Write config to temp file then move

Instead of overwriting in place. This should be safer and reduce
changes of config file corruptions that could cause members to
become unable to restart.

src/ra_log.erl

@@ -724,15 +724,17 @@ overview(#?MODULE{last_index = LastIndex,
 -spec write_config(ra_server:config(), state()) -> ok.
 write_config(Config0, #?MODULE{cfg = #cfg{directory = Dir}}) ->
     ConfigPath = filename:join(Dir, "config"),
+    TmpConfigPath = filename:join(Dir, "config.tmp"),
     % clean config of potentially unserialisable data
     Config = maps:without([parent,
                            counter,
                            has_changed,
                            %% don't write system config to disk as it will
                            %% be updated each time
                            system_config], Config0),
-    ok = ra_lib:write_file(ConfigPath,
+    ok = ra_lib:write_file(TmpConfigPath,
                            list_to_binary(io_lib:format("~p.", [Config]))),
+    ok = prim_file:rename(TmpConfigPath, ConfigPath),
     ok.
 
 -spec read_config(state() | file:filename()) ->