Merge branch 'no-unnecessary-overwrite-of-same-gpb_version.hrl'

* Avoid unnecessary update of gpb_version.hrl
  when the contents will be the same. This should avoid unnecessary
  recompilations in some setups.

build/mk_version_hrl

@@ -25,18 +25,29 @@ main(Argv) ->
             {<<"@vsn@">>, Vsn},
             {<<"@vsn-source@">>, VsnSource},
             {<<"is expected to be">>, <<"was">>},
-            {<<"%% The version below">>, [<<"%% DO NOT EDIT -- generated ">>,
-                <<"from gpb_version.hrl.in\n%% The version below">>]},
+            {<<"%% The version below">>,
+             [<<"%% DO NOT EDIT -- generated from gpb_version.hrl.in\n">>,
+              <<"%% The version below">>]},
             {<<"%% NB: The build.mk_version_hrl depends.*">>, <<"">>}
         ],
-    VersionStr = lists:foldl(fun({Patten, Replacement}, Acc) ->
-        re:replace(Acc, Patten, Replacement, [{return, binary}])
-                             end, BaseStr, SedList),
-    case file:write_file(OutFile, VersionStr) of
-        ok ->
+    Content =
+        iolist_to_binary(
+          lists:foldl(
+            fun({Patten, Replacement}, Acc) ->
+                    re:replace(Acc, Patten, Replacement, [{return, binary}])
+            end, BaseStr, SedList)),
+    case file:read_file(OutFile) of
+        {ok, Content} ->
+            %% Don't overwrite the file with identical data.
+            %% This can cause build systems to rebuild unnecessarily.
             ok;
-        {error, Reason2} ->
-            file_err(write_file, OutFile, Reason2)
+        _ ->
+            case file:write_file(OutFile, Content) of
+                ok ->
+                    ok;
+                {error, Reason2} ->
+                    file_err(write_file, OutFile, Reason2)
+            end
     end.
 
 parse_opts(["--override-version="++Vsn | Rest], Opts, Args) ->