by Leon Rosenshein

Your Bug, Your Customer's Feature

It's not a bug, it's a feature. We've all heard it. We've probably all said it. But think about it. What exactly is a feature. It's the application doing something the customer wants, when the customer wants it done. It doesn't matter if that's what you expected. It doesn't matter if that's what the spec called for or the PM really wanted. It's fulfilling a customer need. And to the customer, that's a feature.

Back when I was working on Falcon 4.0 we kept a lot of internal display information in a couple of buffers. To make runtime debugging easier we turned those buffers into a shared memory segment and would run external tools to display it. It was so useful we created some additional structures and added other internal state. Really handy and let us do things like write to a memory mapped bitmap display (a Hercules graphics card) while DirectX had the main monitor.

The "bug" was leaving it on when we sent out beta versions to testers. Our testers were a creative bunch. Many of them had homemade cockpits. And multiple displays. One of them even had physical gauges that they used with someone else's product. And they found our shared memory. Then they decoded part of it. They used it to drive their own displays and gauges. Then, they thanked us for building such a useful feature and asked if the documentation of the memory layout was ready yet. And suddenly an internal debugging aid that slipped out to our beta testers became a feature. In this case it wasn't a big deal and didn't add a lot of support burden, but it could have.

When I worked on MS FlightSim backward compatibility was a major feature of every release, and not just the "supported" features. As a "platform" we choose to support all add-ons developed with the last 2 or 3 versions, and most of the rest, regardless of which bug/corner case, or side effect the add-on used. It made for great PR, but it was a lot of work.

So be careful what you allow to slip out. It will come back to haunt you.

NB: This is the kind of thing your users can do with your "bugs" (Lines 177-229 are definitely from my original shared mem structure. I think the next 30 or so are. After that it's aftermarket stuff)