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the evenings out here - Thoughts, rants and musings about absolutely everything except photography. Or cats.

Nasty bug squashed

in web design , Monday, May 22, 2006

I finally managed to track down a very elusive bug which was affecting the photo pages on this site. Basically, if a thumbnails page was called directly, from say a link in a gallery update blog post, or a link I sent to someone, the first time it loaded, the gallery was blank. The second time, it was fine. This seemed to be a PHP session issue: if I discarded all the session code, it worked fine - although obviously everything else that depended on sessions was broken. But when I tried to home in on a specific setting, I got nowhere. Finally, I worked out what was happening - I guess if I'd been using a browser which was less "user-friendly" about JavaScript syntax errors I would have got there quicker, but maybe not. I finally discovered that there was a small difference between the page code generated on first and subsequent entries - it is pretty hard to spot: BEFORE before.jpg AFTER after.jpg One extra double-quote was breaking the JavaScript syntax, killing the onLoad event handler. So where did this come from ? Well it seems that if PHP session_start does not find a current session, it rewrites any links it finds in the page to include the new session ID. The problem is, it isn't so clever about what is and isn't a link, and since I am actually writing a JavaScript function on the fly, various escaped quotes and things were confusing it. First, I realised it was doing this elsewhere, although in those cases it was actually harmless. I managed to stop it by using single quotes instead of double quotes. But the line above - where for some reason, no session ID was written, just a superfluous extra " inserted - was harder to fix. Finally, I had to break one string concatenation into several steps, and finally I got the code I was trying to get the page to write, instead of what the PHP interpreter thought I wanted. That has got to be one of the most obscure bugs I've ever had to track down on a web site...