Do not rewrite .app file if it didn't change

src/rebar_file_utils.erl

@@ -29,7 +29,8 @@
 -export([rm_rf/1,
          cp_r/2,
          mv/2,
-         delete_each/1]).
+         delete_each/1,
+         write_file_if_contents_differ/2]).
 
 -include("rebar.hrl").
 
@@ -111,6 +112,17 @@ delete_each([File | Rest]) ->
             ?FAIL
     end.
 
+write_file_if_contents_differ(Filename, Bytes) ->
+    ToWrite = iolist_to_binary(Bytes),
+    case file:read_file(Filename) of
+        {ok, ToWrite} ->
+            ok;
+        {ok,  _} ->
+            file:write_file(Filename, ToWrite);
+        {error,  _} ->
+            file:write_file(Filename, ToWrite)
+    end.
+
 %% ===================================================================
 %% Internal functions
 %% ===================================================================

src/rebar_otp_app.erl

@@ -106,7 +106,7 @@ preprocess(Config, AppSrcFile) ->
 
             %% Setup file .app filename and write new contents
             AppFile = rebar_app_utils:app_src_to_app(AppSrcFile),
-            ok = file:write_file(AppFile, Spec),
+            ok = rebar_file_utils:write_file_if_contents_differ(AppFile, Spec),
 
             %% Make certain that the ebin/ directory is available
             %% on the code path