[tex-live] Please default \pdftrailerid{} to SOURCE_DATE_EPOCH if exported

[Chris Lamb]

Hi tex-live,

Please consider defaulting the \pdftrailerid macro to the value of
SOURCE_DATE_EPOCH if this is exported in the environment.

Whilst we can get a reproducible PDF output in some situations, the
ordering of the elements in these IDs varies. For example:

  -/ID [<A979E943C51DC1AAE48AAFF27BBA5A0D> <EA2B7DBF37C8E3DC2AA0AFDAA1FF4E0C>]
  +/ID [<EA2B7DBF37C8E3DC2AA0AFDAA1FF4E0C> <A979E943C51DC1AAE48AAFF27BBA5A0D>]

This varies depending on the *path* from which you are building,
presumably due to the default pdftrailerid being seeded from the
full path name.

Using SOURCE_DATE_EPOCH would make sense here as it the old behaviour
is unchanged. Another alternative could be to set the default to the
empty string if SOURCE_DATE_EPOCH is exported.

I believe this is the code that actually prints these tokens:

  http://sources.debian.net/src/texlive-bin/2017.20170613.44572-
2/texk/web2c/pdftexdir/pdftex.web/#L20126-L20129

(I would provide a patch but my WEB is very very rusty...)


Best wishes,

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