Fab.js and String Objects

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While messing about with my (fab) game today, I notice that the chat system is completely broken. Sigh.

By completely broken, when I POST a chat message, I get an "empty" response from the fab.js backend:

cstrom@whitefall:~/repos/my_fab_game$ curl http://localhost:4011/chat -i -d '{"id":"foo", "say":"hi"}'
curl: (52) Empty reply from server

Looking at the backend, I find that it has crashed completely:

...
cstrom@whitefall:~/repos/my_fab_game$ ./game.js
/home/cstrom/repos/my_fab_game/game.js:34
            broadcast(comet_player_say(obj.body.substr(0,100)));
                                                ^
TypeError: Object {"id":"foo", "say":"hi"} has no method 'substr'
    at listener (/home/cstrom/repos/my_fab_game/game.js:34:49)
    at IncomingMessage.<anonymous> (/home/cstrom/.node_libraries/fab/apps/fab.nodejs.js:29:36)
    at IncomingMessage.emit (events:25:26)
    at HTTPParser.onBody (http:98:23)
    at Stream.ondata (http:745:22)
    at IOWatcher.callback (net:373:31)
    at node.js:204:9

Like I said...

Sigh.

I added the substr to limit the chat message to 100 characters, but didn't notice that that the body of the message was a JSON string rather than the chat message itself. Since it is a JSON string, I need to parse the JSON into a Javascript object, limit the chat message in that object to 100 characters, then re-stringify it to be broadcast to the game:

  ( /chat/ )
    ( function() {
        var out = this;
        return function listener( obj ) {
          if ( !obj ) out();
          else if ( obj.body ) {
            var msg = JSON.parse(obj.body);
            msg.body = msg.say.substr(0,100);
            broadcast(comet_player_say(JSON.stringify(msg)));
          }
          return listener;
        };
      } )

Sadly, when I restart and POST to the chat resource, I still get an empty resource:

cstrom@whitefall:~/repos/my_fab_game$ curl http://localhost:4011/chat -i -d '{"id":"foo", "say":"hi"}'
curl: (52) Empty reply from server

The backend is still crashing, this time with a different message:

cstrom@whitefall:~/repos/my_fab_game$ ./game.js
/home/cstrom/repos/my_fab_game/game.js:34
            var msg = JSON.parse(obj.body);
                           ^

Hunh? I am not even sure what that means there is not even an error message. Let's have a look at the obj.body just before it crashes:

//...
          else if ( obj.body ) {
            for (var prop in obj.body) {
              puts(prop + ": " + obj.body[prop]);
            }
            var msg = JSON.parse(obj.body);
            msg.body = msg.say.substr(0,100);
            broadcast(comet_player_say(JSON.stringify(msg)));
          }
//...

After I crash the server, I find this in the output:

cstrom@whitefall:~/repos/my_fab_game$ ./game.js
0: 123
1: 34
2: 105
3: 100
4: 34
...
22: 34
23: 125
length: 24
binarySlice: function binarySlice() { [native code] }
asciiSlice: function asciiSlice() { [native code] }
...
toString: function (encoding, start, stop) {
  encoding = (encoding || 'utf8').toLowerCase();
...

Oh man! That is a Javascript String object, not a string literal. Happily, there is a toString() method in that list of properties. I can use that to resolve the problem:

  ( /chat/ )
    ( function() {
        var out = this;
        return function listener( obj ) {
          if ( !obj ) out();
          else if ( obj.body ) {
            var msg = JSON.parse(obj.body.toString());
            msg.body = msg.say.substr(0,100);
            broadcast(comet_player_say(JSON.stringify(msg)));
          }
          return listener;
        };
      } )

Indeed that does resolve the problem. I now have my chat system working again.

That Javascript String object has me a bit worried, though. I know that the fab.js source uses typeof obj == "string" checks in places. Those will fail for String objects (typeof would return "object").

To test this out, I convert the fab app upstream of the chat resource into a binary app. Then, I can delegate the responsibility of converting the supplied JSON string into a JSON object to the built-in fab.parse app:

  ( /chat/ )
    (
      function(app) {
        return function() {
          var out = this;
          return app.call( function listener( obj ) {
            if ( !obj ) out();
            else if ( obj.body ) {
              var msg = {
                id: obj.body.id,
                say: obj.body.say.substr(0,100)
              };
              broadcast(comet_player_say(JSON.stringify(msg)));
            }
            return listener;
          });
        };
      } )
    (fab.parse)
    (fab.echo)

When I access the chat resource now, I get an error similar to the unary version of the chat fab app:

cstrom@whitefall:~/repos/my_fab_game$ ./game.js
/home/cstrom/repos/my_fab_game/game.js:38
                say: obj.body.say.substr(0,100)
                                  ^
TypeError: Cannot call method 'substr' of undefined
    at listener (/home/cstrom/repos/my_fab_game/game.js:38:35)
    at listener (/home/cstrom/.node_libraries/fab/apps/fab.map.js:11:19)
    at IncomingMessage.<anonymous> (/home/cstrom/.node_libraries/fab/apps/fab.nodejs.js:29:36)
    at IncomingMessage.emit (events:25:26)
    at HTTPParser.onBody (http:98:23)
    at Stream.ondata (http:745:22)
    at IOWatcher.callback (net:373:31)
    at node.js:204:9

If I alter the fab.parse.js app from:

exports.app = fab.map( function( obj ) {
  var body = obj.body;
  if ( typeof body == "string" ) obj.body = JSON.parse( body );
  return obj;
})

To make use of toString() (which works on string literals and String objects):

exports.app = fab.map( function( obj ) {
  var body = obj.body;
  if ( body && body.toString ) obj.body = JSON.parse( body.toString() );
  return obj;
})

Then my chat server works again.

I suspect that recent node.js changes have started sending String objects and string literals interchangeably. I will investigate a bit more tomorrow then address the issue in the remaining built-in fab apps.

Day #128