This issue might be an issue with socket.io itself, but I have only ever produced it using socket.io-server-java. It can be recreated using the chat sample that comes with this code.

Steps to reproduce:
1. Modify chat.html so that it declares xhr polling as the transport. This problem is specific to xhr polling (which is what I need to use for my project for various reasons)

      var socket = io('http://localhost:8080/chat', {
       transports: ['polling']
//       transports: ['websocket']
       });

2. run ChatServer
3. browse to http://localhost:8080/chat.html
4. put /rbinary in over and over again, while watching the log console

Sooner or later, you'll have this exception show up in the logs:
com.codeminders.socketio.server.SocketIOProtocolException: Expected binary marker in the payload
at ... EngineIOProtocol.binaryDecodePayload(EngineIOProtocol.java:299)
And the Socket.IO connection will be terminated and re-established.

There doesn't appear to be any predictability on when the communication will fail. Sometimes it happens first, sometimes it takes a few /rbinary commands to get it to happen.

The code here is clearly where the exception occurred:

com.codeminders.socketio.protocol.EngineIOProtocol
public static List<EngineIOPacket> binaryDecodePayload(InputStream is)
        throws IOException
{
    ArrayList<EngineIOPacket> packets = new ArrayList<>();
    if(is.read() != 1)
        throw new SocketIOProtocolException("Expected binary marker in the payload");

}

I dug in a bit and I learned a few things by going through with a debugger in both Chrome and Firefox and in Java and I learned a few things.

• The binary begins with a 1 for all the request where everything works. For the one with a problem, it will begin with a 0 (which triggers the error).

• What appears to be happening is that socket.io combines multiple sends into a single binary send. A functional send will follow something like this:

[TEXT POST] 25:42/chat,["server binary"]
[TEXT POST] 42:461-/chat,4[{"_placeholder":true,"num":0}]
[BINARY POST] ��ÿ�����   (binary blob)

but when it goes wrong it looks something like this (unfortunately Firefox won't let me copy directly):

[TEXT POST] 25:42/chat,["server binary"]
[BINARY POST] [binarystuffhere]ÿ461-/chat,0[{"_placeholder":true,"num":0}][binarystuffhere]

I copied the latest socket.io.js (1.7.2) down from:
https://github.com/socketio/socket.io-client/blob/master/dist/socket.io.js
and ran with it, and it seemed to have the same problem.

Is this an issue with socket.io creating an invalid message? Or is this an issue with socket.io-server-java not being able to parse a valid message?