>
AuthorDate: 2018-12-14 18:22:42 +0000
Commit: Mart Raudsepp <email@example.com>
CommitDate: 2018-12-14 19:01:44 +0000
gnome-base/gnome-desktop: bump to 3.26.2
Also force seccomp when available, or configure fails. This is not
patched to be optional on purpose - this kind of security is NOT
optional as far as I'm concerned.
Compared to gnome overlay version, this one:
* adds a big patchset for more sandboxed thumbnailing fixes,
memory leak fixes and other misc changes that don't break ABI
* patch to hopefully fix thumbnailing on non-seccomp architectures
(bwrap was still enabled, leading to broken runtime as bwrap
* RDEPENDs on sys-apps/bubblewrap with seccomp as required
* removes USE=dbus dep on glib
* revbumps over overlay revision to ensure upgrades
Signed-off-by: Mart Raudsepp <firstname.lastname@example.org>
Package-Manager: Portage-2.3.52, Repoman-2.3.11
gnome-base/gnome-desktop/Manifest | 2 +
.../gnome-desktop/gnome-desktop-3.26.2-r2.ebuild | 67 ++++++++++++++++++++++
profiles/base/package.use.force | 4 ++
3 files changed, 73 insertions(+)