BEAM files compiled from Erlang source files by erlc contain timestamps and the build path.

[[https://reproducible.debian.net/issues/timestamps_in_beam_files_issue.html|Known affected packages]]

= Detection =

`debbindiff` sees `.beam` files that differ shortly after the word "time":

{{{
 69 0000440: 0264 0004 7469 6d65 6806 6200 0007 de61  .d..timeh.b....a
 70 0000450: 0c61 0661 1361 3661 1568 0264 0006 736f  .a.a.a6a.h.d..so   
}}}

This is in the `Attr` chunk of the file, which (I believe) could be anywhere (this typically doesn't make it into the diff).

[[attachment:ejabberd-contrib_0.2014.09.22-1.debbindiff.html|Example debbindiff output]]

= Work-around =

None yet.

= Solution =

`erlc` supports a `+slim` option which happens to omit the build time.  However, it also drops a load of debug information (line number table, ...) from the file.  Also, it's totally undocumented.  This doesn't seem ideal.

[[http://erlang.org/pipermail/erlang-questions/2015-January/082699.html|Question to upstream about usage of the 'time' attribute]]

Alternatively: Patch `erlc` to honour `DEB_BUILD_DATE`, and make sure that's set?

Patch author (ChrisWest) doesn't really know Erlang:
 * There's no date parsing code, so let's do that outside?  This timestamp is UTC so need to be careful with timezones.
 * `{match, ... re:run` will exit the compiler if it doesn't match, which is probably what's wanted.
 * Old behaviour is maintained if the environment variable isn't set
 * erlang isn't whitespace sensitive so the totally bizarre tab/space mix isn't too worrying

{{{
--- a/lib/compiler/src/beam_asm.erl	2015-01-17 19:05:53.283257792 +0000
+++ b/lib/compiler/src/beam_asm.erl	2015-01-17 19:25:41.000000000 +0000
@@ -225,8 +225,14 @@
 build_attributes(Opts, SourceFile, Attr, MD5) ->
     Misc = case member(slim, Opts) of
 	       false ->
-		   {{Y,Mo,D},{H,Mi,S}} = erlang:universaltime(),
-		   [{time,{Y,Mo,D,H,Mi,S}},{source,SourceFile}];
+		    case os:getenv("DEB_BUILD_DATE") of
+		        false ->
+		            {{Y,Mo,D},{H,Mi,S}} = erlang:universaltime();
+		        Date ->
+		            {match, L} = re:run(Date, "(.*)-(.*)-(.*)T(.*):(.*):(.*)", [{capture, all_but_first, list}]),
+		            [Y,Mo,D,H,Mi,S] = lists:map(fun erlang:list_to_integer/1, L)
+		    end,
+		    [{time,{Y,Mo,D,H,Mi,S}},{source,SourceFile}];
 	       true -> []
 	   end,
     Compile = [{options,Opts},{version,?COMPILER_VSN}|Misc],
}}}

With patch: 
{{{
% ./erlc a.erl && sha1sum a.beam
9f050dc1b19f1fb3dc6cf5ec683d2f5dfd4099b8  a.beam
% ./erlc a.erl && sha1sum a.beam
041f6bb7492aae9331f82afbc39297659497c117  a.beam
% DEB_BUILD_DATE=2014-01-01T13:33:38 ./erlc a.erl && sha1sum a.beam
ee77fcb458195e99fd6d005c0bb06f767db6d1f0  a.beam
% DEB_BUILD_DATE=2014-01-01T13:33:38 ./erlc a.erl && sha1sum a.beam
ee77fcb458195e99fd6d005c0bb06f767db6d1f0  a.beam
% DEB_BUILD_DATE=2014-01-01T13:33:37 ./erlc a.erl && sha1sum a.beam
d9f323277ac1781a32bde5353b1d17770231274d  a.beam
% DEB_BUILD_DATE=a ./erlc a.erl && sha1sum a.beam
a.erl: internal error in beam_asm;
crash reason: {badmatch,nomatch}

  in function  beam_asm:build_attributes/4 (beam_asm.erl, line 232)
  in call from beam_asm:build_file/8 (beam_asm.erl, line 152)
  in call from beam_asm:module/4 (beam_asm.erl, line 30)
  in call from compile:beam_asm/1 (compile.erl, line 1320)
  in call from compile:'-internal_comp/4-anonymous-1-'/2 (compile.erl, line 292)
  in call from compile:fold_comp/3 (compile.erl, line 310)
  in call from compile:internal_comp/4 (compile.erl, line 294)
  in call from compile:'-do_compile/2-anonymous-0-'/2 (compile.erl, line 153)
% 
}}}

Simplified patch filed as [[https://bugs.debian.org/795834]]