Archive for June, 2012

double HTTP requests

We first discovered that our webapp was receiving duplicate HTTP AJAX requests from clients that results in a database insert. Fortunately jQuery had a nocache timestamp as part of the AJAX request so we could recognize it as a duplicate and reject the 2nd request.

As we tried to narrow down the cause we found that it happens on both GET/POST, as well as on a variety of browsers and network providers. After days of trying to reproducing the behavior we resorted to using iMacro to do browser automation and finally manage to reproduce it occasionally. What was surprising was that we were receiving the response of the 2nd response, which was the rejection message! (while the database insert was successful) It was absolutely confusing and we set up the automation again with Wireshark.

The packet analysis confirmed that the client browser was sending the duplicate request. However we also scanned Firebug and confirmed that only a single AJAX call was made! It didn’t make any sense to any of us until I noticed a pattern in the Wireshark logs that the 1st request made was always terminated without a response (either a TCP Connection Reset or a proper TCP Connection Close initiated by the server), and the 2nd request would be made on another kept-alive TCP connection. Following that symptom with Google I chanced upon a blog that highlighted HTTP 1.1 RFC 8.2.4.

If an HTTP/1.1 client sends a request which includes a request body, but which does not include an Expect request-header field with the “100-continue” expectation, and if the client is not directly connected to an HTTP/1.1 origin server, and if the client sees the connection close before receiving any status from the server, the client SHOULD retry the request.

Could this be it? We mocked up a HTTP server using raw Java ServerSockets by intentionally closing the first connection and allowing the second. We logged the requests on the server-side and voila! The mock server showed that it received two HTTP requests, Firebug showed that it sent one only, and Wireshark showed that the browser sent two.

Essentially, we just proved that the browsers adhered to HTTP 1.1 RFC 8.2.4…….

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Trimpath unterminated string literal in IE

We’re using an ancient library called TrimPath to render templated data with JavaScript, and a peer hit the “unterminated string literal” in eval() on IE only. I assisted to solve it, by extracting the offending string and isolated the cause to a ‘\n’ within the string. It was easy to cut away the additional newlines and IE is happy. (This is on the assumption that the newline are decorative and not necessary as part of the output)

However, I wasn’t satisfied what caused the newline to be parsed wrongly, and why IE only. The good thing with Open Source is we get to dig in ourselves to troubleshoot issues.

After stepping through the cause is identified: multiple continuous \n not parsed correctly. Apparently someone hit this at least 5 years ago and there was a fix for it, by detecting the additional \n and eating it up. But it still fails if there are more \n.

Why IE only? We were using script tags as the template, and in IE script.innerHTML is prefixed with an additional \n.

The following snippet will fail in IE, but pass in FF.


<html>
    <head>
        <script src="trimpath-template-1.1.2.js"></script>
    </head>
    <body>
        <script id="src" type="text/html">

            Test
        </script>
        <div id="out"></div>
        <script>
            document.getElementById("out").innerHTML = 
                TrimPath.processDOMTemplate("src", null);
        </script>
    </body>
</html>

If another newline is added (with no additional whitepsace), FF will fail with the same error.
If <textarea> instead of <script> is used, it will pass, but adding more newlines will also cause it to fail.

As an anecdote, I’m not sure how our project got to use 1.1.2, when the last on the download site is 1.0.38 (legacy stuff…). But a search on the web shows it’s not just us

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