enforce asynchrony in new database offset methods

lib/storage/file.js

@@ -1222,17 +1222,18 @@ module.exports.create = function (conf, _cb) {
         // OFFSETS
 // these exist strictly as an optimization
 // you can always remove them without data loss
-            clearOffset: function (channelName, cb) {
+            clearOffset: function (channelName, _cb) {
+                var cb = Util.once(Util.mkAsync(_cb));
                 if (!isValidChannelId(channelName)) { return void cb(new Error('EINVAL')); }
                 clearOffset(env, channelName, cb);
             },
             writeOffset: function (channelName, data, _cb) {
-                var cb = Util.mkAsync(_cb);
+                var cb = Util.once(Util.mkAsync(_cb));
                 if (!isValidChannelId(channelName)) { return void cb(new Error('EINVAL')); }
                 writeOffset(env, channelName, data, cb);
             },
             getOffset: function (channelName, _cb) {
-                var cb = Util.mkAsync(_cb);
+                var cb = Util.once(Util.mkAsync(_cb));
                 if (!isValidChannelId(channelName)) { return void cb(new Error('EINVAL')); }
                 getOffset(env, channelName, cb);
             },