为什么会收到错误消息“ http：response.Write on hijacked connection”？

Question

I am trying out the gorilla websocket library to get a feel of how websockets work with Go. But I keep getting this error message when I hit refresh button on the browser.

When I reload the web page that I am using to test the websocket, I get these error messages on the Go console:

2015/09/18 19:04:41 websocket: close 1001
2015/09/18 19:04:41 http: response.Write on hijacked connection

The first one is status code for "going away". I am assuming it is because when I hit refresh it goes away from the websocket connection so that makes sense to me.

But then I get an error message that I don't understand. The hijacked one. Why do I get it and what does it mean?

I am running my code on localhost:8080 on a windows machine.

The code I am using:

func wsHandler(w http.ResponseWriter, r *http.Request, _ httprouter.Params) error {
    conn, err := websocket.Upgrade(w, r, nil, 1024, 1024)
    if err != nil {
        return err
    }
    defer conn.Close()

    for {
        _, msg, err := conn.ReadMessage()
        if err != nil {
            return err
        }
        log.Println(string(msg))
    }
    return nil
}

Client side:

var conn = new WebSocket("ws://localhost:8080/api/messages/websocket");

conn.onclose = function (e) {
    console.log("onclose fired");
};

conn.onopen = function (e) {
    console.log("onopen fired");
};

conn.onmessage = function (e) {
    console.log(e.data);
};              

setTimeout(function () {
    conn.send("foo!");
}, 1500);

When I load the page first time, only foo! is outputted to the console. So all in all, after loading the page once, and then reloading it twice I get an output like this:

2015/09/18 19:04:39 foo!
2015/09/18 19:04:41 websocket: close 1001
2015/09/18 19:04:41 http: response.Write on hijacked connection
2015/09/18 19:04:43 foo!
2015/09/18 19:04:44 websocket: close 1001
2015/09/18 19:04:44 http: response.Write on hijacked connection
2015/09/18 19:04:46 foo!

What does this mean? I'm I doing something wrong?

Answer 1

The websocket.Upgrade function hijacks the the underlying network connection from the http.ResponseWriter.

The ResponseWriter.Write method logs this message when write is called after the connection is hijacked from the writer.

It looks like the router or middleware used by the application is writing to connection after the websocket handler returns.