Duplicate messages emitted after error in onmessage

[jcheng5]

Using xhr-streaming, xhr-polling, or jsonp-polling, if an error is raised in the socket.onmessage event handler, then the next time a message is received SockJS will replay the message that caused the error.

For example, message A is received and the client onmessage event handler throws an uncaught error; later, message B is received, and onmessage is called again with message A.

Repro repo: https://github.com/jcheng5/sockjs-error-test

The problem appears to be here:

sockjs-client/lib/transport/receiver/xhr.js

Lines 40 to 52 in 71876b3

	for (var idx = -1; ; this.bufferPosition += idx + 1) {
	var buf = text.slice(this.bufferPosition);
	idx = buf.indexOf('\n');
	if (idx === -1) {
	break;
	}
	var msg = buf.slice(0, idx);
	if (msg) {
	debug('message', msg);
	this.emit('message', msg);
	}
	}
	};

When this.emit('message', msg); throws an exception, this.bufferPosition doesn't get updated (third clause of the for-loop). If you update this.bufferPosition before emitting the message, the problem goes away.

[aronatkins]

This appears to be the 0.3.4 equivalent code, which updates the buffer position ahead of sending an event and does not suffer from duplicate messages after error:

sockjs-client/lib/trans-receiver-xhr.js

Lines 14 to 24 in 0c70698

	that.xo.onchunk = function(status, text) {
	if (status !== 200) return;
	while (1) {
	var buf = text.slice(buf_pos);
	var p = buf.indexOf('\n');
	if (p === -1) break;
	buf_pos += p+1;
	var msg = buf.slice(0, p);
	that.dispatchEvent(new SimpleEvent('message', {data: msg}));
	}
	};

[github-actions]

This issue has been inactive for 30 days. It will be in closed in 5 days without any new activity.

[aronatkins]

Discussion has been happening in #564; false no-activity.

[github-actions]

This issue has been inactive for 30 days. It will be in closed in 5 days without any new activity.

[aronatkins]

False no-activity. Discussion is in #564; waiting for feedback from @brycekahle.

[aronatkins]

Ping to keep open; #564 is still waiting for feedback from @brycekahle.