[erlang-questions] reltool: sys-level {mod_cond, derived}

[Tuncer Ayaz]

As discussed previously[1] it was decided to make the default
rebar reltool.config template safer to use by removing[2] the
non-default sys-level mod_cond.

While this doesn't produce releases with potentially missing
modules, that one-line patch also pulls in many seemingly unused
applications resulting in a release that's at least twice as big:

With sys-level {mod_cond, derived} (aka non-default settings):
$ ls -1 rel/exemplar/lib
compiler-4.9.1
crypto-2.3
erts-5.10.1
exemplar
hipe-3.10.1
kernel-2.16.1
sasl-2.3.1
stdlib-1.19.1
tools-2.6.10

Without sys-level {mod_cond, derived} (aka default settings):
$ g diff
diff --git a/rel/reltool.config b/rel/reltool.config
index 2dd47be..e65c53c 100644
--- a/rel/reltool.config
+++ b/rel/reltool.config
@@ -19,7 +19,6 @@
        {boot_rel, "exemplar"},
        {profile, embedded},
        {incl_cond, derived},
-       {mod_cond, derived},
        {excl_archive_filters, [".*"]}, %% Do not archive built libs
        {excl_sys_filters, ["^bin/.*", "^erts.*/bin/(dialyzer|typer)",
                            "^erts.*/(doc|info|include|lib|man|src)"]},

$ ls -1 rel/exemplar/lib
asn1-2.0.1
compiler-4.9.1
crypto-2.3
edoc-0.7.12
erts-5.10.1
et-1.4.4.3
exemplar
gs-1.5.15.2
hipe-3.10.1
inets-5.9.4
kernel-2.16.1
mnesia-4.8
observer-1.3
public_key-0.18
runtime_tools-1.8.10
sasl-2.3.1
ssl-5.2.1
stdlib-1.19.1
syntax_tools-1.6.11
tools-2.6.10
webtool-0.8.9.2
wx-1.0
xmerl-1.3.3

To reproduce, follow the steps found in
https://github.com/rebar/rebar/wiki/Release-handling
and make sure to delete the sys-level mod_cond.

If you want to keep the sys-level {mod_cond, derived} and use the
nodetool escript, remember to add the following line:
{app, compiler, [{mod_cond, app}]},
Otherwise reltool might not include[3] some compiler modules which are
used due to the `-mode(compile)` directive. As I don't think the
nodetool escript has anything to gain by being compiled, I've posted a
patch to delete[4] the directive.

So, the question is, is this expected reltool behaviour? If so,
is this really desired or not possibly including much more than
is actually used. Shouldn't the sys-level {incl_cond, derived}
make sure only used applications are included (regardless of
mod_cond)?

[1] http://erlang.org/pipermail/erlang-questions/2012-December/071135.html
[2] https://github.com/rebar/rebar/pull/43
[3] https://github.com/rebar/rebar/issues/64#issuecomment-15998750
[4] https://github.com/rebar/rebar/pull/80