Parse transforms and behaviours are compiled first

The previous code in rebar that was trying to ensure that parse
transforms and behaviours were compiled first doesn't work with multiple
compiler workers because of the possiblity of one of the workers
compiling a file that needs a parse transform or a behaviour at the same
time another worker is compiling that same parse transform or behaviour.

The solution this patch implements is to append any parse transforms and
any behaviours (in that order) to erl_first_files to ensure that they
are compiled before any regular files. This patch won't break any
currently working uses of erl_first files because we only append to the
list, so anything in erl_first_files is still compiled before anything
else.

src/rebar_erlc_compiler.erl

@@ -110,17 +110,27 @@ doterl_compile(Config, OutDir, MoreSources) ->
     RestErls  = [Source || Source <- gather_src(SrcDirs, []) ++ MoreSources,
                            lists:member(Source, FirstErls) == false],
 
-    % Sort RestErls so that parse_transforms and behaviours are first
+    % Split RestErls so that parse_transforms and behaviours are instead added
+    % to erl_first_files, parse transforms first.
     % This should probably be somewhat combined with inspect_epp
-    SortedRestErls = [K || {K, _V} <- lists:keysort(2,
-        [{F, compile_priority(F)} || F <- RestErls ])],
-
+    [ParseTransforms, Behaviours, OtherErls] = lists:foldl(fun(F, [A, B, C]) ->
+                case compile_priority(F) of
+                    parse_transform ->
+                        [[F | A], B, C];
+                    behaviour ->
+                        [A, [F | B], C];
+                    _ ->
+                        [A, B, [F | C]]
+                end
+        end, [[], [], []], RestErls),
+
+    NewFirstErls = FirstErls ++ ParseTransforms ++ Behaviours,
 
     %% Make sure that ebin/ exists and is on the path
     ok = filelib:ensure_dir(filename:join("ebin", "dummy.beam")),
     CurrPath = code:get_path(),
     code:add_path("ebin"),
-    rebar_base_compiler:run(Config, FirstErls, SortedRestErls,
+    rebar_base_compiler:run(Config, NewFirstErls, OtherErls,
                             fun(S, C) -> internal_erl_compile(S, C, OutDir,
                                                               ErlOpts)
                             end),
@@ -297,11 +307,11 @@ compile_priority(File) ->
             F2 = fun({tree,arity_qualifier,_,
                         {arity_qualifier,{tree,atom,_,behaviour_info},
                             {tree,integer,_,1}}}, _) ->
-                    2;
+                    behaviour;
                 ({tree,arity_qualifier,_,
                         {arity_qualifier,{tree,atom,_,parse_transform},
                             {tree,integer,_,2}}}, _) ->
-                    1;
+                    parse_transform;
                 (_, Acc) ->
                     Acc
             end,
@@ -313,7 +323,7 @@ compile_priority(File) ->
                     Acc
             end,
 
-            lists:foldl(F, 10, Trees)
+            lists:foldl(F, normal, Trees)
     end.
 
 %%