[Fontconfig] [PATCH] Please make the output of cache files reproducible

[Chris Lamb]

Hi,

Whilst working on the Reproducible Builds[0] effort, we noticed that
fontconfig — in its usual usage — generates un-reproducible cache files.

This is due to it using the timestamps of each directory in the
"checksum" member of the _FcCache struct. This is so that it can identify
which cache files remain valid and/or require regeneration.

Whilst we could zero-out the checksum, we would actually need it still
function correctly. Therefore, I've attached a patch that adds an option
to list all the configured font directories; that way, distributions,
etc. can modify the mtimes of these files in advance

For example, I'm about to propose the following to Debian's fontconfig
"postinst" script:

  if [ -n "$SOURCE_DATE_EPOCH" ]; then
    fc-cache -s --list-dirs | \
      xargs -I{} find {} -type d -follow -newermt "@$SOURCE_DATE_EPOCH" -print0 2>/dev/null | \
      xargs -0r touch --date="@$SOURCE_DATE_EPOCH"
  fi

… and then we make the usual calls to, for example:

  $ fc-cache -s -f -v


(This work was sponsored by Tails[1].)


 [0] https://reproducible-builds.org/
 [1] https://tails.boum.org/


Regards,

-- 
      ,''`.
     : :'  :     Chris Lamb
     `. `'`      lamby at debian.org / chris-lamb.co.uk
       `-
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