Notice anyway that one of the problems was that originally the
thumbnailer called

     /usr/bin/composite

which, do to the alternative setup in .deb packages, was a link to
/etc/alternatives/composite that pointed back to the real binary;
probably the "/etc/alternatives/" directory should be added to the bind-
ed directories...

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https://bugs.launchpad.net/bugs/1808702

Title:
  Bubblewrap integration broke user thumbnailers

Status in gnome-desktop3 package in Ubuntu:
  New

Bug description:
  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
  
  
IHDRĉd�EtEXtThumb::URIfile:///home/romano/software/thumbnailers/test/rgttiwm.xcf����tEXtThumb::MTime1380735765�y�O
 tEXtSoftwareGNOME::ThumbnailFactoryo 
  [...]

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