Avoid guessing on utf8 decoding of app files

Rather than trying one method and then the other, allow the caller to
specify the encoding of the expected file. All other schemes are risky
and won't work well.

Rollback the function's default interface to the binary format in case
any plugin used it for non-unicode content, preserving backwards compat.

src/rebar_file_utils.erl

@@ -35,6 +35,7 @@
          mv/2,
          delete_each/1,
          write_file_if_contents_differ/2,
+         write_file_if_contents_differ/3,
          system_tmpdir/0,
          system_tmpdir/1,
          reset_dir/1,
@@ -334,15 +335,19 @@ delete_each([File | Rest]) ->
             ?FAIL
     end.
 
+%% @doc backwards compat layer to pre-utf8 support
 write_file_if_contents_differ(Filename, Bytes) ->
-    %% first try to convert directly to binaries,
-    %% but if it fails, we likely contain unicode and
-    %% need special treatment
-    ToWrite = try
-                  iolist_to_binary(Bytes)
-              catch
-                  error:badarg -> unicode:characters_to_binary(Bytes)
-              end,
+    write_file_if_contents_differ(Filename, Bytes, raw).
+
+%% @doc let the user pick the encoding required; there are no good
+%% heuristics for data encoding
+write_file_if_contents_differ(Filename, Bytes, raw) ->
+    write_file_if_contents_differ_(Filename, iolist_to_binary(Bytes));
+write_file_if_contents_differ(Filename, Bytes, utf8) ->
+    write_file_if_contents_differ_(Filename, unicode:characters_to_binary(Bytes, utf8)).
+
+%% @private compare raw strings and check contents
+write_file_if_contents_differ_(Filename, ToWrite) ->
     case file:read_file(Filename) of
         {ok, ToWrite} ->
             ok;

src/rebar_otp_app.erl

@@ -127,7 +127,7 @@ preprocess(State, AppInfo, AppSrcFile) ->
             EbinDir = rebar_app_info:ebin_dir(AppInfo),
             filelib:ensure_dir(filename:join(EbinDir, "dummy.beam")),
             AppFile = rebar_app_utils:app_src_to_app(OutDir, AppSrcFile),
-            ok = rebar_file_utils:write_file_if_contents_differ(AppFile, Spec),
+            ok = rebar_file_utils:write_file_if_contents_differ(AppFile, Spec, utf8),
 
             AppFile;
         {error, Reason} ->