Enhance rebar.config.script for dev only dependencies

ibrowse and meck are only used for testing and should not be imposed
as dependencies on projects using webmachine.

rebar.config.script is used to support a dev_only_deps key in
rebar.config. The mechanism is triggered by a marker file DEV_MODE
written to the top-level of the project directory. If found, the
dev_only_deps are merged into deps. The project's Makefile is modified
to create the DEV_MODE file. Those doing dev work on webmachine can
continue to run builds via standard make commands. Projects pulling in
a dep on webmachine via rebar will receive just the needed bits.

.gitignore

@@ -4,3 +4,4 @@ deps/*
 ebin/*.app
 doc/*
 .local_dialyzer_plt
+/.rebar

Makefile

@@ -11,14 +11,17 @@ all: deps compile
 compile: deps
 	./rebar compile
 
-deps:
+deps: DEV_MODE
 	@(./rebar get-deps)
 
 clean:
 	@(./rebar clean)
 
-distclean: clean
-	@(./rebar delete-deps)
+# nuke deps first to avoid wasting time having rebar recurse into deps
+# for clean
+distclean:
+	@rm -rf deps ./rebar/DEV_MODE
+	@(./rebar clean)
 
 edoc:
 	@$(ERL) -noshell -run edoc_run application '$(APP)' '"."' '[{preprocess, true},{includes, ["."]}]'
@@ -34,3 +37,6 @@ verbosetest: all
 travisupload:
 	tar cvfz ${ARTIFACTSFILE} --exclude '*.beam' --exclude '*.erl' test.log .eunit
 	travis-artifacts upload --path ${ARTIFACTSFILE}
+
+DEV_MODE:
+	@touch ./.rebar/DEV_MODE

rebar.config

@@ -6,7 +6,10 @@
 {xref_checks, [undefined_function_calls]}.
 
 {deps,
- [{mochiweb, "2.9.0", {git, "git://github.com/basho/mochiweb.git", {tag, "v2.9.0"}}},
-  {meck, "0.8.2", {git, "git://github.com/basho/meck.git", {tag, "0.8.2"}}},
-  {ibrowse, "4.0.*", {git, "git://github.com/cmullaparthi/ibrowse.git", {tag, "v4.0.2"}}}
+ [{mochiweb, "2.9.0", {git, "git://github.com/basho/mochiweb.git", {tag, "v2.9.0"}}}
+ ]}.
+
+{dev_only_deps,
+ [{meck, "0.8.2", {git, "git://github.com/basho/meck.git", {tag, "0.8.2"}}},
+  {ibrowse, "4.0.2", {git, "git://github.com/cmullaparthi/ibrowse.git", {tag, "v4.0.2"}}}
  ]}.

rebar.config.script

@@ -1,19 +1,68 @@
 %% -*- mode: erlang;erlang-indent-level: 4;indent-tabs-mode: nil -*-
 %% ex: ft=erlang ts=4 sw=4 et
+OtpVersion = erlang:system_info(otp_release),
+Config1 = case hd(OtpVersion) =:= $R andalso OtpVersion =< "R15B01" of
+              true  ->
+                  HashDefine = [{d,old_hash}],
+                  case lists:keysearch(erl_opts, 1, CONFIG) of
+                      {value, {erl_opts, Opts}} ->
+                          lists:keyreplace(erl_opts,1,CONFIG,{erl_opts,Opts++HashDefine});
+                      false ->
+                          CONFIG ++ [{erl_opts, HashDefine}]
+                  end;
+              false -> CONFIG
+          end.
 
-case erlang:system_info(otp_release) of
-     "R"++_=RVersion ->
-        case RVersion =< "R15B01" of
-            true  ->
-                HashDefine = [{d,old_hash}],
-                case lists:keysearch(erl_opts, 1, CONFIG) of
-                    {value, {erl_opts, Opts}} ->
-                        lists:keyreplace(erl_opts,1,CONFIG,{erl_opts,Opts++HashDefine});
-                    false ->
-                        CONFIG ++ [{erl_opts, HashDefine}]
-                end;
-            false -> CONFIG
-        end;
-    _ ->
-        CONFIG
+%% Merge the list values in `ToAdd' into the list found at key `Key'
+%% in proplist `C'. Don't duplicate items. New Items are added to the
+%% front of existing items. It is an error if the value at `Key' is
+%% not a list in `C'.
+MergeConfig = fun({Key, ToAdd}, C) ->
+                      case lists:keyfind(Key, 1, C) of
+                          false ->
+                              lists:keystore(Key, 1, C, {Key, ToAdd});
+                          {Key, List} when is_list(List) ->
+                              %% remove items in ToAdd already in List
+                              ToAdd1 = [ I || I <- ToAdd, not lists:member(I, List) ],
+                              lists:keystore(Key, 1, C, {Key, ToAdd1 ++ List })
+                      end
+              end.
+
+%% -- Add development only options if we are a top-level build  --
+%%
+%% If a file named `./.rebar/DEV_MODE' exists, we assume we are a
+%% top-level build (not being built as a dep of another project). We
+%% add the deps from dev_only_deps defined in rebar.config, add
+%% concrete as a dep, and define the compiler macro DEV_ONLY.
+
+%% This macro can be used to conditionally enable code (e.g. tests)
+%% that depend on development only dependencies.
+ErlOpts = {erl_opts, [
+    {d, 'DEV_ONLY'}
+]},
+
+%% Development only dependencies can be specified in the main
+%% rebar.config. This file should not need to be edited directly.
+DevOnlyDeps = case lists:keyfind(dev_only_deps, 1, CONFIG) of
+                  false ->
+                      [];
+                  {dev_only_deps, DOD} ->
+                      DOD
+              end,
+
+Deps = {deps, DevOnlyDeps},
+
+ConfigPath = filename:dirname(SCRIPT),
+DevMarker = filename:join([ConfigPath, ".rebar", "DEV_MODE"]),
+_ = filelib:ensure_dir(DevMarker),
+case filelib:is_file(DevMarker) of
+    true ->
+        lists:foldl(fun(I, C) -> MergeConfig(I, C) end,
+                    Config1, [Deps, ErlOpts]);
+    false ->
+        %% If the ./.rebar/DEV_MODE marker is not present, this script simply
+        %% returns the config specified in rebar.config. This will be
+        %% the behavior when the project is built as a dependency of
+        %% another project.
+        Config1
 end.