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Whilst working on the Reproducible Builds[0] effort, we noticed that fontconfig generates unreproducible cache files. This is due to fc-cache uses the modification timestamps of each directory in the "checksum" and "checksum_nano" members of the _FcCache struct. This is so that it can identify which cache files are valid and/or require regeneration. This patch changes the behaviour of the checksum calculations to prefer the value of the SOURCE_DATE_EPOCH[1] environment variable over the directory's own mtime. This variable can then be exported by build systems to ensure reproducible output. If SOURCE_DATE_EPOCH is not set or is newer than the mtime of the directory, the existing behaviour is unchanged. This work was sponsored by Tails[2]. [0] https://reproducible-builds.org/ [1] https://reproducible-builds.org/specs/source-date-epoch/ [2] https://tails.boum.org/
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