[Reproducible-builds] Usage of the 'time' attribute in BEAM files

[Jérémy Bobbio]

Hi!

We are a group of Debian contributors working on “reproducible
builds” [1]: compiling a source package should always result in the same
binaries for a given build environment.

Our tests have shown one difference [2] in Erlang BEAM files produced by
the compiler. The time of the build is recorded in the 'time' attribute.

How important is this attribute?

Chris West has noticed that when using the `+slim` option, it did not
get written to the output [3]. Would it make sense to add a
`+deterministic` build option [4] that would do the same?

 [1]: https://wiki.debian.org/ReproducibleBuilds
 [2]: https://wiki.debian.org/ReproducibleBuilds/TimestampsInBeamFiles?action=AttachFile&do=get&target=ejabberd-contrib_0.2014.09.22-1.debbindiff.html
 [3]: http://sources.debian.net/src/erlang/1:17.3-dfsg-3/lib/compiler/src/beam_asm.erl/?hl=225:233#L225
 [4]: This is inspired by binutils. See the `D` option of `ar` for
      an example: http://manpages.debian.org/ar

Thanks,
-- 
Lunar                                .''`. 
lunar at debian.org                    : :Ⓐ  :  # apt-get install anarchism
                                    `. `'` 
                                      `-   
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