CUPS Snap: Needs fontconfig for text filter, get "Fontconfig error: Cannot load default config file"

snapcraft
1

CUPS can print various file formats, by converting inpout files into PDF and this PDF into the printer’s native language, the conversion filters are provided by cups-filters. For plain text (ASCII, UTF-8) input files there is a texttopdf filter which generates a PDF file from plain text input. To do so it needs a standard, monospace font in case of latin letter text, some special fonts for greek, cyrillic, chinese, … The filter users fontconfig to find a suitable font in the system.

In the CUPS Snap this has to be performed inside a Snap, where one does not have access to all the systemś resources. So I decided to install the fonts into the CUPS Snap. With core18 this worked without problems, using this snapcraft.yaml.

Now I have switched to core20 and after some adjustments it is all working so far except the texttopdf filter which errors with

Fontconfig error: Cannot load default config file

in error_log when one tries to print a plain text file. The filter is called by cupsd when processing a plain text print job. I tried several things to fix this but did not succeed.

Here are the relevant parts of the new snapcraft.yaml:

name: cups
base: core20
[...]
environment:
  [...]
  FONTCONFIG_FILE: /snap/cups/current/etc/fonts/fonts.conf
  FONTCONFIG_PATH: /snap/cups/current/etc/fonts/conf.d
  FONTCONFIG_SYSROOT: /snap/cups/current/etc/fonts
[...]
apps:
  cupsd:
    command: scripts/run-cupsd
    [...]
    daemon: simple
    plugs: [network, network-bind, avahi-control, raw-usb, etc-cups]
[...]
parts:
  [...]
  cups-filters:
    [...]
    build-packages:
      [...]
      - libfontconfig1-dev
      [...]
    stage-packages:
      [...]
      - fonts-freefont-ttf
      - fonts-arphic-uming
      - fontconfig-config
    [...]
    override-prime: |
      set -eux
      perl -p -i \
           -e 's:<dir>\S*/usr/share/fonts</dir>:<dir>/snap/cups/current/usr/share/fonts</dir>:;' \
           -e 's:<cachedir>\S*/var/cache/fontconfig</cachedir>:<cachedir>/var/snap/cups/current/var/cache/fontconfig</cachedir>:;' \
           -e 's:<(cache|)dir>/usr/\S*</(cache|)dir>::;' \
           -e 's:<(cache|)dir>~\S*</(cache|)dir>::;' \
           $SNAPCRAFT_STAGE/etc/fonts/fonts.conf
      snapcraftctl prime
[...]

Especially I have put libfontconfig as build dependency and the fonts into the stage packages, so that they are installed in the Snap. I also have set the environment variables according to man fonts.conf and modified the $SNAP/etc/fonts/fonts.conf to pint to the font directories inside the Snap.

In addition, in run-cupsd, the script which fires up cupsd I have set the environment variables again:

export FONTCONFIG_FILE=$SNAP/etc/fonts/fonts.conf
export FONTCONFIG_PATH=$SNAP/etc/fonts/conf.d
export FONTCONFIG_SYSROOT=$SNAP/etc/fonts

What am I doing wrong?

I do not want to plug interfaces like “desktop” as I am not sure whether they pull in a lot of unneeded stuff (especially if the CUPS Snap is used on a headless server) or opens too many potential attack points.

Anyone has any experience?

2

Here is a font mapping file of texttopdf, /usr/share/cups/charsets/pdf.utf-8 in unsnapped CUPS, it is probably a format specific to texttopdf but it shows which fonts are used in which cases:

charset utf8

#
# This file defines the font mappings used for Unicode/UTF-8 text printing
# through PDF.
#
# Each line consists of:
#
#   first last direction width normal bold italic bold-italic
#
# First and last are the first and last glyphs in the font mapping
# that correspond to that font; contrary to PostScript printing
# they only select the font. To find the glyph the complete unicode
# character will be looked up in the (3,1) resp. (3,0) cmap of the 
# TrueType font. The glyph values are hexadecimal.
#
# Direction is the string "ltor" or "rtol", indicating left-to-right or
# right-to-left text.
#
# Width is the string "single" or "double"; double means that the glyphs
# are twice as wide as ASCII characters in the Courier typeface.
#
# "Normal", "bold", "italic", and "bold-italic" are the typefaces to use
# for each presentation.  If characters are only available in a single
# style then only one typeface should be listed (e.g. "Symbol")
#
# Each font that is listed will be used (and downloaded if needed) when
# printing.
#

0000 04FF ltor single monospace monospace:bold monospace:oblique monospace:bold:oblique
0500 05FF rtol single monospace
3000 9FFF ltor double ARPLUmingCN
3

As @kenvandine asked me yesterday to switch over to core20, to get longer support life mainly, I tried the switch and only got stuck with this problem. I told him today and he gave me the hint of using layouts. And this made it actually working!!

So the relevant lines in snapcraft.yaml look like this now:

name: cups
base: core20
[...]
layout:
  /etc/fonts:
    bind: $SNAP/etc/fonts
  /usr/share/fonts:
    bind: $SNAP/usr/share/fonts
  /var/cache/fontconfig:
    bind: $SNAP_DATA/var/cache/fontconfig
[...]
parts:
  [...]
  cups-filters:
    [...]
    build-packages:
      [...]
      - libfontconfig1-dev
      [...]
    stage-packages:
      [...]
      - fonts-freefont-ttf
      - fonts-arphic-uming
      - fontconfig-config
[...]

I will commit this right now …

Thank you very much, @kenvandine !!