[Reproducible-builds] Bug#795834: erlang: please make the output of erlc reproducible

[Chris Lamb]

Source: erlang
Version: 1:18.0-dfsg-1
Severity: wishlist
Tags: patch
User: reproducible-builds at lists.alioth.debian.org
Usertags: timestamps toolchain
X-Debbugs-Cc: reproducible-builds at lists.alioth.debian.org

Hi,

While working on the "reproducible builds" effort [1], we have noticed
that erlc generates .beam files that include the current timestamp:

  $ echo '-module(test).' > test.erl
  $ while sleep 1; do erlc test.erl && sha1sum test.beam; done
  c82bb744a4665e34bb5cdc320bcc584ed25740b4  test.beam
  7214b49ebc395c5c14df6fc59ac124fa0098fb5e  test.beam
  7353ca62788e505ba5565d4d90daceda1883cc84  test.beam
  ^C                                                                     

The attached patch modifies erlc to optionally use the
SOURCE_DATE_EPOCH environment variable (which we set to the latest entry
from debian/changelog). If the variable is not set or is invalid we
fall back to the original behaviour of using the current UNIX timestamp:

  $ export SOURCE_DATE_EPOCH=1234
  $ while sleep 1; do erlc test.erl && sha1sum test.beam; done
  23da6b9ffae248392455dfa629ab5f3a12846fd0  test.beam
  23da6b9ffae248392455dfa629ab5f3a12846fd0  test.beam
  23da6b9ffae248392455dfa629ab5f3a12846fd0  test.beam
  ^C

  $ export SOURCE_DATE_EPOCH=1234_WHOOPS_BROKEN
  $ while sleep 1; do erlc test.erl && sha1sum test.beam; done
  58f9aec98a3ae9827743ce8ff0785337e4369cf7  test.beam
  11f20e425e546b184fc545961c7d791a218d4df9  test.beam
  63577de11becbcd048a6cd5954e9294b7a3bdaf7  test.beam
  ^C

The patch is based on an original version by Chris West (Faux).

Once applied, erlang packages that ship .beam files can be built
reproducibly in our reproducible toolchain. 

 [1]: https://wiki.debian.org/ReproducibleBuilds


Regards,

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