Question

I have 2 different XMLHttpRequest objects making asynchronous requests to the same web service. The web service sends back a cookie in the HTTP response headers, that I can read with the chrome.cookies.read extension (the request is cross-domain).

Each xhr reads the same cookie by name, but the value of the cookie after each response will be different, as it is a unique token for each requester.

So I want to store each cookie value per request, just after receiving the HTTP response, for later use.

So here’s what my code is currently doing:

var my_cookie_value = [] ;

function begin() 
{
    for (var i = 0 ; i < 2 ; i ++)
    {
        sendRequest(some_url, i);  
    }
}  

function sendRequest(some_url, thread)
{
    var xmlhttp=new XMLHttpRequest();
    xmlhttp.onreadystatechange=function()
    {
        if (xmlhttp.readyState == 4 && xmlhttp.status == 200)
        {
            readCookie(thread) ;
        } 
    }
    xmlhttp.open("GET", some_url, true);
    xmlhttp.send();
}

function readCookie(thread)
{
    chrome.cookies.get(
    {
        "url" : some_domain, 
        "name" : "mycookie"
    }, 
    function(cookie)
    {
       if (cookie != null)
       { 
           my_cookie_value[thread] = cookie.value ;
       }
    }
    );
}

However because chrome.cookies.read uses a callback function with no guaranteed time to fire, I am finding the cookie values get mixed up (or repeated) for each thread, even though they were returned from the server as distinct values.

I think what can happen is:

thread 0 sends request
thread 1 sends request

thread 0 receives response
thread 0 requests cookie value callback

Arrgghh!!…worst case scenario coming up…

thread 1 receives response
thread 1 requests cookie value callback

thread 0 cookie callback fires and cookie value (FOR THREAD 1) is copied into my_cookie_value[0]
thread 1 cookie callback fires and cookie value (FOR THREAD 1) is copied into my_cookie_value[1]

This is obviously not what I wanted. my_cookie_value[0] and [1] both contain the token for thread 1.

So finally to the question: Is there another solution for this? I cannot rewrite the server code, I can only access the client.

I am wondering why oh why is reading a cookie done by a callback function??

Answer 1

Set up var that tracks if you currently have a request active. Something like var currentRequest = false and set it to true before starting a request and set it to false after a request is complete. Before starting a request check if there is an active request and if there is either delay or create a queue of pending requests.