This bug is related to https://bugs.launchpad.net/ubuntu/+source/gnome-desktop3/+bug/1795668 and to the question in Ubuntu Stackexchange https://askubuntu.com/questions/1088539/custom-thumbnailers-don-t-work-on-ubuntu-18-10-and-18-04
The recent hardening on thumbnailers by using bubblewrap is causing a problem with user-installed thumbnailers. Consider this (for example) thumbnailer to add a small Dilbert icon to the GIMP .xcf files:
#!/bin/bash
tmpfile1=$(tempfile --directory /tmp --prefix xcft --suffix .png)
trap 'rm -rf $tmpfile1' INT EXIT TERM
die() {
echo >&2 "$@"
rm -rf $tmpfile1
exit 1
}
# The script will be called with parameters : -s %s %u %o
# -s %s: vertical size, %u: input url, %o: output file
if [ $1 = "-s" ]; then
size=$2
uin=$3
fout=$4
else
size=32 #default size
uin=$1
fout=$2
fi
# uin es una url, eliminar "file://"
fin="${uin#file://*}"
/usr/bin/gnome-xcf-thumbnailer -s $size "$fin" $tmpfile1 || die "gnome-xcf-thumbnailer failed"
/usr/bin/composite-im6.q16 -gravity southeast \
\( -resize $((size/2))x$((size/2)) /usr/share/gimp/2.0/images/wilber.png \) \
\( -resize $sizex$size $tmpfile1 \) "$fout" || die "convert failed"
rm -rf $tmpfile1
exit 0
It used to work before the last update to use bubblewrap, and now doesn't work anymore. I tried to dig into https://github.com/GNOME/gnome-desktop/blob/master/libgnome-desktop/gnome-desktop-thumbnail-script.c to see what happens, and I concluded that the "reinforced" thumbnail generator do this:
mkdir /tmp/temporary
bwrap --ro-bind /usr /usr \
--ro-bind /etc/ld.so.cache /etc/ld.so.cache \
--ro-bind /bin /bin \
--ro-bind /lib64 /lib64 \
--ro-bind /lib /lib \
--ro-bind /sbin /sbin \
--proc /proc \
--dev /dev \
--chdir / \
--setenv GIO_USE_VFS local \
--unshare-all \
--die-with-parent \
--bind /tmp/temporary /tmp \
--ro-bind /home/romano/software/thumbnailers/test/rgttiwm.xcf /tmp/gnome-desktop-file-to-thumbnail.xcf \
/usr/bin/xcf-thumb -s 128 file:///tmp/gnome-desktop-file-to-thumbnail.xcf /tmp/gnome-desktop-thumbnailer.png
#
cp /tmp/temporary/gnome-desktop-thumbnailer.png test.png
...and this way, my thumbnailer did succeed in creating the PNG with the correct preview. But when called internally by gnome, it fails, and creates a strange "~/.cache/thumbnailer/fail" file containing the following:
�PNG
IHDR ĉd�EtEXtThumb::URIfile:///home/romano/software/thumbnailers/test/rgttiwm.xcf����tEXtThumb::MTime1380735765�y�O tEXtSoftwareGNOME::ThumbnailFactoryo
[...]
Notice anyway that one of the problems was that originally the thumbnailer called
/usr/
which, do to the alternative setup in .deb packages, was a link to /etc/alternativ