Software sleuthing

While I was reading Ellen Ullman's novel The Bug last month, life imitated art. The protagonist in that story is a programmer who grapples with a fiendish bug. It strikes intermittently and, to add insult to injury, the testers can never manage to capture the core dump that might yield the clue as to why.

Meanwhile, in real life, an application that I use routinely to create videos, and that has always worked reliably, began failing mysteriously -- and in my case reproducibly -- whenever I tried to produce QuickTime files. In the grand tradition of cryptic error messages, the only clue was an unhelpful -50.

Because I have a relationship with the vendor, and because I'm a fairly technical guy, we were able to collaborate on solving the problem in a way that wouldn't work for a typical customer. The lead developer wound up sending me a specially-built version of the program, one in which verbose logging was enabled, and I used that to capture a trace that led him to the solution.

As is so often true, it was a silly little thing. At some point I'd switched from using an absolute path in the Save As dialog box (c:\jon) to a relative path (\jon) that the QuickTime encoder won't accept. The application should have caught that before calling QuickTime, but didn't. Now, it does.

Ideally, of course, a unit test would have flushed out this problem before it ever got to me. But things will always slip through the safety net. When they do they're often fiendishly difficult to diagnose, and it's worth considering the reasons why.

There are many, but in my post-mortem analysis of this incident I zeroed in on two in particular: provenance and configuration.

By provenance, in this case, I mean the origin of the error code. I was running a Windows application that used a QuickTime component. From which domain did the -50 arise? Windows itself? The application? The QuickTime component? I guessed QuickTime, but searching for "quicktime error -50" yielded no insight.

As I later discovered, -50 is the generic Mac OS "Error in user parameter list". Connecting that to the relative path in my dialog box would, admittedly, still have been a long shot. But there's a chance it would have prompted me to reconsider the parameters I was sending to QuickTime.

Applications increasingly rely on components and services that can turn up in unexpected contexts. I wasn't expecting to see a Mac OS error code on my Windows box, but in a mix-and-match world, that happens. Reporting the provenance of error codes would be a helpful best practice.

If applications recorded snapshots whenever their settings were changed -- in plain text, or even better, in XML -- we'd have audit trails that would help solve a lot of these kinds of problems.

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